1

SECTION CHAPTER

3 Nervous system

The nervous system has two major divisions, the central nervous system with neurones in many different ways; their two-way communication (CNS) and the peripheral nervous system (PNS). The CNS consists of is essential for normal brain activity. the brain, spinal cord, optic nerve and retina, and contains the majority It was thought for many years that glia outnumbered neurones by of neuronal cell bodies. The PNS includes all nervous tissue outside the 10 times in the CNS, but recent studies using the isotropic fractionator CNS and consists of the cranial and spinal nerves, the peripheral auto- method have challenged that popular view, suggesting instead that the nomic nervous system (ANS) and the special senses (taste, olfaction, two cell populations are rather similar in size (Azevedo et al 2009). That vision, hearing and balance). It is composed mainly of the axons of said, the glia : neurone ratio has been reported to be as high as 17 : 1 in sensory and motor neurones that pass between the CNS and the body. the thalamus (Pakkenberg and Gundersen 1988). The ANS is subdivided into sympathetic and parasympathetic compo- The glial population in the CNS consists of microglia and macroglia; nents. It consists of neurones that innervate secretory glands and cardiac the latter are subdivided into oligodendrocytes and astrocytes. The and smooth muscle, and is concerned primarily with control of the principal glial cell in the PNS is the Schwann cell. Satellite cells sur- internal environment. Neurones in the wall of the gastrointestinal tract round each neuronal soma in ganglia. form the enteric nervous system (ENS) and are capable of sustaining For further reading on the nervous system, see Finger (2001), Kandel local reflex activity that is independent of the CNS. The ENS contains et al (2012), Kettenmann and Ransom (2012), Levitan and Kaczmarek as many intrinsic neurones in its ganglia as the entire spinal cord and (2001), Nicholls et al (2011) and Squire et al (2012). is often considered as a third division of the nervous system (Gershon 1998). In the CNS, the cell bodies of neurones are often grouped together NEURONES in discrete areas termed nuclei, or they may form more extensive layers or masses of cells; collectively they constitute the grey matter. Neuronal Most of the neurones in the CNS are either clustered into nuclei, dendrites and synaptic contacts are mostly confined to areas of grey columns or layers, or dispersed within grey matter. Neurones in the PNS matter and form part of its meshwork of neuronal and glial processes, are confined to ganglia. Irrespective of location, neurones share many termed the neuropil. Their axons join bundles of nerve fibres that tend general features, which are discussed here in the context of central to be grouped separately to form tracts. In the spinal cord, cerebellum, neurones. Special characteristics of ganglionic neurones and their adja- cerebral cortices and some other areas, concentrations of tracts consti- cent tissues are discussed on page 57. tute the white matter, so called because the axons are often ensheathed Neurones exhibit great variability in their size (cell bodies range in lipid-rich sheaths of myelin, which is white when fresh (Fig. 3.1; see from 5 to 100 μm diameter) and shape (Spruston 2008). Their surface Fig. 16.9). areas are extensive because most neurones display numerous branched The PNS is composed of the efferent axons (fibres) of motor neu- cell processes. They usually have a rounded or polygonal cell body rones situated inside the CNS, and the cell bodies of sensory neurones (perikaryon or soma). This is a central mass of cytoplasm that encloses (grouped together as ganglia) and their afferent processes. Sensory cells a nucleus and gives off long, branched extensions with which most in dorsal root ganglia give off both centrally and peripherally directed intercellular contacts are made. Typically, one of these processes, the processes; there are no synapses on their cell bodies. Also included are axon, is much longer than the others, the dendrites (Fig. 3.2). Gener- ganglionic neurones of the ANS, which receive synaptic contacts from ally, dendrites conduct electrical signals towards a soma whereas axons the peripheral fibres of preganglionic autonomic neurones whose cell conduct impulses away from it. bodies lie within the CNS. For further details of the organization of the Neurones can be classified according to the number and arrange- nervous system, see Chapter 16. ment of their processes (Bota and Swanson 2007). Multipolar neurones When the neural tube is formed during prenatal development (Sanes (Fig. 3.3) are common; they have an extensive dendritic tree that arises et al 2011), its walls thicken greatly but do not completely obliterate either from a single primary dendrite or directly from the soma, and a the cavity within. The latter remains in the spinal cord as the narrow single axon. Bipolar neurones, which typify neurones of the special central canal and becomes greatly expanded in the brain to form a series sensory systems, have only a single dendrite that emerges from the soma of interconnected cavities called the ventricular system. In the fore- and opposite the axonal pole. Unipolar neurones, which transmit general hindbrains, parts of the neural tube roof do not generate neurones but sensation, e.g. dorsal root ganglion neurones, have a single short process become thin, folded sheets of secretory tissue, which are invaded by that bifurcates into a peripheral and a central process. This arrangement blood vessels and are called the choroid plexuses. The latter secrete arises by the fusion of the proximal axonal and dendritic processes of cerebrospinal fluid (CSF), which fills the ventricles and subarachnoid a bipolar neurone during development, and so such neurones may also spaces, and penetrates the intercellular spaces of the brain and spinal be termed pseudounipolar. Neurones may also be classified according cord to create their interstitial fluid. The CNS has a rich blood supply, to whether their axons terminate locally on other neurones (interneu- which is essential to sustain its high metabolic rate. The blood–brain rones), or transmit impulses over long distances, often to distinct ter- barrier places considerable restrictions on the substances that are able ritories via defined tracts (projection neurones). to diffuse from the blood stream into the neuropil. Neurones are postmitotic cells and, with few exceptions, they are not Neurones encode information, conduct it over considerable dis- replaced when lost. tances, and then transmit it to other neurones or to various non-neural targets such as muscle cells. The propagation of this information within the nervous system depends on rapid electrical signals, the action SOMA potentials. Transmission to other cells is mediated by secretion of neu- rotransmitters at special junctions, either with other neurones (syn- The plasma membrane of the soma is generally unmyelinated and apses), or with cells outside the nervous system, e.g. muscle cells at is contacted by both inhibitory and excitatory axosomatic synapses; neuromuscular junctions, gland cells or adipose tissue, and causes very occasionally, somasomatic and dendrosomatic contacts may be changes in the behaviour of the target cells. made. The non-synaptic surface may contain gap junctions and is partly The nervous system contains large populations of non-neuronal covered by either astrocytic or satellite oligodendrocyte processes. cells, termed neuroglia or glia. These cells do not generate action poten- The cytoplasm of a typical soma (see Fig. 3.2) is rich in rough and tials, but convey information encoded as transient changes in intracel- smooth endoplasmic reticulum and free polyribosomes, indicating 42 lular calcium concentration, termed calcium signalling. Glia interact a high level of protein synthetic activity. Free polyribosomes often 3 Nervous system CHAPTER

WM

GM

Fig. 3.1 A section through the human cerebellum stained to show the arrangement in the brain of the central white matter (WM, deep pink) and the highly folded outer grey matter (GM). In the cerebellum, GM consists of an inner granular layer of tightly packed small neurones (blue) and an outermost molecular layer (pale pink), where neuronal processes make synaptic contacts. (Courtesy of Mr Peter Helliwell and the late Dr Joseph Mathew, Department of Histopathology, Royal Cornwall Hospitals Trust, UK.)

42.e1 3 Neurones

tide subunits, NF-L (68 kDa), NF-M (160 kDa) and NF-H (200 kDa). NF-M and NF-H have long C-terminal domains that project as side arms from the assembled neurofilament and bind to neighbouring filaments. They can be heavily phosphorylated, particularly in the highly stable CHAPTER neurofilaments of mature axons, and are thought to give axons their tensile strength. Some axons are almost filled by neurofilaments. Soma Microtubules are important in axonal transport, although dendrites Nucleolus usually have more microtubules than axons. Centrioles persist in Nucleus mature postmitotic neurones, where they are concerned with the gen- eration of microtubules rather than cell division. Centrioles are associ- ated with cilia on the surfaces of developing neuroblasts. Their significance, other than at some sensory endings (e.g. the olfactory Axon hillock mucosa), is not known. Pigment granules (Fig. 3.5) appear in certain regions, e.g. neurones Dendrite of the substantia nigra contain neuromelanin, which is probably a waste product of catecholamine synthesis. A similar pigment gives a bluish colour to the neurones in the locus coeruleus. Some neurones are unusually rich in metals, which may form components of enzyme Axon systems, e.g. zinc in the hippocampus and iron in the red nucleus. Ageing neurones, especially in spinal ganglia, accumulate granules of Myelin sheath lipofuscin (senility pigment) in residual bodies, which are lysosomes packed with partially degraded lipoprotein material.

Axodendritic DENDRITES synapse Dendrites are highly branched, usually short processes that project from Axosomatic the soma (see Fig. 3.2; Shah et al 2010). The branching patterns of many synapse dendritic arrays are probably established by random adhesive interac- Axon collateral tions between dendritic growth cones and afferent axons that occur Axo-axonal during development. There is an overproduction of dendrites in early synapse development, and this is pruned in response to functional demand as the nervous system matures and information is processed through the dendritic tree. There is evidence that dendritic trees may be plastic structures throughout adult life, expanding and contracting as the traffic of synaptic activity varies through afferent axodendritic contacts (for a Synaptic terminals review, see Wong and Ghosh (2002)). Groups of neurones with similar functions have a similar stereotypic tree structure (Fig. 3.6), suggesting that the branching patterns of dendrites are important determinants of the integration of the afferent inputs that converge on the tree. For a Fig. 3.2 A schematic view of typical neurones featuring one with the review of current research on dendritic trees in the normal and patho- soma cut away to show the nucleus and cytoplasmic organelles, dendritic logical brain, see Kulkarni and Firestein (2012). trees with synaptic contacts, other types of synapse, the axon hillock and Dendrites differ from axons in many respects. They represent the a myelinated axon. afferent rather than the efferent system of the neurone, and receive both excitatory and inhibitory axodendritic contacts. They may also make dendrodendritic and dendrosomatic connections (see Fig. 3.9), some congregate in large groups associated with the rough endoplasmic retic- of which are reciprocal. Synapses occur either on small projections ulum. These aggregates of RNA-rich structures are visible by light micro- called dendritic spines or on the smooth dendritic surface. Dendrites scopy as basophilic Nissl bodies or granules. They are distributed contain ribosomes, smooth endoplasmic reticulum, microtubules, neu- throughout the cell body and large dendrites; the axon hillock is con- rofilaments, actin filaments and Golgi complexes. Their neurofilament spicuously ribosome-free. Nissl bodies are more obvious in large, highly proteins are poorly phosphorylated and their microtubules express the active cells, such as spinal motor neurones (Fig. 3.4), which contain microtubule-associated protein (MAP)-2 almost exclusively in compari- large stacks of rough endoplasmic reticulum and polyribosome aggre- son with axons. gates. Maintenance and turnover of cytoplasmic and membranous com- The shapes of dendritic spines range from simple protrusions to ponents are necessary activities in all cells; the huge total volume of structures with a slender stalk and expanded distal end. Most spines are cytoplasm within the soma and processes of many neurones requires a not more than 2 μm long, and have one or more terminal expansions; considerable commitment of protein synthetic machinery. Neurones they can also be short and stubby, branched or bulbous. Large mush- also synthesize other proteins (enzyme systems, G-protein coupled room spines are assumed to have differentiated in response to afferent receptors, scaffold proteins) involved in the production of neurotrans- activity (‘memory spines’; Matsuzaki et al 2004). These large spines mitters and in the reception and transduction of incoming stimuli. often contain a spine apparatus, an organelle consisting of small Various transmembrane channel proteins and enzymes are located at sacs of endoplasmic reticulum interleaved by electron-dense bars (Gray the surfaces of neurones, where they are associated with movements 1959, Segal et al 2010). Mouse mutants deficient in these organelles of ions. show memory deficits (Deller et al 2003). Free ribosomes and polyri- The nucleus is characteristically large and euchromatic, and contains bosomes are concentrated at the base of the spine. Ribosomal accumu- at least one prominent nucleolus; these are features typical of all cells lations near synaptic sites provide a mechanism for activity-dependent engaged in substantial levels of protein synthesis. The cytoplasm con- synaptic plasticity through the local regulation of protein synthesis. tains many mitochondria and moderate numbers of lysosomes. Golgi complexes are usually close to the nucleus, near the bases of the main dendrites and opposite the axon hillock. AXONS The neuronal cytoskeleton is a prominent feature of its cytoplasm and gives shape, strength and support to the dendrites and axons. A The axon originates either from the soma or from the proximal segment number of neurodegenerative diseases are characterized by abnormal of a dendrite at a specialized region free of Nissl granules, the axon aggregates of cytoskeletal proteins (reviewed in Cairns et al (2004)). hillock (see Fig. 3.2). Action potentials are initiated here, at the junction Neurofilaments (the intermediate filaments of neurones) and microtu- with the proximal axon (axon initial segment). The axonal plasma bules are abundant in the soma and along dendrites and axons; the membrane (axolemma) is undercoated at the hillock by a concentration proportions vary with the type of neurone and cell process. Bundles of of cytoskeletal molecules, including spectrin and actin fibrils, which are neurofilaments constitute neurofibrils, which can be seen by light important in anchoring numerous voltage-sensitive channels to the microscopy in silver-stained or immunolabelled sections. Neurofila- membrane. For details, see Bender and Trussell (2012), and for neural ments are heteropolymers of proteins assembled from three polypep- electrophysiological techniques, see Sakmann and Neher (2009). The 43 1 NERVOUS SYSTEM

Sensory Integrative Motor Dendrites

SECTION Apical dendrites Sensory endings e.g. in skin Nissl bodies in soma

Pyramidal cell soma Unipolar neurone Purkinje cell soma

Basal Large motor dendrites neurone Axon Axon Soma

Peripheral Bipolar axon neurone Presynaptic autonomic neurone Axon Soma

Soma Axon

Soma Postsynaptic autonomic neurone Axon

Central axon

Smooth muscle Axon e.g intestine Soma Interneurone Interneurone

Axon Striated (skeletal) muscle

Fig. 3.3 The variety of shapes of neurones and their processes. The inset shows a human multipolar retinal ganglion cell, filled with fluorescent dye by microinjection. (Inset, Courtesy of Drs Richard Wingate, James Morgan and Ian Thompson, King’s College, London.)

Fig. 3.5 Neurones in the substantia nigra of the human midbrain, S showing cytoplasmic granules of neuromelanin pigment. (Courtesy of Mr Peter Helliwell and the late Dr Joseph Mathew, Department of N Histopathology, Royal G P Cornwall Hospitals Trust, UK.)

P

Fig. 3.4 Spinal motor neurones (toluidine blue stained resin section, rat tissue) showing a group of cell bodies (somata, S), some with the proximal parts of axonal and dendritic processes (P) visible. Their nuclei (N) typically have prominent, deeply staining nucleoli, indicative of metabolically highly active cells; two are visible in the plane of section. Nissl granules (G) are seen in the cytoplasm. Surrounding the neuronal somata is the neuropil, consisting of the interwoven processes of these 44 and other neurones and of glial cells. 3 Neurones

Fig. 3.6 A Purkinje role in Alzheimer’s disease (Cairns et al 2004): formation of tau oli- neurone from the gomers and the subsequent pathological filament arrays are critical cerebellum of a rat steps in the aetiopathogenesis of this condition. Neurofilament proteins stained by the ranging from high to low molecular weights are highly phosphorylated CHAPTER Golgi–Cox method, in mature axons, whereas growing and regenerating axons express a showing the extensive calmodulin-binding membrane-associated phosphoprotein, growth- two-dimensional array associated protein-43 (GAP-43), as well as poorly phosphorylated of dendrites. (Courtesy neurofilaments. of Dr Martin Sadler and Neurones respond differently to injury, depending on whether the Professor M Berry, damage occurs in the CNS or the PNS. The glial microenvironment of Division of Anatomy and Cell Biology, GKT a damaged central axon does not facilitate axonal regrowth; conse- School of Medicine, quently, reconnection with original synaptic targets does not normally London.) occur. In marked contrast, the glial microenvironment in the PNS is capable of facilitating axonal regrowth. However, functional outcome of clinical repair of a large mixed peripheral nerve, especially if the injury occurs some distance from the target organ, or produces a long defect in the damaged nerve, is frequently unsatisfactory (Birch 2011; see also Commentary 1.6). Axoplasmic flow

Neuronal organelles and cytoplasm are in continual motion. Bidirec- tional streaming of vesicles along axons results in a net transport of materials from the soma to the terminals, with more limited movement in the opposite direction. Two major types of transport occur, one slow and one relatively fast. Slow axonal transport is a bulk flow of axoplasm only in the anterograde direction, carrying cytoskeletal proteins and soluble, non-membrane-bound proteins from the soma to the termi- axon hillock is unmyelinated and often participates in inhibitory axo- nals at a rate of approximately 0.1–3 mm a day. In contrast, fast axonal axonal synapses. It is unique because it contains ribosomal aggregates transport carries membrane-bound vesicular material (endosomes and immediately below the postsynaptic membrane (Kole and Stuart 2012). lysosomal autophagic vacuoles) and mitochondria at approximately In the CNS, small, unmyelinated axons lie free in the neuropil, 200 mm a day in the retrograde direction (towards the soma) and whereas in the PNS they are embedded in Schwann cell cytoplasm. approximately 40 mm per day anterogradely (in particular, synaptic Myelin, which is formed around almost all axons of >2 μm diameter vesicles containing neurotransmitters). by oligodendrocytes in the CNS and by Schwann cells in the PNS, Rapid flow depends on microtubules. Vesicles with side projections begins at the distal end of the axon hillock. Nodes of Ranvier are spe- line up along microtubules and are transported along them by their cialized regions of myelin-free axon where action potentials are gener- side arms. Two microtubule-based motor proteins with adenosine 5′- ated and where an axon may branch. In both CNS and PNS, the territory triphosphatase (ATPase) activity are involved in fast transport: kinesin of a myelinated axon between adjacent nodes is called an internode; family proteins are responsible for the fast component of anterograde the region close to a node, where the myelin sheath terminates, is called transport, and cytoplasmic dynein is responsible for retrograde trans- the paranode; and the region just beyond that is the juxtaparanode. port. Retrograde transport mediates the movement of neurotrophic Myelin thickness and internodal lengths are, in general, positively cor- viruses, e.g. herpes zoster, rabies and polio, from peripheral terminals, related with axon diameter. The density of sodium channels in the and their subsequent concentration in the neuronal soma. It has been axolemma is highest at nodes of Ranvier, and very low along internodal suggested that specific pools of endocytic organelles, signalling endo- membranes; sodium channels are spread more evenly within the axo- somes, mediate the long-distance axonal transport of growth factors, lemma of unmyelinated axons. Fast potassium channels are present in such as neurotrophins and their signalling receptors. Defects in axonal the paranodal regions of myelinated axons. Fine processes of glial cyto- and dendritic transport have been linked to various neurodegenerative plasm (astrocytic in the CNS, Schwann cell in the PNS) surround the processes. See Guzik and Goldstein (2004), Hinckelmann et al (2013) nodal axolemma. and Schmieg et al (2014) for reviews of axonal transport in health and The terminals of an axon are unmyelinated. Most expand into presy- disease. naptic boutons, which may form connections with axons, dendrites, neuronal somata or, in the periphery, muscle fibres, glands and lym- phoid tissue. Exceptions include the free afferent sensory endings in, for example, the epidermis, which are unspecialized structurally, and SYNAPSES the peripheral terminals of afferent sensory fibres with encapsulated endings (see Fig. 3.27). Axon terminals contain abundant small clear Transmission of impulses across specialized junctions (synapses) synaptic vesicles and large dense-core vesicles. The former contain a between two neurones is largely chemical and depends on the release of neurotransmitters from the presynaptic terminal. These neurotrans- neurotransmitter (e.g. glutamate, γ-aminobutyric acid (GABA), acetyl- choline) that is released into the synaptic cleft on the arrival of an action mitters bind to cognate receptors in the postsynaptic neuronal mem- potential at the terminal and which then binds to cognate receptors on brane, resulting in a change of membrane conductance and leading to the postsynaptic membrane. Depending on the nature of the transmit- either a depolarization or a hyperpolarization (Ryan and Grant 2009). ter and its receptors, the postsynaptic neurone will become excited or The patterns of axonal termination vary considerably. A single axon inhibited. The dense-core vesicles contain neuropeptides, including may synapse with one neurone (e.g. climbing fibres ending on cere- brain-derived neurotrophic factor (BDNF; Dieni et al 2012). Axon ter- bellar Purkinje neurones), or more often with many neurones (e.g. minals may themselves be contacted by other axons, forming axo- cerebellar parallel fibres, which provide an extreme example of this axonal presynaptic inhibitory circuits. Further details of neuronal phenomenon). In synaptic glomeruli (e.g. in the olfactory bulb), groups microcircuitry are given in Kandel et al (2012) and Haines (2006). of synapses between two or many neurones form interactive units Axons contain microtubules, neurofilaments, mitochondria, mem- encapsulated by neuroglia (Fig. 3.7; Perea et al 2009). brane vesicles, cisternae and lysosomes. They do not usually contain Electrical synapses (direct communication via gap junctions) are rare ribosomes or Golgi complexes, other than at the axon hillock; excep- in the human CNS and are confined largely to groups of neurones with tionally, the neurosecretory fibres of hypothalamo-hypophysial neu- tightly coupled activity, e.g. the inspiratory centre in the medulla. They rones contain the mRNA of neuropeptides. Organelles are differentially will not be discussed further here. distributed along axons, e.g. there is a greater density of mitochondria and membrane vesicles in the axon hillock, at nodes and in presynaptic Classification of chemical synapses endings. Axonal microtubules are interconnected by cross-linking MAPs, of which tau is the most abundant. Microtubules have an intrin- Chemical synapses have an asymmetric structural organization (Figs sic polarity, and in axons all microtubules are uniformly orientated with 3.8–3.9) in keeping with the unidirectional nature of their transmis- their rapidly growing ends directed away from the soma towards the sion. Typical chemical synapses share a number of important features. axon terminal. The microtubule binding protein tau plays an important They all display an area containing a presynaptic density apposed to a 45 1 NERVOUS SYSTEM

Axon of Golgi cell Soma of granule cell corresponding postsynaptic density; the two are separated by a narrow (20–30 nm) gap, the synaptic cleft. Synaptic vesicles containing the

SECTION appropriate neurotransmitter are found on the presynaptic side, clus- tered near the presynaptic density at the presynaptic membrane. Together these define the active zone, the area of the synapse where neurotransmission takes place (Eggermann et al 2012, Gray 1959). Chemical synapses can be classified according to a number of dif- ferent parameters, including the neuronal regions forming the synapse, their ultrastructural characteristics, the chemical nature of their neurotransmitter(s) and their effects on the electrical state of the post- synaptic neurone. The following classification is limited to associations between neurones. Neuromuscular junctions share many (though not – all) of these parameters, and are often referred to as peripheral synapses. – + They are described separately on page 63. + Synapses can occur between almost any surface regions of the par- ticipating neurones. The most common type occurs between an axon and either a dendrite or a soma, when the axon is expanded as a small bulb or bouton (see Figs 3.8–3.9). This may be a terminal of an axonal branch (terminal bouton) or one of a row of bead-like endings, when the axon makes contact at several points, often with more than one + neurone (bouton de passage). Boutons may synapse with dendrites, – including thorny protrusions named dendritic spines or the flat surface + of a dendritic shaft; a soma (usually on its flat surface, but occasionally on spines); the axon hillock; and the terminal boutons of other axons. The connection is classified according to the direction of transmis- sion, and the incoming terminal region is named first. Most common are axodendritic synapses, although axosomatic connections are fre- quent. All other possible combinations are found but are less common, i.e. axo-axonal, dendro-axonal, dendrodendritic, somatodendritic or somatosomatic. Axodendritic and axosomatic synapses occur in all regions of the CNS and in autonomic ganglia, including those of the ENS. The other types appear restricted to regions of complex inter- action between larger sensory neurones and microneurones, e.g. in the thalamus. Neuroglial cell Mossy fibre Dendrite of Ultrastructurally, synaptic vesicles may be internally clear or dense, axon terminal Golgi cell and of different size (loosely categorized as small or large) and shape Fig. 3.7 The arrangement of a complex synaptic unit. A cerebellar (round, flat or pleomorphic, i.e. irregularly shaped). The submembra- synaptic glomerulus with excitatory (‘+’) and inhibitory (‘−’) synapses nous densities may be thicker on the postsynaptic than on the presyn- grouped around a central axonal bouton. The directions of transmission aptic side (asymmetric synapses), or equivalent in thickness (symmetrical are shown by the arrows. synapses), and can be perforated or non-perforated. Synaptic ribbons

Fig. 3.8 Electron micrographs demonstrating various types of synapse. A, A cross-section of a dendrite (D) on which two synaptic boutons (B) end. The upper bouton contains round vesicles, and the lower bouton contains flattened vesicles of the small type. A number of pre- and B postsynaptic (P) thickenings mark the specialized zones of contact. B, A type I synapse (S, postsynaptic site) containing both small, round, P B clear vesicles and also large, dense-cored vesicles of the neurosecretory type. C, A large terminal * bouton (B) of an optic nerve afferent fibre, which D is making contact with a number of postsynaptic processes, in the dorsal lateral geniculate nucleus Bf of the rat. One of the postsynaptic processes (*) also receives a synaptic contact from a bouton P P (Bf) containing flattened vesicles. D, Reciprocal synapses (S) between two neuronal processes in the olfactory bulb. (Courtesy of Professor AR B Lieberman, Department of Anatomy, University College, London.)

C A

S

S

S

B D 46 3 Neurones

A Excitatory synapses Serial synapses B Axosomatic synapses Bouton With small With dense Excitatory Inhibitory

de passage clear spherical catecholamine- to dendrite axo-axonal CHAPTER vesicles containing vesicles synapse

Capillary

Dendrite Nucleus

C Ribbon synapse Axo-initial segment Retinal synapse rod

With small With large Inhibitory to dendrite flattened flattened Excitatory in opposite vesicles vesicles direction Neurosecretory Inhibitory synapses Reciprocal synapse ending Fig. 3.9 The structural arrangements of different types of synaptic contact. are found at sites of neurotransmission in the retina and inner ear. They instances, types I and II synapses are found in close proximity, orien- have a distinctive morphology, in that the synaptic vesicles are grouped tated in opposite directions across the synaptic cleft (a reciprocal around a ribbon- or rod-like density orientated perpendicular to the synapse). cell membrane (see Fig. 3.9). Synaptic boutons make obvious close contacts with postsynaptic Mechanisms of synaptic activity structures but many other terminals lack specialized contact zones. Synaptic activation begins with arrival of one or more action potentials Areas of transmitter release occur in the varicosities of unmyelinated at the presynaptic bouton, which causes the opening of voltage-sensitive axons, where effects are sometimes diffuse, e.g. the aminergic pathways calcium channels in the presynaptic membrane. The response time in of the basal ganglia, and in autonomic fibres in the periphery. In some typical fast-acting synapses is then very rapid; classic neurotransmitter instances, such axons may ramify widely throughout extensive areas of (e.g. ACh, glutamate or GABA) is released in less than a millisecond. the brain and affect the behaviour of very large populations of neu- Release-ready synaptic vesicles are docked to the presynaptic membrane rones, e.g. the diffuse cholinergic innervation of the cerebral cortices. and primed through processes not yet fully understood. On Ca2+ influx Pathological degeneration of these pathways can therefore cause wide- through voltage-sensitive channels, their membranes fuse to open a spread disturbances in neural function. pore through which neurotransmitter diffuses into the synaptic cleft Neurones express a variety of neurotransmitters, either as one class (Eggermann et al 2012; Gray 1959). of neurotransmitter per cell or more often as several. Good correlations Once the vesicle has discharged its contents, its membrane is incor- exist between some types of transmitter and specialized structural porated into the presynaptic plasma membrane and is then recycled features of synapses. In general, asymmetric synapses with relatively back into the bouton by endocytosis near the edges of the active zone. small spherical vesicles are associated with acetylcholine (ACh), gluta- The recycling time for a synaptic vesicle may be in the range of a few mate, serotonin (5-hydroxytryptamine, 5-HT) and some amines; those seconds to minutes; newly recycled vesicles may be used instantly for with dense-core vesicles include many peptidergic synapses and the next cycle of neurotransmitter release (cycling pool of vesicles). The other amines (e.g. noradrenaline (norepinephrine), adrenaline (epine- fusion of vesicles with the presynaptic membrane is responsible for the phrine), dopamine). Symmetrical synapses with flattened or pleomor- observed quantal behaviour of neurotransmitter release, both during phic vesicles have been shown to contain either GABA or glycine. neural activation and spontaneously, in the slightly leaky resting condi- Neurosecretory endings found in various parts of the brain and in tion (Neher and Sakaba 2008; Suedhof 2012). neuroendocrine glands and cells of the dispersed neuroendocrine Postsynaptic events vary greatly, depending on the receptor mole- system share many features with presynaptic boutons. They all contain cules and their related molecular complexes (Murakoshi and Yasuda peptides or glycoproteins within dense-core vesicles. The latter are of 2012). Receptors are generally classed as either ionotropic or metabo- characteristic size and appearance: they are often ellipsoidal or irregular tropic. Ionotropic receptors are multimeric protein complexes that in shape, and relatively large, e.g. oxytocin and vasopressin vesicles in harbour intrinsic ion channels that can be operated by conformational the neurohypophysis may be up to 200 nm in diameter. changes induced when neurotransmitter molecules bind the receptor Synapses may cause depolarization or hyperpolarization of the post- complex, causing a voltage change within the postsynaptic cell. Exam- synaptic membrane, depending on the neurotransmitter released and ples are the nicotinic ACh receptor and the related GABAA receptor, the classes of receptor molecule in the postsynaptic membrane. Depo- which are formed from five subunits, and the tetrameric ionotropic larization of the postsynaptic membrane results in excitation of the glutamate receptors, such as the N-methyl-D-aspartate (NMDA) postsynaptic neurone, whereas hyperpolarization has the effect of tran- receptor or the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid siently inhibiting electrical activity. Subtle variations in these responses (AMPA) receptor. Alternatively, the receptor and ion channel may be may also occur at synapses where mixtures of neuromediators are separate molecules, coupled by G-proteins, some via a complex cascade present and their effects are integrated. For details of the synaptic organ- of chemical interactions (a second messenger system), e.g. the adenylate ization of the brain, see Shepherd (2003). cyclase pathway. Postsynaptic effects are generally rapid and short-lived, because the transmitter is quickly inactivated either by an extracellular Type I and II synapses enzyme (e.g. acetylcholinesterase, AChE), or by uptake into neurones There are two broad categories of synapse, type I and type II. In active or glial cells. Examples of such metabotropic receptors are the mus- zones of type I synapses the cytoplasmic density is thicker on the post- carinic ACh receptor and the dopamine receptor. synaptic side. In type II synapses the pre- and postsynaptic densities are thinner and more symmetrical. Type I boutons contain a predominance Neurohormones of small spherical vesicles approximately 50 nm in diameter, and type Neurohormones are included in the class of molecules with II boutons contain oval or flattened vesicles. Throughout the CNS, type neurotransmitter-like activity. They are synthesized in neurones and I synapses are generally excitatory and type II are inhibitory. In a few released into the blood circulation by exocytosis at synaptic bouton-like 47 1 NERVOUS SYSTEM

structures. As with classic endocrine gland hormones, they may act at lie mainly in the brainstem, although their axons ramify widely into all great distances from their site of secretion. Neurones secrete into the parts of the nervous system. Monoaminergic cells are also present in

SECTION CSF or local interstitial fluid to affect other cells, either diffusely or at the retina. a distance. To encompass this wide range of phenomena the general Noradrenaline is the chief transmitter present in sympathetic gangli- term neuromediation has been used, and the chemicals involved are onic neurones with endings in various tissues, notably smooth muscle called neuromediators. and glands, and in other sites including adipose and haemopoietic tissues and the corneal epithelium. It is also found at widely distributed Neuromodulators synaptic endings within the CNS, many of them the terminals of neu- Some neuromediators do not appear to affect the postsynaptic mem- ronal somata situated in the locus coeruleus in the medullary floor. The brane directly but they can affect its responses to other neuromediators, actions of noradrenaline depend on its site of action and vary with the either enhancing their activity (by increasing or prolonging the immedi- type of postsynaptic receptor, e.g. it strongly inhibits neurones of ate response), or perhaps limiting or inhibiting their action. These the submucosal plexus of the intestine and of the locus coeruleus via substances are called neuromodulators. A single synaptic terminal may α2-adrenergic receptors, whereas it mediates depolarization, producing contain one or more neuromodulators in addition to a neurotransmit- vasoconstriction, via β-receptors in vascular smooth muscle. Adrenaline ter, usually (though not always) in separate vesicles. Neuropeptides (see is present in central and peripheral nervous pathways and occurs below) are nearly all neuromodulators, at least in some of their actions. with noradrenaline in the suprarenal medulla. Both adrenaline and They are stored within dense granular synaptic vesicles of various sizes noradrenaline are found in dense-cored synaptic vesicles approximately and appearances. 50 nm in diameter. Dopamine is a neuromediator of considerable clinical importance, found mainly in neurones with cell bodies in the telencephalon, Development and plasticity of synapses diencephalon and mesencephalon. A major dopaminergic neuronal population in the midbrain constitutes the substantia nigra, so called Embryonic synapses first appear as inconspicuous dense zones flanking because its cells contain neuromelanin, a black granular by-product synaptic clefts. Immature synapses often appear after birth, suggesting of dopamine synthesis. Dopaminergic endings are particularly numer- that they may be labile, and are reinforced if transmission is function- ous in the corpus striatum, limbic system and cerebral cortex. Structur- ally effective, or withdrawn if redundant. This is implicit in some theo- ally, dopaminergic synapses contain numerous dense-cored vesicles ries of memory (Squire and Kandel 2008), which postulate that synapses that resemble those containing noradrenaline. Pathological reduction are modifiable by frequency of use, to establish preferential conduction in dopaminergic activity has widespread effects on motor control, pathways. Evidence from hippocampal neurones suggests that even affective behaviour and other neural activities, as seen in Parkinson’s brief synaptic activity can increase the strength and sensitivity of the syndrome. synapse for some hours or longer (long-term potentiation, LTP). During Serotonin and histamine are found in neurones mainly within the early postnatal life, the normal developmental increase in numbers and CNS. Serotonin is typically synthesized in small midline neuronal clus- sizes of synapses and dendritic spines depends on the degree of neural ters in the brainstem, mainly in the raphe nuclei; the axons from these activity and is impaired in areas of damage or functional deprivation. neurones ramify extensively throughout the entire brain and spinal cord. Synaptic terminals contain rounded, clear vesicles approximately Neurotransmitter molecules 50 nm in diameter and are of the asymmetrical type. Histaminergic neurones appear to be relatively sparse and are restricted largely to the Until recently, the molecules known to be involved in chemical syn- hypothalamus. apses were limited to a fairly small group of classic neurotransmitters, e.g. ACh, noradrenaline (norepinephrine), adrenaline (epinephrine), Amino acids dopamine and histamine, all of which had well-defined rapid effects There are three major amino acids: GABA, glutamate and glycine, which on other neurones, muscle cells or glands. However, many synaptic bind to specific receptors (Barrera and Edwardson 2008). GABA is a interactions cannot be explained on the basis of classic neurotransmit- major inhibitory transmitter released at the terminals of local circuit ters, and it is now known that other substances, particularly some neurones within the brainstem and spinal cord (e.g. the recurrent inhib- amino acids such as glutamate, glycine, aspartate, GABA and the itory Renshaw loop), cerebellum (where it is the main transmitter of monoamine, serotonin, also function as transmitters. Substances first Purkinje cells), basal ganglia, cerebral cortex, thalamus and subthala- identified as hypophysial hormones or as part of the dispersed neuroen- mus. It is stored in flattened or pleomorphic vesicles within symmetrical docrine system (see below) of the alimentary tract, can be detected synapses. GABA may be inhibitory to postsynaptic neurones, or may widely throughout the CNS and PNS, often associated with functionally mediate either presynaptic inhibition or facilitation, depending on the integrated systems. Many of these are peptides; more than one hundred synaptic arrangement (Gassmann and Bettler 2012). (together with other candidates) function mainly as neuromodulators Glutamate is the major excitatory transmitter present widely within and influence the activities of classic transmitters. the CNS, including the major projection pathways from the cortex to the thalamus, tectum, substantia nigra and pontine nuclei. It is found Acetylcholine in the central terminals of the auditory and trigeminal nerves, and in Acetylcholine (ACh) is perhaps the most extensively studied neuro- the terminals of parallel fibres ending on Purkinje cells in the cerebel- transmitter of the classic type. Its precursor, choline, is synthesized in lum. Structurally, glutamate is associated with asymmetrical synapses the neuronal soma and transported to the axon terminals, where it is containing small (approximately 30 nm), round, clear synaptic vesicles acetylated by the enzyme choline acetyl transferase (ChAT), and stored (Contractor et al 2011). For further reading, see Willard and Koochek- in clear spherical vesicles 40–50 nm in diameter. ACh is synthesized by pour (2013). motor neurones and released at all their motor terminals on skeletal Glycine is a well-established inhibitory transmitter of the CNS, par- muscle. It is released by preganglionic fibres at synapses in parasympa- ticularly the lower brainstem and spinal cord, where it is mainly found thetic and sympathetic ganglia, and many parasympathetic, and some in local circuit neurones. Recent evidence suggests that glycine may also sympathetic, ganglionic neurones are cholinergic. ACh is also associ- be released from glutamatergic axon terminals to participate in activa- ated with the degradative extracellular enzyme AChE, which inactivates tion of NMDA receptors, and from astrocytes into the synaptic cleft via the transmitter by converting it to choline. activation of non-NMDA-type glutamatergic ionotropic receptors in The effects of ACh on nicotinic receptors (i.e. those in which nicotine the glial cell membrane (see Harsing and Matyus (2013) for further is an agonist) are rapid and excitatory. In the CNS, the nicotinic ACh references). receptor mediates the effect of tobacco (for review, see Albuquerque et al (2009)). In the peripheral autonomic nervous system, the slower, ATP and adenosine more sustained excitatory effects of cholinergic autonomic endings are ATP serves not only as a universal energy substrate, but also as an extra- mediated by muscarinic receptors via a second messenger system. cellular signalling molecule. Specific ionotropic (P2X) and metabo- tropic (P2Y) receptors are expressed in neurones and even more Monoamines prominently on all types of glial cell. Adenosine is a degradation Monoamines include the catecholamines (noradrenaline (norepine- product of ATP and has specific metabotropic receptors that may be phrine), adrenaline (epinephrine) and dopamine), the indoleamine located presynaptically (Burnstock et al 2011). serotonin (5-hydroxytryptamine, 5-HT) and histamine (Haas et al 2008). They are synthesized by neurones in sympathetic ganglia and by Nitric oxide their homologues, the chromaffin cells of the suprarenal medulla and Nitric oxide (NO) is of considerable importance at autonomic and 48 paraganglia. Within the CNS, the somata of monoaminergic neurones enteric synapses, where it mediates smooth muscle relaxation. It 3 Central glia functions in several types of synaptic plasticity, including hippocampal Subpial end-foot Neurone Tanycyte Perineuronal long-term potentiation (LTP), when it may act as a retrograde messen- Pia mater Astrocyte end-foot Microglial cell Ependymal cell Ventricle ger after postsynaptic NMDA receptor activation. NO is able to diffuse freely through cell membranes, and so is not under such tight quantal CHAPTER control as vesicle-mediated neurotransmission. Neuropeptides Many neuropeptides coexist with other neuromediators in the same synaptic terminals. As many as three peptides often share a particular ending with a well-established neurotransmitter, in some cases within the same synaptic vesicles. Some peptides occur in both the CNS and the PNS, particularly in the ganglion cells and peripheral terminals of the ANS, whilst others are entirely restricted to the CNS. Only a few examples are given here. Most of the neuropeptides are classified according to the site where they were first discovered. For example, the gastrointestinal peptides were initially found in the gut wall, and a group that includes releasing Myelinated Oligodendrocyte Capillary Pericapillary Astrocyte hormones, adenohypophysial and neurohypophysial hormones was axon end-foot first associated with the pituitary gland. Some of these peptides are closely related to each other in their chemistry because they are derived Fig. 3.10 The different types of non-neuronal cell in the CNS and their from the same gene products (e.g. the pro-opiomelanocortin group), structural organization and interrelationships with each other and with neurones. which are cleaved to produce smaller peptides. Substance P (SP) was the first of the peptides to be characterized as a gastrointestinal neuromediator and is considered to be the proto- typic neuropeptide. It is an 11-amino-acid polypeptide that belongs to the tachykinin neuropeptide family, and is a major neuromediator in ASTROCYTES the brain and spinal cord. Contained within large granular synaptic vesicles, SP is found in approximately 20% of dorsal root and trigemi- Astrocytes are the most abundant and diverse glial cell type but their nal ganglion cells, in particular in small nociceptive neurones, and in identity is not well defined (Matyash and Kettenmann 2010). There is some fibres of the facial, glossopharyngeal and vagal nerves. Within no common marker that labels all astrocytes, in the way that myelin the CNS, SP is present in several apparently unrelated major pathways, basic protein is a marker for oligodendrocytes or the calcium-binding and has been described in the limbic system, basal ganglia, amygdala protein Iba1 is a marker for microglia. A commonly used marker is the and hypothalamus. Its known action is prolonged postsynaptic expression of glial fibrillary acidic protein (GFAP), which forms inter- excitation, particularly from nociceptive afferent terminals, which sus- mediate filaments, but GFAP is not expressed in all astrocytes. tains the effects of noxious stimuli. SP is one of the main neuropep- The morphology of astrocytes is extremely diverse. Classically, two tides that trigger an inflammatory response in the skin and has forms were distinguished: protoplasmic and fibrous astrocytes. Proto- also been implicated in the vomiting reflex, changes in cardiovascular plasmic astrocytes (star-shaped cells) are found in grey matter, possess tone, stimulation of salivary secretion, smooth muscle contraction, several stem processes that branch further into a very complex network, and vasodilation. and contact synapses, both at the pre- and postsynaptic membranes. Vasoactive intestinal polypeptide (VIP), another gastrointestinal Fibrous astrocytes are predominantly found in white matter and their peptide, is widely present in the CNS, where it is probably an excitatory processes are often orientated in parallel with the axons. Radial glial neurotransmitter or neuromodulator. It is found in distinctive bipolar cells are found early in development and serve as stem cells for neu- neurones of the cerebral cortex; small dorsal root ganglion cells, par- rones and glial cells. They may be categorized as astrocytes because they ticularly of the sacral region; the median eminence of the hypothala- transform later in development into typical astrocytes. There are a mus, where it may be involved in endocrine regulation; intramural number of other types of astrocyte with specialized morphologies. Berg- ganglion cells of the gut wall; and sympathetic ganglia. mann glial cells in the cerebellum have somata in the Purkinje cell layer, Somatostatin (ST, somatotropin release inhibiting factor) has a processes that intermingle with the dendritic trees of the Purkinje broad distribution within the CNS, and may be a central neurotransmit- neurones and terminal end-feet at the pial surface. Müller cells in the ter or neuromodulator. It also occurs in small dorsal root ganglion cells. retina have a radial morphology and span the entire retina. Other Beta-endorphin, leu- and metenkephalins, and the dynorphins belong astrocytic cells are tanycytes, velate astrocytes (cerebellum) and pitui- to a group of peptides called the naturally occurring opiates that possess cytes (infundibulum and neurohypophysis of the pituitary gland). analgesic properties. They bind to opiate receptors in the brain where, Pituicyte processes end mostly on endothelial cells in the neurohypo- in general, their action seems to be inhibitory. Enkephalins have been physis and tuber cinereum. localized in many areas of the brain. Their particular localization in the Astrocyte complexity and morphological diversity has reached the septal nuclei, amygdaloid complex, basal ganglia and hypothalamus highest evolutionary level in humans (Fig. 3.11). A single astrocyte may suggests that they are important mediators in the limbic system and in enwrap several neuronal somata and make contacts with tens of thou- the control of endocrine function. They have also been implicated sands of individual synapses; bipolar astrocytes located in layer 5 and strongly in the central control of pain pathways, because they are found 6 of the cortex may extend processes up to 1 mm long. in the peri-aqueductal grey matter of the midbrain, a number of reticu- Astrocytes in grey matter form a syncytium in which cells are inter- lar raphe nuclei, the spinal nucleus of the trigeminal nerve and the connected by gap junctions, permitting the exchange of ions (e.g. substantia gelatinosa of the spinal cord. The enkephalinergic pathways calcium, propagated in waves) and small molecules such as ATP or exert an important presynaptic inhibitory action on nociceptive affer- glucose. They are the only cells in the brain capable of converting ents in the spinal cord and brainstem. Like many other neuromediators, glucose into glycogen, which serves as an energy store. Before re-releasing enkephalins also occur widely in other parts of the brain in lower glucose, astrocytes convert it to lactate, which is taken up by neurones; concentrations. failure in glucose flow through the astrocytic network results in impair- ment of neuronal function. Astrocytes not only respond to neuronal activity but also modulate that activity. They enwrap all penetrating and intracerebral arterioles CENTRAL GLIA and capillaries, control the ionic and metabolic environment of the neuropil and mediate neurovascular coupling. They form specialized Glial (neuroglial) cells (Fig 3.10) vary considerably in type and number structures that contact either the pial surface (as the glia limitans) or in different regions of the CNS. There are two major groups, macroglia blood vessels; their end-feet entirely enwrap blood vessels and are (astrocytes and oligodendrocytes) and microglia, classified according to instrumental in the induction of the blood–brain barrier. origin. Macroglia arise within the neural plate, in parallel with neu- Traumatic injury to the CNS induces astrogliosis, seen as a local rones, and constitute the great majority of glial cells. Their functions increase in the number and size of GFAP-positive cells and a character- are diverse and are now known to extend beyond a passive supporting istic extensive meshwork of processes. The microenvironment of this role (reviewed in Kettenmann and Ransom (2012)). Microglia have a glial scar, which may also include cells of oligodendrocyte lineage and small soma (see Fig. 3.19) and are derived from a distinct lineage of myelin debris, plays an important role in inhibiting regrowth of monocytic cells originating from the yolk sac. damaged CNS axons (Robel et al 2011, Seifert et al 2006). 49 3 Nervous system

Astrocytes control the diameter of the vessels they contact and can trigger either their dilation or their contraction, depending on the sub- stances they release and the levels of associated neuronal activity. They express water channels (aquaporins) at the end-feet covering the capil- CHAPTER laries; it has been suggested that this may represent the means by which astrocytes control brain volume (Tait et al 2008), and it may be relevant to understanding mechanisms of brain tissue swelling, a major clinical complication. Astrocytes express different glutamate transporters that efficiently maintain low levels of extracellular glutamate, which is excitotoxic. Internalized glutamate is converted into glutamine and released from astrocytes to be taken up by local neurones and recon- verted to glutamate via the glutamate–glutamine cycle. They play a similar role in controlling extracellular GABA levels via expression of GABA transporters. Astrocytes possess both passive and active mecha- nisms to control extracellular potassium levels at a resting level of about 3 mmol. They also express transporters that regulate pH and are thought to play an important role in controlling extracellular pH in the brain. For further reading on the concept of the ‘tripartite synapse’, where astrocytic processes interact with pre- and postsynaptic neuronal ele- ments, see Haydon and Carmignoto (2006). It has become evident that astrocytes are involved in the modulation of long-term potentiation (considered as a cellular mechanism of memory formation) and heterosynaptic depression. They modulate neuronal activity by releasing neuroactive substances such as D-serine, ATP or glutamate; it is unclear whether they express all the elements required for neurotransmitter release by a vesicular mechanism (Parpura and Zorec 2010).

49.e1 1 NERVOUS SYSTEM

Fig. 3.11 Human protoplasmic astrocytes are larger and more complicated than their rodent counterparts. A, A typical mouse protoplasmic SECTION astrocyte. Glial fibrillary acidic protein (GFAP) immunostain; white. SB = 20 μm. B, A typical human protoplasmic astrocyte to the same scale. SB = 20 μm. (From Oberheim NA, Takano T, Han X, et al 2009 Uniquely hominid features of adult human astrocytes. J Neurosci 29:3276–87.)

A B

A which means that a free exchange of molecules occurs between blood and adjacent brain. Most of these areas are situated close to the ventri- cles and are known as circumventricular organs; these areas make up Astrocyte end-foot less than 1% of the total area of the brain. Elsewhere, unrestricted Basal lamina diffusion through the blood–brain barrier is only possible for sub- stances that can cross biological membranes because of their lipophilic Pia mater character. Lipophilic molecules may be actively re-exported by the (larger vessels only) brain endothelium. Breakdown of the blood–brain barrier occurs when the brain is Pericyte under damaged by ischaemia or infection, and is also associated with primary the basal lamina and metastatic cerebral tumours. Reduced blood flow to a region of the brain alters the permeability and regulatory transport functions of the barrier locally; the increased stress on compromised endothelial cells results in leakage of fluid, ions, serum proteins and intracellular sub- stances into the extracellular space of the brain. The integrity of the barrier can be evaluated clinically using computed tomography and B functional magnetic resonance imaging. Breakdown of the blood–brain barrier may be seen at postmortem in jaundiced patients who have had Pericyte Astrocyte an infarction. Normally, the brain, spinal cord and peripheral nerves end-foot remain unstained by the bile post mortem, although the choroid plexus Perivascular cell is often stained a deep yellow. However, areas of recent infarction (1–3 (macrophage) days) will also be stained by bile pigment because of the localized breakdown of the blood–brain barrier.

Endothelium OLIGODENDROCYTES

Oligodendrocytes myelinate CNS axons and are most commonly seen as intrafascicular cells in myelinated tracts (Figs 3.13–3.14). They Basal lamina usually have round nuclei and their cytoplasm contains numerous mitochondria, microtubules and glycogen. They display a spectrum of Fig. 3.12 The relationship between the glia limitans, perivascular cells morphological variation, from cells with large euchromatic nuclei and and blood vessels within the brain, in longitudinal (A) and transverse pale cytoplasm, to cells with heterochromatic nuclei and dense cyto- (B) sections. A sheath of astrocytic end-feet wraps around the vessel and, plasm. In contrast to Schwann cells, which myelinate only one axonal in vessels larger than capillaries, its investment of pial meninges. Vascular segment, individual oligodendrocytes myelinate up to 50 axonal seg- endothelial cells are joined by tight junctions and supported by pericytes; ments. Some oligodendrocytes are not associated with axons, and are perivascular macrophages lie outside the endothelial basal lamina (light either precursor cells or perineuronal (satellite) oligodendrocytes with blue). processes that ramify around neuronal somata. Within tracts, interfascicular oligodendrocytes are arranged in long rows interspersed at regular intervals with single astrocytes. Since oli- Blood–brain barrier godendrocyte processes are radially aligned to the axis of each row, myelinated tracts typically consist of cables of axons myelinated by a Proteins circulating in the blood enter most tissues of the body except row of oligodendrocytes running down the axis of each cable. those of the brain, spinal cord and peripheral nerves. This concept of a Oligodendrocytes originate from the ventricular neurectoderm and blood–brain or a blood–nerve barrier applies to many substances – the subependymal layer in the fetus, and continue to be generated from some are actively transported across the blood–brain barrier, others are the subependymal plate postnatally. Stem cells migrate and seed into actively excluded. The blood–brain barrier is located at the capillary white and grey matter to form a pool of adult progenitor cells, which endothelium within the brain and is dependent on the presence of tight can later differentiate to replenish defunct oligodendrocytes, and pos- junctions (occluding junctions, zonulae adherentes) between endothe- sibly remyelinate axons in pathologically demyelinated regions. These lial cells coupled with a relative lack of transcytotic vesicular transport. cells display a highly branching morphology and express a specific The tightness of the barrier is substantially supported by the close chondroitin sulphate proteoglycan (Neuron Glia 2 (NG2) in rats and apposition of astrocytes, which direct the formation of endothelial tight its homologue, melanoma cell surface chondroitin sulphate proteo- junctions, to blood capillaries (reviewed in Abbott et al (2006), Cardoso glycan (MSCP), in humans). The name NG2 cell is used to describe the et al (2010); Fig. 3.12). cells in both species: several different names have also been used since The blood–brain barrier develops during embryonic life but may it was first recognized, including polydendrocyte (Nishiyama et al not be fully completed by birth. There are certain areas of the adult 2009) and syantocyte (Butt et al 2005). NG2 cells express a complex 50 brain where the endothelial cells are not linked by tight junctions, set of voltage-gated channels and ionotropic receptors for glutamate 3 Central glia

Oligodendrocyte Lateral loop

Node of Ranvier Nucleus Outer loop CHAPTER

A

N

Fig. 3.15 A node of Ranvier (N) in the central nervous system of a rat. The pale-staining axon (A) is ensheathed by oligodendrocyte myelin (arrow), apart from a short, exposed region at the node. Toluidine blue stained resin section. (Courtesy of Dr Clare Farmer, King’s College, London.)

and GABA; they form direct synapses with axons, enabling transient activation of these receptors (Hill and Nishiyama 2014). There is con- siderable support for the view that the NG2 cell is a distinct glial type. Nodes of Ranvier and incisures of Schmidt–Lanterman

The territory ensheathed by an oligodendrocyte (or Schwann cell) process defines an internode, the interval between internodes is called a node of Ranvier (Fig. 3.15) and the territory immediately adjacent to the nodal gap is a paranode, where loops of oligodendrocyte cytoplasm Axon Longitudinal Inner loop Myelin sheath abut the axolemma. Nodal axolemma is contacted by fine filopodia of incisures perinodal cells, which have been shown in animal studies to have a Fig. 3.13 The ensheathment of a number of axons by the processes of presumptive adult oligodendrocyte progenitor phenotype; their func- an oligodendrocyte. The oligodendrocyte soma is shown in the centre tion is unknown. Schmidt–Lanterman incisures are helical decompac- and its myelin sheaths are unfolded to varying degrees to show their tions of internodal myelin where the major dense line of the myelin extensive surface area. (Modified from Morell P, Norton WT (1980, May). sheath splits to enclose a spiral of oligodendrocyte cytoplasm. Their Myelin, Scientific American, 242(5), 88–90, 92, 96 and Raine CS (1984), structure suggests that they may play a role in the transport of molecules Morphology of Myelin and Myelination. In Myelin, 2nd ed. P Morell (ed) across the myelin sheath, but their function is not known. New York (Plenum Press), by permission.)

MYELIN AND MYELINATION

Myelin is formed by oligodendrocytes (CNS) and Schwann cells (PNS). A single oligodendrocyte may ensheathe up to 50 separate axon seg- ments, depending on calibre, whereas myelinating Schwann cells ensheathe axons on a 1 : 1 basis. In general, myelin is laid down around axons above 2 μm in diam- eter. However, the critical minimal axon diameter for myelination is smaller and more variable in the CNS than in the PNS (approximately 0.2 μm in the CNS compared with 1–2 μm in the PNS). There is con- siderable overlap between the size of the smallest myelinated and the largest unmyelinated axons, and so axonal calibre is unlikely to be the only factor in determining myelination. Moreover, the first axons to become ensheathed ultimately attain larger diameters than those that are ensheathed at a later date. There is a reasonable linear relationship between axon diameter and internodal length and myelin sheath thick- ness: as the sheath thickens from a few lamellae to up to 200, the axon may also grow from 1 to 15 μm in diameter. Internodal lengths increase A about 10-fold during the same time (Nave 2010). It is not known precisely how myelin is formed in either PNS or CNS. Akt/mTOR (mammalian (or mechanistic) target of rapamycin) signalling has emerged as one of the major pathways involved in myeli- nation; it has been implicated in the regulation of several steps during the development of myelinating Schwann cells and oligodendrocytes (Norrmén and Suter 2013). In the CNS, myelination also depends in part on expression of a protein (Wiskott–Aldrich syndrome protein B family verprolin homologous; WAVE), which influences the actin cytoskeleton, oligodendrocyte lamellipodia formation and myelination Fig. 3.14 A, An oligodendrocyte enwrapping several axons with myelin, (Kim et al 2006). The ultrastructural appearance of myelin is usually demonstrated in a whole-mounted rat anterior medullary velum, explained in terms of the spiral wrapping of an extensive, flat glial immunolabelled with antibody to an oligodendrocyte membrane antigen. process (lamellipodium) around an axon, and the subsequent extrusion B, A confocal micrograph of a mature myelin-forming oligodendrocyte in of cytoplasm from the sheath at all points other than incisures and an adult rat optic nerve, iontophoretically filled with an immunofluorescent paranodes. In this way, the compacted external surfaces of the plasma dye by intracellular microinjection. (A, Courtesy of Fiona Ruge. membrane of the ensheathing glial cell are thought to produce the B, Prepared by Professor A Butt, Portsmouth, and Kate Colquhoun, minor dense lines, and the compacted inner cytoplasmic surfaces, the formerly Division of Physiology, GKT School of Medicine, London.) major dense lines, of the mature myelin sheath (Fig. 3.16). These lines, first described in early electron microscope studies of the myelin sheath, correspond to the intraperiod and period lines respectively, defined in X-ray studies of myelin. The inner and outer zones of occlusion of the spiral process are continuous with the minor dense line and are called 51 1 NERVOUS SYSTEM

Fig. 3.16 Suggested stages in myelination of a peripheral axon by an ensheathing Schwann cell. SECTION

Schwann cell Basal Inner Outer cytoplasm lamina mesaxon mesaxon Axon

the inner and outer mesaxons. For further reading on aspects of myeli- nation, see Bakhti et al (2013). V There are significant differences between central and peripheral myelin, reflecting the fact that oligodendrocytes and Schwann cells express different proteins during myelinogenesis. The basic dimensions of the myelin membrane are different. CNS myelin has a period repeat C thickness of 15.7 nm whereas PNS myelin has a period to period line thickness of 18.5 nm, and the major dense line space is approximately E 1.7 nm in CNS myelin, compared with 2.5 nm in PNS myelin. Myelin membrane contains protein, lipid and water, which forms at least 20% of the wet weight. It is a relatively lipid-rich membrane and contains SVZ 70–80% lipid. All classes of lipid have been found; perhaps not surpris- ingly, the precise lipid composition of PNS and CNS myelin is different. The major lipid species are cholesterol (the most common single mol- ecule), phospholipids and glycosphingolipids. Minor lipid species include galactosylglycerides, phosphoinositides and gangliosides. The major glycolipids are galactocerebroside and its sulphate ester, sul- Fig. 3.17 Ciliated columnar epithelial lining of the lateral ventricle (V), phatide; these lipids are not unique to myelin but they are present overlying the subventricular zone (SVZ). C, cilia; E, ependymal cells. in characteristically high concentrations. CNS and PNS myelin also Mouse tissue, toluidine blue stained resin section. contain low concentrations of acidic glycolipids, which constitute important antigens in some inflammatory demyelinating states. Gan- gliosides, which are glycosphingolipids characterized by the presence ependymal lining of the ventricles but four major types have been of sialic acid (N-acetylneuraminic acid), account for less than 1% of the described. These are: general ependymal, which overlies grey matter; lipid in myelin. general ependymal, which overlies white matter; specialized areas of A relatively small number of protein species account for the majority ependyma in the third and fourth ventricles; and choroidal epithelium. of myelin protein. Some of these proteins are common to both PNS The ependymal cells overlying areas of grey matter are cuboidal. and CNS myelin, but others are different. Proteolipid protein (PLP) Each cell bears approximately 20 central apical cilia, surrounded by and its splice variant DM20 are found only in CNS myelin, whereas short microvilli. The cells are joined by gap junctions and desmosomes. myelin basic protein (MBP) and myelin associated glycoprotein (MAG) Beneath them there may be a subependymal (or subventricular) zone, occur in both. MAG is a member of the immunoglobulin supergene from two to three cells deep, consisting of cells that generally resemble family, and is localized specifically at those regions of the myelin ependymal cells. In rodents, the subventricular zone contains neural segment where compaction starts: namely, the mesaxons and inner progenitor cells, which can give rise to new neurones, but the existence periaxonal membranes, paranodal loops and incisures, in both CNS of these stem cells in the adult human brain is controversial (Sanai et al and PNS sheaths. It is thought to have a functional role in membrane 2011, Kempermann 2011). The capillaries beneath the ependymal cells adhesion. have no fenestrations and few transcytotic vesicles, which is typical of In the developing CNS, axonal outgrowth precedes the migration of the CNS. Where the ependyma overlies myelinated tracts of white oligodendrocyte precursors, and oligodendrocytes associate with and matter, the cells are much flatter and few are ciliated. There are gap myelinate axons after their phase of elongation; oligodendrocyte myelin junctions and desmosomes between these cells, but their lateral margins gene expression is not dependent on axon association. In marked con- interdigitate, unlike their counterparts overlying grey matter. No sub- trast, Schwann cells in the developing PNS are associated with axons ependymal zone is present. during the entire phase of axonal growth. Myelination does not occur Specialized areas of ependymal cells called the circumventricular simultaneously in all parts of the body in late fetal and early postnatal organs are found in four areas around the margins of the third ventricle: development. White matter tracts and nerves in the periphery have their namely, the lining of the median eminence of the hypothalamus; the own specific temporal patterns that relate to their degree of functional subcommissural organ; the subfornical organ; and the vascular organ maturity. of the lamina terminalis. The area postrema, at the inferoposterior limit Mutations of the major myelin structural proteins have now been of the fourth ventricle, has a similar structure. In all of these sites the recognized in a number of inherited human neurological diseases. As ependymal cells are only rarely ciliated and their ventricular surfaces would be expected, these mutations produce defects in myelination and bear many microvilli and apical blebs. They have numerous mitochon- in the stability of nodal and paranodal architecture that are consistent dria, well-formed Golgi complexes and rather flattened basal nuclei. with the suggested functional roles of the relevant proteins in maintain- They are joined laterally by tight junctions, which form a barrier to the ing the integrity of the myelin sheath. passage of materials across the ependyma, and by desmosomes. Many of the cells are tanycytes (ependymal astrocytes) and have basal processes that project into the perivascular space surrounding the EPENDYMA underlying capillaries. Significantly, these capillaries are fenestrated and therefore do not form a blood–brain barrier. It is believed that neu- Ependymal cells line the ventricles (Fig. 3.17; see Fig. 3.10) and central ropeptides can pass from nervous tissue into the CSF by active transport canal of the spinal cord. They form a single-layered epithelium that through the ependymal cells in these specialized areas, and so access a varies from squamous to columnar in form. At the ventricular surface, wide population of neurones via the permeable ependymal lining of cells are joined by gap junctions and occasional desmosomes. Their the rest of the ventricle. 52 apical surfaces have numerous microvilli and/or cilia, the latter contrib- The ependyma is highly modified where it lies adjacent to the vas- uting to the flow of CSF. There is considerable regional variation in the cular layer of the choroid plexuses. 3 Central glia

Choroid plexus MICROGLIA

The choroid plexus forms the CSF and actively regulates the concentra- Microglia are the endogenous immune cells of the brain (Kettenmann CHAPTER tion of molecules in the CSF. It consists of highly vascularized masses et al 2011, Eggen et al 2013). They originate from an embryonic mono- of pia mater enclosed by pockets of ependymal cells. The ependymal cyte precursor and invade the brain early during development. While cells resemble those of the circumventricular organs, except that they the invading cells have an ameboid morphology, microglial cells in a do not have basal processes, but form a cuboidal epithelium that rests mature brain are highly ramified cells. They have elongated nuclei, scant on a basal lamina adjacent to the enclosed fold of meningeal pia mater cytoplasm and several highly branched processes. They occupy a defined and its capillaries (Fig. 3.18). The cells have numerous long microvilli territory in the brain parenchyma and are found in all areas of the CNS with only a few cilia interspersed between them. They also have many including optic nerve, retina and spinal cord. Their density shows little mitochondria, large Golgi complexes and basal nuclei, features consist- variation. ent with their secretory activity; they produce most components of the Resting microglia, a term used to refer to microglia in the normal CSF. They are linked by tight junctions forming a transepithelial barrier brain, should more accurately be described as surveying microglia. (a component of the blood–CSF barrier), and by desmosomes. Their Microglial processes are fast-moving structures that rapidly scan their lateral margins are highly folded. territory while the soma remains fixed in position. Microglial cells The choroid plexus has a villous structure where the stroma is com- express receptors for neurotransmitters and thus can sense neuronal posed of pial meningeal cells, and contains fine bundles of collagen activity. It is likely that they interact with synapses, from which it has and blood vessels. Choroidal capillaries are lined by a fenestrated been inferred that they may influence synaptic transmission. endothelium. During fetal life, erythropoiesis occurs in the stroma, All pathological changes in the brain result in the activation of which is occupied by bone marrow-like cells. In adult life, the stroma microglial cells (Fig. 3.19), e.g. activated microglia are found in the contains phagocytic cells, which, together with the cells of the choroid brain tissue of multiple sclerosis, Alzheimer’s disease and stroke plexus epithelium, phagocytose particles and proteins from the ven- patients. The most common indication of their activation is a change tricular lumen. from a ramified to an ameboid morphology, which may occur within Age-related changes occur in the choroid plexus, which can be a few hours of the onset of injury or disease process. detected by neuroimaging. Calcification of the choroid plexus can be In general, microglia respond to two types of signal: ‘on’ signals, which detected by X-ray or CT scan very rarely in individuals in the first decade either appear de novo or are strongly upregulated, e.g. cell wall compo- of life and in the majority in the eighth decade. The incidence of calci- nents of invading bacteria; and ‘off’ signals, which are normally present fication rises sharply, from 35% of CT scans in the fifth decade to 75% but disappear or decrease in pathological states, e.g. defined cytokines in the sixth decade. Visible calcification is usually restricted to the or neurotransmitters. Both types of event are interpreted as signals for glomus region of the choroid plexus, i.e. the vascular bulge in the activation. The functional repertoire of activated microglia includes pro- choroid plexus as it curves to follow the anterior wall of the lateral liferation; migration to the site of injury; expression of major histocom- ventricle into the temporal horn. patibility complex (MHC) II molecules to interact with infiltrating lymphocytes; and the release of a variety of different substances including chemokines, cytokines and growth factors. These cells are therefore capa- ble of significantly influencing ongoing pathological processes. Microglial cells are the professional phagocytes of the nervous system and actively migrate through tissue. A number of factors such as ATP and complement factors act as chemoattractants. This behaviour is relevant not only in pathology but also during development where microglial cells remove apoptotic cells. After a pathological insult, microglial cells revert to their surveying phenotype, re-acquiring a rami- fied morphology. Entry of inflammatory cells into the brain

Although the CNS has long been considered to be an immunologically privileged site, lymphocyte and macrophage surveillance of the brain may be a normal, but very low-grade, activity that is enhanced in disease. Lymphocytes can enter the brain in response to virus infections A and as part of the autoimmune response in multiple sclerosis. Activated, but not resting, lymphocytes pass through the endothelium of small venules, a process that requires the expression of recognition and adhe- sion molecules (induced following cytokine activation), and subse- Arachnoid Pia mater Choroid Subarachnoid Capillary mater fissure space quently migrate into the brain parenchyma. Within the CNS, microglia can be induced by T-cell cytokines to act as efficient antigen-presenting cells. After leaving the CNS, lymphocytes probably drain along lym- phatic pathways to regional cervical lymph nodes.

CNS CNS

Choroid epithelium Choroid capillary Ependyma

B Ventricle Fig. 3.18 A, A choroid plexus within the lateral ventricle. Frond-like projections of vascular stroma derived from the pial meninges are Fig. 3.19 Activated microglial cells in the human central nervous system, covered with a low columnar epithelium that secretes cerebrospinal fluid. in a biopsy from a patient with Rasmussen’s encephalitis, visualized using Mouse tissue, toluidine blue stained resin section. B, The arrangement of MHC class II antigen immunohistochemistry. (Courtesy of Dr Norman 53 tissues forming the choroid plexus. Gregson, Division of Neurology, GKT School of Medicine, London.) 1 NERVOUS SYSTEM

Monocytes enter the CNS in the early stages of infarction and fibres. B fibres are myelinated autonomic preganglionic efferent fibres. autoimmune disease and, in particular, in pyogenic infections, probably C fibres are unmyelinated and have thermoreceptive, nociceptive and

SECTION by passing through the endothelium of local vessels. Once in the brain, interoceptive functions, including the perception of slow, burning pain monocytes are difficult to distinguish from intrinsic microglia because and visceral pain. This scheme can be applied to fibres of both spinal both cell types express a similar marker profile. During the inflamma- and cranial nerves except perhaps those of the olfactory nerve, where tory phase of meningitis, polymorphonuclear leukocytes and lym- the fibres form a uniquely small and slow group. The largest somatic phocytes pass into the CSF through the endothelium of large veins in efferent fibres (Aα) innervate extrafusal muscle fibres (at motor end- the subarachnoid space. Recent developments in research on brain plates) exclusively and conduct at a maximum of 120 m/s. Fibres to fast inflammatory disorders are reviewed in Anthony and Pitossi (2013). twitch muscles are larger than those to slow twitch muscle. Smaller (Aγ) fibres of gamma motor neurones, and autonomic preganglionic (B) and postganglionic (C) efferent fibres conduct, in order, progressively more PERIPHERAL NERVES slowly (40 m/s to less than 10 m/s). A different classification, used for afferent fibres from muscles, Afferent nerve fibres connect peripheral receptors to the CNS; they are divides fibres into groups I–IV on the basis of their calibre; groups I–III derived from neuronal somata located either in special sense organs are myelinated and group IV is unmyelinated. Group I fibres are large (e.g. the olfactory epithelium) or in the sensory ganglia of the cranio- (12–22 μm), and include primary sensory fibres of muscle spindles spinal nerves. Efferent nerve fibres connect the CNS to the effector cells (group Ia) and smaller fibres of Golgi tendon organs (group Ib). Group and tissues and are the peripheral axons of neurones with somata in II fibres are the secondary sensory terminals of muscle spindles, with the central grey matter. diameters of 6–12 μm. Group III fibres, 1–6 μm in diameter, have free Peripheral nerve fibres are grouped in widely variable numbers into sensory endings in the connective tissue sheaths around and within bundles (fasciculi). The size, number and pattern of fasciculi vary in muscles and are nociceptive and, in skin, also thermosensitive. Group different nerves and at different levels along their paths (Fig. 3.20). IV fibres are unmyelinated, with diameters below 1.5 μm; they include Their number increases and their size decreases some distance proximal free endings in skin and muscle, and are primarily nociceptive. to a point of branching. Where nerves are subjected to pressure, e.g. deep to a retinaculum, fasciculi are increased in number but reduced in size, and the amount of associated connective tissue and degree of CONNECTIVE TISSUE SHEATHS vascularity also increase. At these points, nerves may occasionally show a pink, fusiform dilation, sometimes termed a pseudoganglion or gan- Nerve trunks, whether uni- or multifascicular, are limited externally by gliform enlargement. an epineurium, which is connected to surrounding tissues by mesoneu- rium. Mesoneurium is a loose connective tissue sheath (see Ch. 2) containing the extrinsic, segmental blood supply of the nerve, and so CLASSIFICATION OF PERIPHERAL NERVE FIBRES is of clinical importance in nerve injury. Individual fasciculi of the nerve trunk are enclosed by a multilayered perineurium, which in turn sur- Classification of peripheral nerve fibres is based on various parameters rounds the endoneurium or intrafascicular connective tissue (see such as conduction velocity, function and fibre diameter. Of two clas- Fig. 3.20). sifications in common use, the first divides fibres into three major classes, designated A, B and C, corresponding to peaks in the distribu- Epineurium tion of their conduction velocities. In humans, this classification is used mainly for afferent fibres from the skin. Group A fibres are subdivided Epineurium is a condensation of loose (areolar) connective tissue into α, β, γ and δ subgroups; fibre diameter and conduction velocity derived from mesoderm. As a general rule, the more fasciculi present are proportional in most fibres. Group Aα fibres are the largest and in a peripheral nerve, the thicker the epineurium. Epineurium contains conduct most rapidly, and C fibres are the smallest and slowest. fibroblasts, collagen (types I and III) and variable amounts of fat, and The largest afferent axons (Aα fibres) innervate encapsulated cutane- it cushions the nerve it surrounds. Loss of this protective layer may be ous mechanoreceptors, Golgi tendon organs and muscle spindles, and associated with pressure palsies seen in wasted, bedridden patients. The some large alimentary enteroceptors. Aβ fibres form secondary endings epineurium also contains lymphatics (which probably pass to regional on some muscle spindle (intrafusal) fibres and also innervate cutaneous lymph nodes) and blood vessels, vasa nervorum, that pass across the and joint capsule mechanoreceptors. Aδ fibres innervate thermorecep- perineurium to communicate with a network of fine vessels within the tors, stretch-sensitive free endings, hair receptors and nociceptors, endoneurium, forming the intrinsic system of vascular plexuses. including those in dental pulp, skin and connective tissue. Aγ fibres are exclusively fusimotor to plate and trail endings on intrafusal muscle Perineurium

Perineurium extends from the CNS–PNS transition zone to the periph- ery, where it is continuous with the capsules of muscle spindles and encapsulated sensory endings, but ends openly at unencapsulated endings and neuromuscular junctions. It consists of alternating layers of flattened polygonal cells (thought to be derived from fibroblasts) and collagen. It can often contain 15–20 layers of such cells, each layer enclosed by a basal lamina up to 0.5 μm thick. Within each layer the cells interdigitate along extensive tight junctions; their cytoplasm typi- cally contains vesicles and bundles of microfilaments and their plasma membrane often shows evidence of pinocytosis. These features are con- E sistent with the function of the perineurium as a metabolically active P diffusion barrier; together with the blood–nerve barrier, the perineu- rium is thought to play an essential role in maintaining the osmotic milieu and fluid pressure within the endoneurium. Lymphatic vessels Ep have not been detected in the perineurium. Endoneurium

Strictly speaking, the term endoneurium is restricted to intrafascicular connective tissue and excludes the perineurial partitions within fasci- cles. Endoneurium consists of a fibrous matrix composed predomi- Fig. 3.20 A transverse section of a biopsied human sural nerve, showing nantly of type III collagen (reticulin) fibres, characteristically organized the arrangement of the connective tissue sheaths. Individual axons, in fine bundles lying parallel to the long axis of the nerve, and con- myelinated and unmyelinated, are arranged in a small fascicle bounded densed around individual Schwann cell–axon units and endoneurial by a perineurium. Abbreviations: P, perineurium; Ep, epineurium; vessels. The fibrous and cellular components of the endoneurium are E, endoneurium. (Courtesy of Professor Susan Standring, GKT School bathed in endoneurial fluid at a slightly higher pressure than that 54 of Medicine, London.) outside in the surrounding epineurium. The major cellular constituents 3 Peripheral nerves of the endoneurium are Schwann cells and endothelial cells; minor components are fibroblasts (constituting approximately 4% of the total endoneurial cell population), resident macrophages and mast cells. Schwann cell–axon units and blood vessels are enclosed within indi- CHAPTER vidual basal laminae and therefore isolated from the other cellular and A extracellular components of the endoneurium. Endoneurial arterioles have a poorly developed smooth muscle layer and do not autoregulate well. In sharp contrast, epineurial and perineu- rial vessels have a dense perivascular plexus of peptidergic, serotonin- ergic and adrenergic nerves. There are free nerve endings in all layers of neural connective tissue sheaths and there are some encapsulated (Pacinian) corpuscles in the endoneurium. These probably contribute A to the acute sensitivity of nerves trapped in fibrosis after injury or surgery. S

SCHWANN CELLS

Schwann cells are the major glial type in the PNS. In vitro they are A fusiform in appearance. Both in vitro and in vivo, Schwann cells ensheathe S peripheral axons, and myelinate those greater than 2 μm in diameter. In a mature peripheral nerve, they are distributed along the axons in longitudinal chains; the geometry of their association depends on whether the axon is myelinated or unmyelinated. In myelinated axons Fig. 3.21 An electron micrograph of a transverse section of biopsied the territory of a Schwann cell defines an internode. human sural nerve, showing a myelinated axon and several unmyelinated The molecular phenotype of mature myelin-forming Schwann cells axons (A), ensheathed by Schwann cells (S). (Courtesy of Professor Susan is different from that of mature non-myelin-forming Schwann cells. Standring, GKT School of Medicine, London.) Adult myelin-forming Schwann cells are characterized by the presence of several myelin proteins, some, but not all, of which are shared with oligodendrocytes and central myelin. In contrast, expression of the low-affinity neurotrophin receptor (p75NTR) and GFAP intermediate et al 2008). The region under the compact myelin sheath that extends filament protein (which differs from the CNS form in its post- between two juxtaparanodes is the internode. The molecular domains translational modification) characterizes adult non-myelin-forming of myelinated axons, including that of the axon initial segment are Schwann cells. reviewed in Buttermore et al (2013)). Schwann cells arise from Schwann cell precursors that, in turn, are Schwann cell cytoplasm forms a continuous layer only in the peri- generated from multipotent cells of the neural crest. Neuronal signals nuclear (mid-internodal) and paranodal regions, where it forms a collar regulate many aspects of Schwann cell behaviour in developing and from which microvilli project into the nodal gap substance. Elsewhere postnatal nerves. Axon-associated signals appear to control the prolif- it is dispersed as a lace-like network over the inner (adaxonal) and outer eration of developing Schwann cells and their precursors; the develop- (abaxonal) surfaces of the myelin sheath. mentally programmed death of those precursors in order to match numbers of axons and glia within each peripheral nerve bundle; the Nodes of Ranvier production of basal laminae by Schwann cells; and the induction and The nodal compartment consists of a short length of exposed axo- maintenance of myelination. Axonal neuregulin 1 signalling via ErbB2/ lemma, typically 0.8–1.1 μm long, surrounded by a nodal gap sub- B3 receptors on Schwann cells is essential for Schwann cell myelination stance composed of various extracellular components, some of which and also determines myelin thickness. An extensive literature supports may possess nerve growth-repulsive characteristics. Multiple processes the view that Schwann cells are key players in the acute injury response (microvilli) protrude into the gap substance from the outer collar of in the PNS (see Commentary 1.6), helping to provide a microenviron- Schwann cytoplasm and contact the nodal axolemma. Voltage-gated + ment that facilitates axonal regrowth (Birch 2011). Few Schwann cells Na channels, the cell adhesion molecules NrCAM and neurofascin-186, persist in chronically denervated nerves. For further reading about the cytoskeletal adaptor ankyrin G25,26 and the actin-binding protein Schwann cells, see Kidd et al (2013). spectrin βIV are clustered at nodes. The calibre of the nodal axon is usually significantly less than that of the internodal axon, particularly Unmyelinated axons in large-calibre fibres. Paranodes Unmyelinated axons are commonly 1.0 μm in diameter, although some The axolemma on either side of a node is contacted by paranodal loops may be 1.5 μm or even 2 μm in diameter. Groups of up to 10 or more of Schwann cell cytoplasm via specialized septate junctions that spiral small axons (0.15–2.0 μm in diameter) are enclosed within a chain of around the axon. The junctions are thought to form a partial diffusion overlapping Schwann cells that is surrounded by a basal lamina. Within barrier into the peri-axonal space; restrict the movement of K+ channels each Schwann cell, individual axons are usually sequestered from their from under the compact myelin; and limit lateral diffusion of mem- neighbours by delicate processes of cytoplasm. It seems likely, on the brane components. Caspr, contactin and their putative ligand NF155 basis of quantitative studies in subhuman primates, that axons from (an isoform of neurofascin) are concentrated in paranodes. adjacent cord segments may share Schwann cell columns; this phenom- enon may play a role in the evolution of neuropathic pain after nerve Juxtaparanodes injury. In the absence of a myelin sheath and nodes of Ranvier, action The region of the axon lying just beyond the innermost paranodal junc- potential propagation along unmyelinated axons is not saltatory but tion is now recognized as a distinct domain defined by the localization continuous, and relatively slow (0.5–4.0 m/s). of voltage-gated K+ channels (delayed-rectifier K+ channels Kv1.1, Kv1.2 and their Kvb2 subunit). Clustering of Kv1 channels at the juxtapara- Myelinated axons nodal region depends on their association with the Caspr2/TAG-1 adhe- sion complex. Myelinated axons (Fig. 3.21) have a 1 : 1 relationship with their ensheathing Schwann cells. The territorial extent of individual Schwann Schmidt–Lanterman incisures cells varies directly with the diameter of the axon they surround, from Schmidt–Lanterman incisures are helical decompactions of internodal 150 to 1500 μm. Specialized domains of axo-glial interaction define myelin that appear as funnel-like profiles in teased fibre preparations nodes of Ranvier and their neighbouring compartments, paranodes and labelled for markers of non-compacted myelin (e.g. MAG, Cx32). At an juxtaparanodes (Pereira et al 2012; Fig. 3.22). These domains contain incisure the major dense line of the myelin sheath splits to enclose a multiprotein complexes characterized by unique sets of transmembrane continuous spiral band of cytoplasm passing between abaxonal and and cytoskeletal proteins and clusters of ion channels; the mechanisms adaxonal layers of Schwann cell cytoplasm. The minor dense line of regulating channel clustering and node formation remain a subject of incisural myelin is also split, creating a channel connecting the peri- intense scrutiny (Peles and Salzer 2000, Poliak and Peles 2003, Horresh axonal space with the endoneurial extracellular fluid. The function of 55 1 NERVOUS SYSTEM

A B

SECTION BL

CM PNL PNL PNL MV

CM

Internode Juxtaparanode Paranode Node • Caspr 2 • Caspr • Na+ ch EP • Kv1.1, 1.2, β2 • Contactin • ank G • NF155 • NrCAM • NF186

Fig. 3.22 The general plan of a peripheral myelinated nerve fibre in longitudinal section, including one complete internodal segment and two adjacent paranodal bulbs, used as a key for the more detailed microarchitecture of specific subregions. A, A transverse electron microscope section through the centre of a node of Ranvier, with numerous finger-like processes of adjacent Schwann cells converging towards the nodal axolemma. Many microtubules and neurofilaments are visible within the axoplasm. B, The arrangement of the axon, myelin sheath and Schwann cell cytoplasm at the node of Ranvier, in the paranodal bulbs and in the juxtaparanodal region. The axon is myelinated by a Schwann cell surrounded by a basal lamina (BL). Only a portion of the internode, which is located beneath the compact myelin (CM) sheath, is shown. A spiral of paranodal (green) and juxtaparanodal (blue) proteins extends into the internode; this spiral is apposed to the inner mesaxon of the myelin sheath (not shown). K+ channels and Caspr2 are concentrated in the juxtaparanodal region. In the paranodal region, the compact myelin sheath opens up into a series of paranodal cytoplasmic loops (PNL) that invaginate and closely appose the axon, forming a series of septum-like junctions that spiral around the axon. Caspr, contactin and an isoform of neurofascin (NF155) are concentrated in this region. At the node, numerous microvilli (MV) project from the outer collar of the Schwann cell to contact the axolemma. The axon is enormously enriched in intramembranous particles at the node that correspond to Na+ channels (Na+ ch). Ankyrin G (ank G) isoforms and the cell adhesion molecules NrCAM and NF186 are also concentrated in this region. (A, Courtesy of Professor Susan Standring, GKT School of Medicine, London. B, Redrawn from Peles and Salzer 2000.)

incisures is not known; their structure suggests that they may participate from horizontal basal stem cells in the olfactory epithelium (Leung et al in transport of molecules across the myelin sheath. 2007, Forni and Wray 2012). They extend new axons through the lamina propria and cribriform plate into the CNS environment of the olfactory bulb, where they synapse with second-order neurones. Olfac- SATELLITE CELLS tory ensheathing cells (OECs, also known as olfactory ensheathing glia) accompany olfactory axons from the lamina propria of the olfactory Many non-neuronal cells of the nervous system have been called satel- epithelium to their synaptic contacts in the glomeruli of the olfactory lite cells, including small, round extracapsular cells in peripheral bulbs and are thought to play a role in directing them to their correct ganglia, ganglionic capsular cells, Schwann cells, any cell that is closely position in the olfactory bulb (Higginson and Barnett 2011). This associated with neuronal somata, and precursor cells associated with unusual arrangement is unique; elsewhere in the nervous system the striated muscle fibres (Hanani 2010). Within the nervous system, the territories of peripheral and central glia are clearly demarcated at CNS– term is most commonly reserved for flat, epithelioid cells (ganglionic PNS transition zones. OECs and the end-feet of astrocytes lying between glial cells, capsular cells) that surround the neuronal somata of periph- the bundles of olfactory axons both contribute to the glia limitans at eral ganglia (see Fig. 3.23). Their cytoplasm resembles that of Schwann the pial surface of the olfactory bulbs. cells, and their deep surfaces interdigitate with reciprocal infoldings in OECs share many properties with Schwann cells and express similar the membranes of the enclosed neurones. antigenic and morphological properties. They ensheathe olfactory sensory axons in a manner comparable to the relationship that exists Enteric glia transitorily between Schwann cells and axons in very immature periph- eral nerves, i.e. they surround, but do not segregate, bundles of up to Enteric nerves lack an endoneurium and so do not have the collagenous 50 fine unmyelinated axons to form approximately 20 fila olfactoria. coats of other peripheral nerves. The enteric ganglionic neurones are Both OECs and Schwann cells can myelinate axons, even though nor- supported by glia that closely resemble astrocytes; they contain more mally none of the axons in the olfactory nerve is myelinated. It was GFAP than non-myelinating Schwann cells and do not produce a basal thought that OECs shared a common origin with olfactory receptor lamina. Evidence for their roles in gut function is reviewed in Gul- neurones in the olfactory placode, but recent fate-mapping experiments bransen and Sharkey (2012). in chicken embryos and genetic linkage-tracing studies in mice have shown that OECs are derived from neural crest cells (Forni and Wray Olfactory ensheathing glia 2012). OECs have a malleable phenotype. There may be several subtypes: The olfactory system is unusual because it supports neurogenesis some OECs express GFAP as either fine filaments or more diffusely in 56 throughout life. Olfactory receptor neurones are continuously renewed their cytoplasm, and some express p75NTR and the O4 antigen. 3 Peripheral nerves

BLOOD SUPPLY OF PERIPHERAL NERVES processes; in myelinated fibres the junction occurs at a node of Ranvier. The peripheral process terminates in a sensory ending and, because it conducts impulses towards the soma, it functions as an elongated den-

The blood vessels supplying a nerve, end in a capillary plexus that CHAPTER pierces the perineurium. The branches of the plexus run parallel with drite, strictly speaking. However, it has the typical structural and func- the fibres, connected by short transverse vessels, forming narrow, rec- tional properties of a peripheral axon, and is conventionally described tangular meshes similar to those found in muscle. The blood supply of as an axon. peripheral nerves is unusual. Endoneurial capillaries have atypically Each neuronal soma is surrounded by a sheath of satellite glial cells large diameters and intercapillary distances are greater than in many (SGCs). (A notable exception is the spiral, or cochlear, ganglion, where other tissues. Peripheral nerves have two separate, functionally inde- most neuronal somata are myelinated, presumably contributing to fast pendent vascular systems: an extrinsic system (regional nutritive vessels electrical transmission.) The axodendritic process and its peripheral and and epineurial vessels) and an intrinsic system (longitudinally running central divisions, ensheathed by Schwann cells, lie outside the SGC microvessels in the endoneurium). Anastomoses between the two sheath. All the cells in the ganglion lie within a highly vascularized systems produce considerable overlap between the territories of the connective tissue that is continuous with the endoneurium of the nerve segmental arteries. This unique pattern of vessels, together with a high root. In dorsal root ganglia there is no clear regional mapping of the basal nerve blood flow relative to metabolic requirements, means that innervated body regions. In contrast, each of the three nerve branches peripheral nerves possess a high degree of resistance to ischaemia. (ophthalmic, maxillary and mandibular) of the trigeminal nerve is mapped to a different part of the trigeminal ganglion. Although sensory Blood–nerve barrier neurones receive no synapses, they are endowed with receptors for numerous neurotransmitters and hormones, and can thus communi- cate chemically amongst themselves and with SGCs. Just as the neuropil within the CNS is protected by a blood–brain SGCs are the main type of glial cell in sensory ganglia. They share barrier, the endoneurial contents of peripheral nerve fibres are protected several properties with astrocytes, including expression of glutamine by a blood–nerve barrier and by the cells of the perineurium. The synthetase and various neurotransmitter transporters. In addition, like blood–nerve barrier operates at the level of the endoneurial capillary astrocytes, the SGCs that surround a neurone are coupled by gap junc- walls, where the endothelial cells are joined by tight junctions, and are tions and express receptors for ATP. Unlike astrocytes, SGCs completely non-fenestrated and surrounded by continuous basal laminae. The surround individual sensory neurones (and more rarely two or three barrier is much less efficient in dorsal root ganglia and autonomic neurons) in a glial sheath. They undergo major changes as a result of ganglia and in the distal parts of peripheral nerves. injury to peripheral nerves, and appear to contribute to chronic pain in a number of animal pain models. GANGLIA Herpes zoster Primary infection with the varicella zoster virus causes Ganglia are aggregations of neuronal somata and are of varying form chickenpox. Following recovery, the virus remains dormant within and size. They occur in the dorsal roots of spinal nerves; in the sensory dorsal root ganglia or trigeminal ganglia, mostly in the neurones, and roots of the trigeminal, facial, vestibulocochlear, glossopharyngeal and less commonly in the SGCs. Reactivation of the virus leads to herpes vagal cranial nerves; and in the peripheral autonomic nervous system zoster (shingles), which involves the dermatome(s) supplied by the (ANS). Each ganglion is enclosed within a capsule of fibrous connective affected sensory nerve(s). Diagnostic signs are severe pain, erythema tissue and contains neuronal somata and neuronal processes. Enteric and blistering as occurs in chickenpox, often confined to one of the ganglia are an exception to this rule; they resemble the CNS in both divisions of the trigeminal nerve or to a spinal nerve dermatome. structure and function, and are not covered by a connective tissue Herpes zoster involving the geniculate ganglion compresses the facial capsule. Some ganglia, particularly in the ANS, contain axons that nerve and results in a lower motor neurone facial paralysis, known as originate from neuronal somata that lie elsewhere in the nervous system Ramsay Hunt syndrome. Occasionally, if the vestibulocochlear nerve and which pass through the ganglia without synapsing. becomes involved, there is vertigo, tinnitus and some deafness. The most important complication of herpes zoster is post-herpetic neural- Sensory ganglia gia, a severe and persistent pain that is highly refractory to treatment.

The sensory ganglia of dorsal spinal roots (Fig. 3.23) and the ganglia Autonomic ganglia of the trigeminal, facial, glossopharyngeal and vagal cranial nerves are enclosed in a periganglionic connective tissue capsule that resembles The main types of cell in autonomic ganglia are the ganglionic neu- the perineurium surrounding peripheral nerves. Ganglionic neurones rones, small intensely fluorescent (SIF) cells and satellite glial cells are unipolar (sometimes called pseudounipolar, see above). They have (SGCs). Most of the neurones have somata ranging from 25 to 50 μm spherical or oval somata of varying size, aggregated in groups between and complex dendritic fields; dendritic glomeruli have been observed fasciculi of myelinated and unmyelinated nerve fibres. For each neurone, in ganglia in experimental animals. Ganglionic neurones receive many a single axodendritic process bifurcates into central and peripheral axodendritic synapses from preganglionic axons; axosomatic synapses are less numerous. Postganglionic fibres commonly arise from the initial stem of a large dendrite and produce few or no collateral proc- esses. Given their close relationship to the ganglionic neurones, auto- nomic SGCs may have the potential to influence synaptic transmission. SIF cells are characterized by being smaller than the neurones and by having numerous granules that contain noradrenaline (norepine- phrine), dopamine and serotonin. They are almost completely invested by a sheath of SGCs and receive and make synapses; their physiological role is currently obscure, but they lend credence to the idea that auto- S nomic ganglia are far more than simple relay stations. N Sympathetic neurones are multipolar and their dendritic trees, on which preganglionic motor axons synapse, are more elaborate than those of parasympathetic neurones (Fig. 3.24). The neurones are sur- rounded by a mixed neuropil of afferent and efferent fibres, dendrites, S synapses and non-neural cells. There is considerable variation in the ratio of pre- and postganglionic fibres in different types of ganglion. Preganglionic sympathetic axons may synapse with many postgangli- onic neurones for the wide dissemination and perhaps amplification of sympathetic activity, a feature not found to the same degree in para- sympathetic ganglia. Dissemination may also be achieved by connec- Fig. 3.23 Sensory neurones in a dorsal root ganglion (rat). Neurones (N) tions with ganglionic interneurones or by the diffusion within the are typically variable in size but all are encapsulated by satellite cells (S). ganglion of transmitter substances produced either locally (paracrine Myelinated axons are seen above and below the neuronal somata. effect) or elsewhere (endocrine effect). Some axons within a ganglion Toluidine blue stained resin section. (Courtesy of Dr Clare Farmer, King’s may be efferent fibres en route to another ganglion, or afferents from College, London.) viscera and glands. These fibres may synapse with neurones in the 57 1 NERVOUS SYSTEM

the neonatal period. Treatment usually consists of removing the dis- eased intestinal segment.

SECTION The enteric plexuses consist of sensory neurones, interneurones and a variety of motor neurones. These neurones are endowed with recep- tors for a large number of neurotransmitters and also release a variety of neurotransmitters. All classes of enteric neurone are equally distrib- uted along the entire ganglionic network; consequently the ENS con- sists of numerous repeating modules. The myenteric plexus contains the motor neurones that control the movements of gastrointestinal smooth muscle. The main excitatory neurotransmitter is acetylcholine, which may be co-localized with an excitatory peptide (usually a tachykinin, such as substance P). The main inhibitory neurotransmitter is nitric oxide (NO), released from neu- rones that may also release the inhibitory peptide vasoactive intestinal peptide (VIP). An important function of myenteric neurones is to mediate the peri- staltic reflex, which is induced by intestinal wall distension or by mechanical stimulation of the mucosa. These stimuli initiate contrac- tion oral to the site of the stimulus, and relaxation anal to the site, creat- ing a pressure gradient that propels the intestinal contents. Interstitial cells of Cajal (ICC) are pacemaker cells believed to integrate neuronal Fig. 3.24 A parasympathetic autonomic ganglion from a human stomach. signals with rhythmic oscillations of muscle contraction; disturbance of Large neuronal somata, some with nuclei and prominent nucleoli in the ICC function may be a factor in a number of gastrointestinal disorders plane of section, are encapsulated by satellite cells and surrounded by (Huizinga et al 2009). nerve fibres and non-neuronal cells. (Courtesy of Mr Peter Helliwell and Enteric glia are the main type of glial cell in the ENS. In some the late Dr Joseph Mathew, Department of Histopathology, Royal respects they resemble astrocytes, e.g. they form end-feet with blood Cornwall Hospitals Trust, UK.) vessels, respond to numerous chemical mediators, and are extensively coupled among themselves by gap junctions. They appear to play an important role in neuroprotection and in maintaining the integrity of the intestinal mucosal barrier.

M DISPERSED NEUROENDOCRINE SYSTEM

Although the nervous, neuroendocrine and endocrine systems all operate by intercellular communication, they differ in the mode, speed and degree or localization of the effects produced (Day and Salzet 2002). The autonomic nervous system uses impulse conduction and neurotransmitter release to transmit information, and the responses induced are rapid and localized. The dispersed neuroendocrine system uses only secretion. It is slower and the induced responses are less local- ized, because the secretions, e.g. neuromediators, can act either on contiguous cells, or on groups of nearby cells reached by diffusion, or on distant cells via the blood stream. Many of its effector molecules operate in both the nervous system and the neuroendocrine system. M The endocrine system proper, which consists of clusters of cells and discrete, ductless, hormone-producing glands, is even slower and less localized, although its effects are specific and often prolonged. These regulatory systems overlap in function, and can be considered as a Fig. 3.25 An enteric ganglion (outlined) of the myenteric (Auerbach’s) single neuroendocrine regulator of the metabolic activities and internal plexus between the inner circular and outer longitudinal layers of smooth environment of the organism, acting to provide conditions in which it muscle (M) in the wall of the human intestine. An enteric ganglionic neurone is arrowed. (Courtesy of Mr Peter Helliwell and the late Dr can function successfully. Neural and neuroendocrine axes appear to Joseph Mathew, Department of Histopathology, Royal Cornwall Hospitals cooperate to modulate some forms of immunological reaction; the Trust, UK.) extensive system of vessels, circulating hormones and nerve fibres that link the brain with all viscera are thought to constitute a neuroimmune network (Fig. 3.26). ganglion, e.g. substance P-containing axons of dorsal root neurones Some cells can take up and decarboxylate amine precursor com- synapse on neurones in prevertebral ganglia, thereby enabling interac- pounds (amine precursor uptake and decarboxylation, or APUD, cells). tions between the sensory system and the ANS. They are characterized by dense-core cytoplasmic granules (see Fig. 2.6), similar to the neurotransmitter vesicles seen in some types of neuronal Enteric ganglia terminal. The group includes cells described as chromaffin cells (phaeo- chromocytes), derived from neuroectoderm and innervated by pregan- The enteric nervous system (ENS) lies within the walls of the gastroin- glionic sympathetic nerve fibres. Chromaffin cells synthesize and secrete testinal tract (see Fig. 2.15 for the layers of a typical viscus) and includes catecholamines (dopamine, noradrenaline (norepinephrine) or adren- the myenteric and submucosal plexuses and associated ganglia (Furness aline (epinephrine)). Their name refers to the finding that their cyto- 2012, Neunlist et al 2013). The ganglionic neurones (Fig. 3.25) serve plasmic store of catecholamines is sufficiently concentrated to give an different functions, including the regulation of gut motility (in conjunc- intense yellow–brown colouration, the positive chromaffin reaction, tion with interstitial cells of Cajal (Huizinga et al 2009)), mucosal when they are treated with aqueous solutions of chromium salts, par- transport and mucosal blood flow. Unlike the other two divisions of ticularly potassium dichromate. Classic chromaffin cells include clus- the ANS, the ENS is largely independent of the CNS, and the extrinsic ters of cells in the suprarenal medulla; the para-aortic bodies, which autonomic fibres that supply the gut wall exert only modulatory effects secrete noradrenaline; paraganglia; certain cells in the carotid bodies; on it. Submucosal neurones, together with sympathetic axons, regulate and small groups of cells irregularly dispersed among the paravertebral the local blood flow. sympathetic ganglia, splanchnic nerves and prevertebral autonomic Hirschsprung’s disease is a congenital disease in which dysfunctional plexuses. neural crest migration means that the ganglia of both the myenteric and The alimentary tract contains a large population of cells of a similar submucosal plexuses in the distal bowel fail to develop. The resulting type (previously called neuroendocrine or enterochromaffin cells) lack of propulsive activity in the aganglionic bowel leads to functional in its wall. These cells act as sensory transducers, activating intrinsic obstruction and megacolon, which can be life-threatening. Around 1 in and extrinsic primary afferent neurones via their release of 5- 58 5,000 infants is born with the condition and is typically diagnosed in hydroxytryptamine (5-HT, serotonin). The neonatal respiratory tract 3 Sensory endings

Brainstem receptor and partly in the neurone that innervates it, in the case of epithelial receptors. Transduction varies with the modality of the stimu- Sensory vagal neurone lus, and usually causes depolarization of the receptor membrane (or hyperpolarization, in the retina). In mechanoreceptors, transduction CHAPTER may involve the deformation of membrane structure, which causes Prevertebral sympathetic ganglion either strain or stretch-sensitive ion channels to open. In chemorecep- tors, receptor action may resemble that for ACh at neuromuscular Spinal sensory neurone Immune and tissue junctions. Visual receptors share similarities with chemoreceptors: defence signals: light causes changes in receptor proteins, which activate G proteins, Spinal cord local and systemic Intestinofugal resulting in the release of second messengers and altered membrane neurone permeability. Neurocrine signals: The quantitative responses of sensory endings to stimuli vary greatly local and circulating and increase the flexibility of the functional design of sensory systems. Although increased excitation with increasing stimulus level is a Intrinsic common pattern (‘on’ response), some receptors respond to decreased sensory stimulation (‘off’ response). Even unstimulated receptors show varying neurone degrees of spontaneous background activity against which an increase or decrease in activity occurs with changing levels of stimulus. In all ch et receptors studied, when stimulation is maintained at a steady level, tr Gut lumen S there is an initial burst (the dynamic phase) followed by a gradual adaptation to steady level (the static phase). Though all receptors show Signals from lumen these two phases, one or other may predominate, providing a distinc- e.g. nutrients, antigens, tion between rapidly adapting endings that accurately record the rate irritants, secretions of stimulus onset, and slowly adapting endings that signal the constant amplitude of a stimulus, e.g. position sense. Dynamic and static phases are reflected in the amplitude and duration of the receptor potential Fig. 3.26 The ways in which the nervous system, neuroendocrine system and also in the frequency of action potentials in the sensory fibres. The and immune system are integrated, demonstrated in the intestine. stimulus strength necessary to elicit a response in a receptor, i.e. its Neurocrine signals from enteric neuroendocrine cells and signals from threshold level, varies greatly between receptors, and provides an extra immune defence cells (e.g. lymphocytes, macrophages and mast cells) level of information about stimulus strength. act on other cells of those systems and on neurones with sensory For further information on sensory receptors, see Nolte (2008). endings in the intestinal wall, either locally or at a distance. Some neuronal soma lie within enteric ganglia in the gut wall; others have their bodies in peripheral ganglia. Neuronal signals may act locally, be transmitted to the CNS or enter a reflex pathway via sympathetic ganglia. FUNCTIONAL CLASSIFICATION OF RECEPTORS Receptors have been classified in several ways. They may be classified by the modalities to which they are sensitive, such as mechanoreceptors contains a prominent system of neuroendocrine cells, both dispersed (which are responsive to deformation, e.g. touch, pressure, sound waves, and aggregated (neuroepithelial bodies); the numbers of both types etc.), chemoreceptors, photoreceptors and thermoreceptors. Some re- decline during childhood. Merkel cells (see Commentary 1.3) in the ceptors are polymodal, i.e. they respond selectively to more than one basal epidermis of the skin store neuropeptides, which they release modality; they usually have high thresholds and respond to damaging to associated nerve endings or other cells in a neuroendocrine role, in stimuli associated with irritation or pain (nociceptors). response to pressure and possibly other stimuli (Lucarz and Brand Another widely used classification divides receptors on the basis of 2007). Experimental animal studies have revealed 5-HT-containing their distribution in the body into exteroceptors, proprioceptors and intraepithelial paraneurones in the urothelial lining of the urethra; interoceptors. Exteroceptors and proprioceptors are receptors of the these cells are thought to relay information from the luminal surface somatic afferent components of the nervous system, while interoceptors of the urethra to underlying sensory nerves. are receptors of the visceral afferent pathways. A number of descriptions and terms have been applied to cells of Exteroceptors respond to external stimuli and are found at, or close this system in the older literature (see online text for details). to, body surfaces. They can be subdivided into the general or cutaneous For further reading, see Day and Salzet (2002). sense organs and special sensory organs. General sensory receptors include free and encapsulated terminals in skin and near hairs; none of these has absolute specificity for a particular sensory modality. SENSORY ENDINGS Special sensory organs are the olfactory, visual, acoustic, vestibular and taste receptors. GENERAL FEATURES OF SENSORY RECEPTORS Proprioceptors respond to stimuli to deeper tissues, especially of the locomotor system, and are concerned with detecting movement, There are three major forms of sensory receptor: neuroepithelial, epi- mechanical stresses and position. They include Golgi tendon organs, thelial and neuronal (Fig. 3.27). muscle spindles, Pacinian corpuscles, other endings in joints, and ves- A neuroepithelial receptor is a neurone with a soma lying near a tibular receptors. Proprioceptors are stimulated by the contraction of sensory surface and an axon that conveys sensory signals into the CNS muscles, movements of joints and changes in the position of the body. to synapse on second-order neurones. This is an evolutionarily primi- They are essential for the coordination of muscles, the grading of mus- tive arrangement, and the only examples remaining in humans are the cular contraction, and the maintenance of equilibrium. sensory neurones of the olfactory epithelium. Interoceptors are found in the walls of the viscera, glands and vessels, An epithelial receptor is a cell that is modified from a non-nervous where their terminations include free nerve endings, encapsulated ter- sensory epithelium and innervated by a primary sensory neurone with minals and endings associated with specialized epithelial cells. Nerve a soma lying near the CNS, e.g. auditory receptors and taste buds. When terminals are found in the layers of visceral walls and the adventitia of activated, this type of receptor excites its neurone by neurotransmission blood vessels, but the detailed structure and function of many of these across a synaptic gap. endings are not well established. Encapsulated (lamellated) endings A neuronal receptor is a primary sensory neurone that has a soma occur in the heart, adventitia and mesenteries. Free terminal arboriza- in a craniospinal ganglion and a peripheral axon ending in a sensory tions occur in the endocardium, the endomysium of all muscles, and terminal. All cutaneous sensors and proprioceptors are of this type; connective tissue generally. Tension produced by excessive muscular their sensory terminals may be encapsulated or linked to special meso- contraction or by visceral distension often causes pain, particularly in dermal or ectodermal structures to form a part of the sensory apparatus. pathological states, which is frequently poorly localized and of a deep- The extraneural cells are not necessarily excitable, but create an appro- seated nature. Visceral pain is often referred to the corresponding der- priate environment for the excitation of the neuronal process. matome (see Fig. 16.10). Polymodal nociceptors (irritant receptors) The receptor stimulus is transduced into a graded change of electrical respond to a variety of stimuli such as noxious chemicals or damaging potential at the receptor surface (receptor potential), and this initiates mechanical stimuli. They are mainly the free endings of fine, unmyeli- an all-or-none action potential that is transmitted to the CNS. This may nated fibres that are widely distributed in the epithelia of the alimentary occur either in the receptor, when this is a neurone, or partly in the and respiratory tracts; they may initiate protective reflexes. 59 3 Nervous system

They include: clear cells (so named because of their poor staining properties in routine preparations); argentaffin cells (reduce silver salts); argyrophil cells (absorb silver); small intensely fluorescent cells; peptide-producing cells (particularly of the hypothalamus, hypophysis, CHAPTER pineal and parathyroid glands, and placenta); Kulchitsky cells in the lungs; and paraneurones. Many cells of the dispersed (or diffuse) neu- roendocrine system are derived embryologically from the neural crest. Some – in particular, cells from the gastrointestinal system – are now known to be endodermal in origin.

59.e1 1 NERVOUS SYSTEM

Free endings: SECTION

Rapidly adapting mechanoreceptor

Thermoreceptor (hot and cold)

Type I Slowly adapting mechanoreceptor (Merkel cell ending)

Nociceptor

Type II Slowly adapting mechanoreceptor Rapidly adapting lamellated (Pacinian) corpuscle Rapidly adapting ‘field’ mechanoreceptor (Ruffini ending) (Meissner’s corpuscle) Fig. 3.27 Some major types of sensory ending of general afferent fibres (omitting neuromuscular, neurotendinous and hair-related types). The traces below each type of ending indicate (top) their response (firing rate (vertical lines) and adaption with time) to an appropriate stimulus (below) of the duration indicated. The Pacinian corpuscle’s response to vibration (rapid sequence of on–off stimuli) is also shown.

Interoceptors include vascular chemoreceptors, e.g. the carotid body, Special types of free ending are associated with epidermal structures and baroceptors, which are concerned with the regulation of blood flow in the skin. They include terminals associated with hair follicles (peri- and pressure and the control of respiration. trichial receptors), which branch from myelinated fibres in the deep dermal cutaneous plexus; the number, size and form of the endings are related to the size and type of hair follicle innervated. These endings FREE NERVE ENDINGS respond mainly to movement when hair is deformed and belong to the rapidly adapting mechanoreceptor group. Sensory endings that branch to form plexuses occur in many sites (see Merkel tactile endings (see Commentary 1.3) lie either at the base Fig. 3.27). They occur in all connective tissues, including those of the of the epidermis or around the apical ends of some hair follicles, and dermis, fasciae, capsules of organs, ligaments, tendons, adventitia of most are innervated by large myelinated axons. Each axon expands into blood vessels, meninges, articular capsules, periosteum, perichon- a disc that is applied closely to the base of a Merkel cell in the basal drium, Haversian systems in bone, parietal peritoneum, walls of viscera layer of the epidermis. The cells are believed to be derived from the and the endomysium of all types of muscle. They also innervate the epidermis, although a neural crest origin remains possible. They contain epithelium of the skin, cornea, buccal cavity, and the alimentary and many large (50–100 nm) dense-core vesicles, presumably containing respiratory tracts and their associated glands. Within epithelia, free transmitters. Merkel endings are thought to be slow-adapting mech- sensory endings lack Schwann cell ensheathment and are enveloped anoreceptors, responsive to sustained pressure and sensitive to the instead by epithelial cells. Afferent fibres from free terminals may be edges of applied objects. Their functions are controversial, however, and myelinated or unmyelinated but are always of small diameter and low likely to be more varied. conduction velocity. When afferent axons are myelinated, their termi- nal arborizations lack a myelin sheath. These terminals serve several sensory modalities. In the dermis, they may be responsive to moderate ENCAPSULATED ENDINGS cold or heat (thermoreceptors); light mechanical touch (mechanore- ceptors); damaging heat, cold or deformation (unimodal nociceptors); Encapsulated endings are a major group of special endings that includes and damaging stimuli of several kinds (polymodal nociceptors). lamellated corpuscles of various kinds (e.g. Meissner’s, Pacinian), Golgi Similar fibres in deeper tissues may also signal extreme conditions, tendon organs, neuromuscular spindles and Ruffini endings (see Fig. which are experienced, as with all nociceptors, as ache or pain. Free 3.27). They exhibit considerable variety in their size, shape and distribu- endings in the cornea, dentine and periosteum may be exclusively tion but share a common feature: namely, that each axon terminal is 60 nociceptive. encapsulated by non-excitable cells (Proske and Gandevia 2012). 3 Sensory endings

Meissner’s corpuscles into the capsule or core, so that it is not clearly defined in mature cor- puscles. The core consists of approximately 60 bilateral, compacted lamellae lying on both sides of a central nerve terminal.

Meissner’s corpuscles are found in the dermal papillae of all parts of CHAPTER the hand and foot, the anterior aspect of the forearm, the lips, palpebral Each corpuscle is supplied by a myelinated axon, which initially conjunctiva and mucous membrane of the apical part of the tongue. loses its myelin sheath and subsequently loses its ensheathing Schwann They are most concentrated in thick hairless skin, especially of the finger cell at its junction with the core. The naked axon runs through the pads, where there may be up to 24 corpuscles per cm2 in young adults. central axis of the core and ends in a slightly expanded bulb. It is in Mature corpuscles are cylindrical in shape, approximately 80 μm long contact with the innermost core lamellae, is transversely oval and sends and 30 μm across, with their long axes perpendicular to the skin surface. short projections of unknown function into clefts in the lamellae. The Each corpuscle has a connective tissue capsule and central core com- axon contains numerous large mitochondria, and minute vesicles, posed of a stack of flat modified Schwann cells (Fig. 3.28). Meissner’s approximately 5 nm in diameter, which aggregate opposite the clefts. corpuscles are rapidly adapting mechanoreceptors, sensitive to shape The cells of the capsule and core lamellae are thought to be specialized and textural changes in exploratory and discriminatory touch; their fibroblasts but some may be Schwann cells. Elastic fibrous tissue forms acute sensitivity provides the neural basis for reading Braille text. an overall external capsule to the corpuscle. Pacinian corpuscles are supplied by capillaries that accompany the axon as it enters the capsule. Pacinian corpuscles act as very rapidly adapting mechanoreceptors. Pacinian corpuscles They respond only to sudden disturbances and are especially sensitive to very-high-frequency vibration. The rapidity may be partly due to the Pacinian corpuscles are situated subcutaneously in the palmar and lamellated capsule acting as a high pass frequency filter, damping slow plantar aspects of the hand and foot and their digits, the external geni- distortions by fluid movement between lamellar cells. Groups of cor- talia, arm, neck, nipple, periosteal and interosseous membranes, and puscles respond to pressure changes, e.g. on grasping or releasing an near joints and within the mesenteries (Fig. 3.29). They are oval, spheri- object. cal or irregularly coiled and measure up to 2 mm in length and 100–500 μm or more across; the larger ones are visible to the naked eye. Each corpuscle has a capsule, an intermediate growth zone and a Ruffini endings central core that contains an axon terminal. The capsule is formed by approximately 30 concentrically arranged lamellae of flat cells approxi- Ruffini endings are slowly adapting mechanoreceptors. They are found mately 0.2 μm thick (see Fig. 3.28). Adjacent cells overlap and succes- in the dermis of thin, hairy skin, where they function as dermal stretch sive lamellae are separated by an amorphous proteoglycan matrix that receptors and are responsive to maintained stresses in dermal collagen. contains circularly orientated collagen fibres, closely applied to the They consist of the highly branched, unmyelinated endings of myeli- surfaces of the lamellar cells. The amount of collagen increases with nated afferents. They ramify between bundles of collagen fibres within age. The intermediate zone is cellular and its cells become incorporated a spindle-shaped structure, which is enclosed partly by a fibrocellular sheath derived from the perineurium of the nerve. Ruffini endings appear electrophysiologically similar to Golgi tendon organs, which they resemble, although they are less organized structurally. Similar structures appear in joint capsules (see below). Golgi tendon organs Epidermis Golgi tendon organs are found mainly near musculotendinous junc- tions (Fig. 3.30), where more than 50 may occur at any one site. Each terminal is closely related to a group of muscle fibres (up to 20) as they insert into the tendon. Golgi tendon organs are approximately 500 m Tactile corpuscle μ long and 100 μm in diameter, and consist of small bundles of tendon

Fig. 3.30 The structure and innervation of a Golgi tendon organ. For clarity, the perineurium and endoneurium have been omitted to show the distribution of nerve fibres ramifying between the collagen fibre bundles of the tendon.

Fig. 3.28 A tactile Meissner’s corpuscle in a dermal papilla in the skin, demonstrated using the modified Bielschowsky silver stain technique. (Courtesy of Professor N Cauna, University of Pittsburgh.)

Fig. 3.29 A Pacinian corpuscle in human dermis. (Courtesy of Mr Peter Helliwell and the late Dr Joseph Mathew, Department of Histopathology, Royal Cornwall Hospitals Trust, UK.) 61 1 NERVOUS SYSTEM

fibres enclosed in a delicate capsule. The collagen bundles (intrafusal fasciculi) are less compact than elsewhere in the tendon, the collagen

SECTION fibres are smaller and the fibroblasts larger and more numerous. A single, thickly myelinated 1b afferent nerve fibre enters the capsule and divides. Its branches, which may lose their ensheathing Schwann cells, terminate in leaf-like enlargements containing vesicles and mitochon- dria, which wrap around the tendon. A basal lamina or process of Schwann cell cytoplasm separates the nerve terminals from the collagen bundles that constitute the tendon. Golgi tendon organs are activated External capsule by passive stretch of the tendon but are much more sensitive to active contraction of the muscle. They are important in providing propriocep- tive information that complements the information coming from neu- Internal capsule romuscular spindles. Their responses are slowly adapting and they signal maintained tension.

Neuromuscular spindles

Neuromuscular spindles are mechanosensors essential for propriocep- tion (Boyd 1985). Each spindle contains a few small, specialized Nuclear bag fibre intrafusal muscle fibres, innervated by both sensory and motor nerve fibres (Figs 3.31–3.32). The whole is surrounded equatorially by a Nuclear chain fibre fusiform spindle capsule of connective tissue, consisting of an outer perineurium-like sheath of flattened fibroblasts and collagen, and an Subcapsular space inner sheath that forms delicate tubes around individual intrafusal fibres (Fig. 3.33). A gelatinous fluid rich in glycosaminoglycans fills the Primary (anulospiral) space between the two sheaths. ending of group 1a There are usually 5–14 intrafusal fibres (the number varies between afferent fibre muscles) and two major types of fibre, nuclear bag and nuclear chain fibres, which are distinguished by the arrangement of nuclei in their sarcoplasm. In nuclear bag fibres, an equatorial cluster of nuclei makes the fibre bulge slightly, whereas the nuclei in nuclear chain fibres form a single axial row. Nuclear bag fibres are subdivided into bag1 and bag2 fibres, are greater in diameter than chain fibres and extend beyond the surrounding capsule to the endomysium of nearby extrafusal muscle Secondary (flower spray) ending of fibres. Nuclear chain fibres are attached at their poles to the capsule or group II afferent fibre to the sheaths of nuclear bag fibres. The intrafusal fibres resemble typical skeletal muscle fibres, except that the zone of myofibrils is thin around the nuclei. Dynamic bag1 fibres generally lack M lines, possess little sarcoplasmic reticulum, and have an abundance of mitochondria and oxidative enzymes, but little glycogen. Static bag2 fibres have distinct M lines and abundant glyco- gen. Nuclear chain fibres have marked M lines, sarcoplasmic reticulum Trail ending of γ-efferent fibre and T-tubules, and abundant glycogen, but few mitochondria. Each fibre type carries distinct myosin heavy chain isoforms. These variations reflect the contractile properties of different intrafusal fibres. Muscle spindles receive two types of sensory innervation via the Plate ending of γ-efferent fibre unmyelinated terminations of large myelinated axons. Primary (anulo- spiral) endings are equatorially placed and form spirals around the nucleated parts of intrafusal fibres. They are the endings of large sensory fibres (group Ia afferents), each of which sends branches to a number of intrafusal muscle fibres. Each terminal lies in a deep sarcolemmal groove in the spindle plasma membrane beneath its basal lamina. M Secondary (flower spray) endings, which may be spray-shaped or anular, Plate ending of are largely confined to bag2 and nuclear chain fibres, and are the β-efferent fibre branched terminals of somewhat thinner myelinated (group II) affer- ents. They are varicose and spread in a narrow band on both sides of the primary endings. They lie close to the sarcolemma, though not in grooves. In essence, primary endings are rapidly adapting, while second- Fig. 3.31 A neuromuscular spindle, showing nuclear bag and nuclear ary endings have a regular, slowly adapting response to static stretch. chain fibres within the spindle capsule (green); these are innervated by There are three types of motor endings in muscle spindles. Two are the sensory anulospiral and ‘flower spray’ afferent fibre endings (blue) and from fine, myelinated, fusimotor (γ) efferents and one is from myeli- by the γ and β fusimotor (efferent fibre) endings (red). The β efferent fibres nated (β) efferent collaterals of axons that supply extrafusal slow twitch are collaterals of fibres innervating extrafusal slow twitch muscle cells (M). muscle fibres. The fusimotor efferents terminate nearer the equatorial region, where their terminals either resemble the motor end-plates of extrafusal fibres (plate endings) or are more diffuse (trail endings). acceleration. Moreover, they are under complex central control; efferent Stimulation of the fusimotor and β-efferents causes contraction of the (fusimotor) nerve fibres, by regulating the strength of contraction, can intrafusal fibres and, consequently, activation of their sensory endings. adjust the length of the intrafusal fibres and thereby the responsiveness Muscle spindles signal the length of extrafusal muscle both at rest of spindle sensory endings. In summary, the organization of spindles and throughout contraction and relaxation, the velocity of their con- allows them to monitor muscle conditions actively in order to compare traction and changes in velocity. These modalities may be related to the intended and actual movements, and to provide a detailed input to different behaviours of the three major types of intrafusal fibre and their spinal, cerebellar, extrapyramidal and cortical centres about the state of sensory terminals. The sensory fusimotor endings of one type of nuclear the locomotor apparatus. bag fibre (dynamic bag1) are particularly concerned with signalling rapid changes in length that occur during movement, whilst those of the second bag fibre type (static bag2) and of chain fibres are less JOINT RECEPTORS responsive to movement. These elements can therefore detect complex changes in the state of the extrafusal muscle surrounding spindles and The arrays of receptors situated in and near articular capsules provide 62 can signal fluctuations in length, tension, velocity of length change and information on the position, movements and stresses acting on joints. 3 Neuromuscular junctions

Static bag2 deeper layers and other articular structures (e.g. the fat pad of the tem- fibre poromandibular joint). They are rapidly adapting, low-threshold mech- anoreceptors, sensitive to movement and pressure changes, and they

Dynamic bag1 CHAPTER fibre Long-chain respond to joint movement and transient stresses in the joint capsule. fibre Short-chain They are supplied by myelinated afferent axons but are probably not fibres involved in the conscious awareness of joint sensation. Type III endings are identical to Golgi tendon organs in structure and function; they occur in articular ligaments but not in joint capsules. They are high-threshold, slowly adapting receptors and may serve, at least in part, to prevent excessive stresses at joints by reflex inhibition of the adjacent muscles. They are innervated by large myelinated affer- Dynamic γ-efferent ent axons. Static γ-efferent Type IV endings are free terminals of myelinated and unmyelinated axons that ramify in articular capsules and the adjacent fat pads, and II around the blood vessels of the synovial layer. They are high-threshold, II slowly adapting receptors and are thought to respond to excessive Afferent fibres movements, providing a basis for articular pain.

Ia NEUROMUSCULAR JUNCTIONS Static γ-efferent Static β-efferent SKELETAL MUSCLE Dynamic β-efferent The most intensively studied effector endings are those that innervate muscle, particularly skeletal muscle. All neuromuscular (myoneural) junctions are axon terminals of motor neurones. They are specialized for the release of neurotransmitter on to the sarcolemma of skeletal muscle fibres, causing a change in their electrical state that leads to contraction. Each axon branches near its terminal to innervate from several to hundreds of muscle fibres, the number depending on the precision of motor control required (Shi et al 2012). The detailed structure of a motor terminal varies with the type of muscle innervated. Two major types of ending are recognized, innervat- Collaterals to extrafusal muscle ing either extrafusal muscle fibres or the intrafusal fibres of neuromus- cular spindles. In the former type, each axonal terminal usually ends Fig. 3.32 Nuclear bag and nuclear chain fibres in a neuromuscular midway along a muscle fibre in a discoidal motor end-plate (Fig. spindle. Dynamic β- and γ-efferents innervate dynamic bag1 intrafusal 3.34A), and usually initiates action potentials that are rapidly con- fibres, whereas static β- and γ-efferents innervate static bag2 and nuclear ducted to all parts of the muscle fibre. In the latter type, the axon gives chain intrafusal fibres. off numerous branches that form a cluster of small expansions extend- ing along the muscle fibre; in the absence of propagated muscle excita- tion, these excite the fibre at several points. Both types of ending are associated with a specialized receptive region of the muscle fibre, the sole plate, where a number of muscle cell nuclei are grouped within the granular sarcoplasm. M The sole plate contains numerous mitochondria, endoplasmic retic- ulum and Golgi complexes. The terminal branches of the axon are C plugged into shallow grooves in the surface of the sole plate (primary clefts), from where numerous pleats extend for a short distance into the underlying sarcoplasm (secondary clefts) (Fig. 3.34B,C). The axon ter- minal contains mitochondria and many clear, 60 nm spherical vesicles similar to those in presynaptic boutons, which are clustered over the zone of membrane apposition. It is ensheathed by Schwann cells whose cytoplasmic projections extend into the synaptic cleft. The plasma IF membranes of the axon terminal and the muscle cell are separated by a 30–50 nm gap and an interposed basal lamina, which follows the C surface folding of the sole-plate membrane into the secondary clefts. Endings of fast and slow twitch muscle fibres differ in detail: the sarco- lemmal grooves are deeper, and the presynaptic vesicles more numer- ous, in the fast fibres. Fig. 3.33 A neuromuscular spindle in transverse section in a human Junctions with skeletal muscle are cholinergic: the release of ACh extraocular muscle. The spindle capsule (C) encloses intrafusal fibres (IF) changes the ionic permeability of the muscle fibre (Sine 2012). Cluster- of varying diameters. Typical muscle fibres (M) in transverse section are ing of ACh receptors at the neuromuscular junction depends in part on shown above the spindle. Toluidine blue stained resin section. the presence in the muscle basal lamina of agrin, which is secreted by the motor neurone, and is important in establishing the postjunctional Structural and functional studies have demonstrated at least four types molecular machinery. When the depolarization of the sarcolemma of joint receptor; their proportions and distribution vary with site. Three reaches a particular threshold, it initiates an action potential in the are encapsulated endings, the fourth a free terminal arborization. sarcolemma, which is then propagated rapidly over the whole cell Type I endings are encapsulated corpuscles of the slowly adapting surface and also deep within the fibre via the invaginations (T-tubules) mechanoreceptor type and resemble Ruffini endings. They lie in the of the sarcolemma, causing contraction. The amount of ACh released superficial layers of the fibrous capsules of joints in small clusters and by the arrival of a single nerve impulse is sufficient to trigger an action are innervated by myelinated afferent axons. Being slowly adapting, potential. However, because ACh is very rapidly hydrolysed by the they provide awareness of joint position and movement, and respond enzyme AChE, present at the sarcolemmal surface of the sole plate, a to patterns of stress in articular capsules. They are particularly common single nerve impulse only gives rise to one muscle action potential, i.e. in joints where static positional sense is necessary for the control of there is a one-to-one relationship between neuronal and muscle action posture (e.g. hip, knee). potentials. Thus the contraction of a muscle fibre is controlled by the Type II endings are lamellated receptors and resemble small versions firing frequency of its motor neurone. of the large Pacinian corpuscles found in general connective tissue. They Neuromuscular junctions are partially blocked by high concentra- occur in small groups throughout joint capsules, particularly in the tions of lactic acid, as in some types of muscle fatigue. 63 1 NERVOUS SYSTEM

A Fig. 3.34 The neuromuscular junction. A, Whole-mount preparation of teased skeletal muscle fibres (pale, faintly striated, diagonally orientated structures). The terminal part of the axon (silver-stained, brown) branches to form motor SECTION end-plates on adjacent muscle fibres. The sole plate recesses in the sarcolemma, into which the motor end-plates fit, are demonstrated by the presence of acetylcholinesterase (shown by enzyme histochemistry, blue). B, The axonal motor end-plate and the deeply infolded sarcolemma. C, Electron micrograph showing the expanded motor end-plate of an axon filled with vesicles containing synaptic transmitter (ACh) (above); the deep infoldings of the sarcolemmal sole plate (below) form subsynaptic gutters. (A, Courtesy of Dr Norman Gregson, Division of Neurology, GKT School of Medicine, London. C, Courtesy of Professor DN Landon, Institute of Neurology, University College London.)

AUTONOMIC MOTOR TERMINATIONS

Autonomic neuromuscular junctions differ in several important ways from the skeletal neuromuscular junction and from synapses in the Motor axon Schwann cell CNS and PNS. There is no fixed junction with well-defined pre- and postjunctional specializations. Unmyelinated, highly branched, post- ganglionic autonomic axons become beaded or varicose as they reach the effector smooth muscle. These varicosities are not static but move along axons. They are packed with mitochondria and vesicles contain- ing neurotransmitters, which are released from the varicosities during B conduction of an impulse along the axon. The distance (cleft) between the varicosity and smooth muscle membrane varies considerably depending on the tissue, from 20 nm in densely innervated structures such as the vas deferens to 1–2 μm in large elastic arteries. Unlike skel- etal muscle, the effector tissue is a muscle bundle rather than a single cell. Gap junctions between individual smooth muscle cells are low- resistance pathways that allow electronic coupling and the spread of activity within the effector bundle; they vary in size from punctate junc- tions to junctional areas of more than 1 μm in diameter. Adrenergic sympathetic postganglionic terminals contain dense- cored vesicles. Cholinergic terminals, which are typical of all parasym- pathetic and some sympathetic endings, contain clear spherical vesicles like those in the motor end-plates of skeletal muscle. A third category Muscle cell of autonomic neurones has non-adrenergic, non-cholinergic endings Muscle nucleus that contain a wide variety of chemicals with transmitter properties. sole plate Motor end-plate with ATP is a neurotransmitter at these terminals, which express purinergic synaptic vesicles receptors (Burnstock et al 2011). The axons typically contain large, Motor end-plate 80–200 nm, dense opaque vesicles, congregated in varicosities at inter- vals along their length. These terminals are formed in many sites, including the lungs, blood vessel walls, the urogenital tract and the C external muscle layers and sphincters of the gastrointestinal tract. In the intestinal wall, neuronal somata lie in the myenteric plexus, and their axons spread caudally for a few millimetres, mainly to innervate circular muscle. Purinergic neurones are under cholinergic control from pregan- glionic sympathetic neurones. Their endings mainly hyperpolarize smooth muscle cells, causing relaxation, e.g. preceding peristaltic waves, opening sphincters and, probably, causing reflex distension in gastric filling. Autonomic efferents innervate exocrine glands, myoepithelial cells, adipose tissue (noradrenaline (norepinephrine) released from postganglionic sympathetic axons binds to β3-receptors on adipocytes to stimulate lipolysis) and the vasculature and parenchymal fields of lymphocytes and associated cells in several lymphoid organs, including the thymus, spleen and lymph nodes.

CNS–PNS TRANSITION ZONE

The transition between CNS and PNS usually occurs some distance from the point at which nerve roots emerge from the brain or the spinal cord. The segment of root that contains components of both CNS and PNS tissue is called the CNS–PNS transition zone (TZ). All axons in the PNS, other than postganglionic autonomic neurones, cross such a TZ. Macroscopically, as a nerve root is traced towards the spinal cord or the brain, it splits into several thinner rootlets that may, in turn, subdivide into minirootlets. The TZ is located within either rootlet or minirootlet (Fig. 3.35). The arrangement of roots and rootlets varies according to whether the root trunk is ventral, dorsal or cranial. Thus, in dorsal roots, the main root trunk separates into a fan of rootlets and minirootlets that enter the spinal cord in sequence along the dorsolateral sulcus. In certain cranial nerves, the minirootlets come together central to the TZ 64 and enter the brain as a stump of white matter. 3 Conduction of the nervous impulse

R R Nerve root R CNS tissue

Glial fringe CHAPTER

r r r Mantle zone TZ 1 2 3 R

RRR

SC BS A BCD E F G Fig. 3.35 The nerve root–spinal cord junction. A–G, Different CNS–PNS borderline arrangements. A, A pointed borderline. The extent of the transitional zone (TZ) is indicated. B–G, Glial fringe omitted. B, A concave borderline (white line) and inverted TZ. C, A flat borderline situated at the level of the root (R)–spinal cord junction. D and E, A convex, dome-shaped borderline; the CNS expansion into the rootlet is moderate in D and extensive in E. F, The root (R) splits into rootlets (r), each with its own TZ and attaching separately to the spinal cord (SC). G, The arrangement found in several cranial nerve roots (e.g. vestibulocochlear nerve). The PNS component of the root separates into a bundle of closely packed minirootlets, each equipped with a TZ. The minirootlets reunite centrally. BS, brainstem. (Adapted with permission from Dyck PJ, Thomas PK, Griffin JW, et al (eds) Peripheral Neuropathy, 3rd ed. Philadelphia: Saunders, 1993.)

All-or-none action potentials Graded potentials generated at nodes along axon

Membrane potential

Excitation Inhibition Net effect at axon (depolarization) (hyperpolarization) hillock is excitation +40 mV Time 0

-80 mV

Inhibitory axon

Excitatory axon Conduction

Fig. 3.36 The types of change in electrical potential that can be recorded across the cell membrane of a motor neurone at the points indicated. Excitatory and inhibitory synapses on the surfaces of the dendrites and soma cause local graded changes of potential that summate at the axon hillock and may initiate a series of all-or-none action potentials, which in turn are conducted along the axon to the effector terminals.

Microscopically, the TZ is characterized by an axial CNS compart- thought to prevent cell mixing at these interfaces not only by helping ment surrounded by a PNS compartment. The zone lies more peripher- dorsal root ganglion afferents navigate their path to targets in the spinal ally in sensory nerves than in motor nerves, but in both, the apex of cord but also by inhibiting motoneurone cell bodies exiting to the the TZ is described as a glial dome, whose convex surface is usually periphery. For further reading, see Zujovic et al (2011). directed distally. The centre of the dome consists of fibres with a typical CNS organization, surrounded by an outer mantle of astrocytes (cor- responding to the glia limitans). From this mantle, numerous glial CONDUCTION OF THE NERVOUS IMPULSE processes project into the endoneurial compartment of the peripheral nerve, where they interdigitate with its Schwann cells. The astrocytes All cells generate a steady electrical potential across their plasma mem- form a loose reticulum through which axons pass. Peripheral myeli- brane (the membrane potential). This potential is generated by an nated axons usually cross the zone at a node of Ranvier, which is here uneven distribution of potassium ions across the membrane (higher in termed a PNS–CNS compound node. the intracellular compartment than in the extracellular compartment), Boundary cap (BC) cells are neural crest derivatives that form tran- and by a selective permeability of the membrane for potassium (Fig. sient, discrete clusters localized at the presumptive dorsal root entry 3.36). The distribution of sodium ions is opposite to that for potassium zones and motor exit points of the embryonic spinal cord. They are ions, but at rest the sodium conductance of the membrane is low. In 65 1 NERVOUS SYSTEM

neurones this membrane potential is known as the resting potential, tion of neighbouring membrane. Sodium channels within the newly and amounts to approximately −60 mV (potential inside the cell meas- depolarized segment open and positively charged sodium ions enter,

SECTION ured relative to the outside of the cell). Non-excitable cells have an even driving the local potential inside the axon towards positive values. This higher membrane potential. Neurones receive, conduct and transmit inward current in turn depolarizes the neighbouring, downstream, non- information by changes in membrane conductance for sodium, potas- depolarized membrane, and the cyclic propagation of the action poten- sium, calcium or chloride ions. Increase in the sodium or calcium tial is completed. Several milliseconds after the action potential, the conductance causes an influx of these ions and results in a depolariza- sodium channels are inactivated, a period known as the refractory tion of the cell, while chloride influx or potassium efflux results in period. The length of the refractory period determines the maximum hyperpolarization. Plasma membrane permeability to these ions is frequency at which action potentials can be conducted along a nerve altered by the opening or closing of ion-specific transmembrane chan- fibre; it varies in different neurones and affects the amount of informa- nels, triggered by voltage changes or chemical signals such as transmit- tion that can be carried by an individual fibre. ters (Catterall 2010). Myelinated fibres are electrically insulated by their myelin sheaths Chemically triggered ionic fluxes may be either direct, where the along most of their lengths, except at nodes of Ranvier. The distance chemical agent (neurotransmitter) binds to the channel itself to cause between nodes, referred to as the internodal distance, is directly related it to open, or indirect, where the neurotransmitter is bound by a trans- to axon diameter and varies between 0.2 and 2.0 mm. Voltage-gated membrane receptor molecule that activates a complex second messen- sodium channels are clustered at nodes, and the nodal membrane is ger system within the cell to open separate transmembrane channels. the only place where high densities of inward sodium current can be Electrically induced changes in membrane potential depend on the generated across the axon membrane. Conduction in myelinated axons presence of voltage-sensitive ion channels, which, when the transmem- is self-propagating, but instead of physically adjacent regions of mem- brane potential reaches a critical level, open to allow the influx or efflux brane acting to excite one another (as occurs along unmyelinated of specific ions. In all cases, the channels remain open only transiently, axons), it is the depolarization occurring in the neighbouring upstream and the numbers that open and close determine the total flux of ions node that excites a node to threshold. Reaching threshold causes the across the membrane (Bezanilla 2008). sodium channels at the node to open and generate inward sodium The types and concentrations of transmembrane channels and current, but instead of this acting on the adjacent membrane, the high related proteins, and therefore the electrical activity of the membranes, resistance and low capacitance of the myelin sheath directs the current vary in different parts of the cell. Dendrites and neuronal somata depend towards the next downstream node, exciting it to threshold and com- mainly on neurotransmitter action and show graded potentials, whereas pleting the cycle. The action potential thus jumps from node to node, axons have voltage-gated channels that give rise to action potentials. a process known as saltatory conduction, which greatly increases the In graded potentials, a flow of current occurs when a synapse is conduction velocity. activated; the influence of an individual synapse on the membrane A number of disorders of the CNS and PNS include demyelination potential of neighbouring regions decreases with distance. Thus syn- as a characteristic feature. Perhaps most common amongst these is apses on the distal tips of dendrites may, on their own, have relatively multiple sclerosis, which is characterized by primary demyelination at little effect on the membrane potential of the cell body. The electrical scattered sites within the CNS (it is now recognized that axonal loss state of a neurone therefore depends on many factors, including the also contributes to the progression of multiple sclerosis). Primary numbers and positions of thousands of excitatory and inhibitory syn- demyelination is the loss of the myelin sheath with axonal preservation, apses, their degree of activation, and the branching pattern of the den- and is usually segmental, i.e. it rarely extends along the entire length of dritic tree and geometry of the cell body. The integrated activity directed an affected axon. The phenomenon is associated with conduction block towards the neuronal cell body is converted to an output directed away because the newly exposed, previously internodal, axolemma contains from the soma at the site where the axon leaves the cell body, at its relatively few voltage-sensitive Na+ channels. There is experimental evi- junction with the axon hillock. Voltage-sensitive channels are concen- dence that conduction can be restored in some demyelinated axons, trated at this trigger zone, the axon initial segment, and when this and experimental and clinical evidence that remyelinated axons can region is sufficiently depolarized, an action potential is generated and conduct at near-normal speeds, because even though their sheaths are is subsequently conducted along the axon. thinner than the original myelin sheaths, the safety factor (i.e. the factor by which the outward current at a quiescent node next to an excited node exceeds the minimum current required to evoke a response) is ACTION POTENTIAL greater than 1. The myelin loss that occurs in the early stages of Walle- rian degeneration in both CNS and PNS, usually distal to a site of The action potential is a brief, self-propagating reversal of membrane trauma but also in response to a prolonged period of ischaemia or polarity. It depends on an initial influx of sodium ions, which causes a exposure to a neuronotoxic substance, is accompanied by axonal degen- reversal of polarity to about +20 mV, followed by a rapid return towards eration (the term secondary demyelination is sometimes used to the resting potential as potassium ions flow out. The rapid reversal describe this form of myelin loss). process is completed in approximately 0.5 msec, followed by a slower Axonal conduction is naturally unidirectional, from dendrites and recovery phase of up to 5 msec, when the resting potential is even soma to axon terminals. When an action potential reaches the axonal hyperpolarized. Once the axon hillock reaches threshold, propagation terminals, it causes depolarization of the presynaptic membrane, and of the action potential is independent of the initiating stimulus; thus as a result, quanta of neurotransmitter (which correspond to the content the size and duration of action potentials are always the same (described of individual vesicles) are released to change the degree of excitation of as all-or-none) for a particular neurone, no matter how much a stimulus the next neurone, muscle fibre or glandular cell. may exceed the threshold value. Once initiated, an action potential spreads spontaneously and at a relatively constant velocity, within the range of 4–120 m/s. Conduction velocity depends on a number of factors related to the way in which Bonus e-book image the current spreads, e.g. axonal cross-sectional area, the numbers and positioning of ion channels, and membrane capacitance (influenced particularly by the presence of myelin). In axons lacking myelin, action Fig. 3.1 A section through the human cerebellum stained to show potential conduction is analogous to a flame moving along a fuse. Just the arrangement in the brain of the central white matter and the as each segment of fuse is ignited by its upstream neighbour, each highly folded outer grey matter. segment of axon membrane is driven to threshold by the depolariza-

66 3 Key references

KEY REFERENCES

Finger S 2001 Origins of Neuroscience: A History of Explorations into Brain Sakmann B, Neher E 2009 Single-channel Recording. New York: Springer. CHAPTER Function. New York: Oxford University Press. An introduction to the patch-clamp technique and electrophysiology. A historic introduction to the neuroscience field. Sanes DH, Reh TA, Harris WA 2011 Development of the Nervous System, Kandel ER, Schwartz JH, Jessell TM et al 2012 Principles of Neural Science, 3rd ed. Oxford: Elsevier, Academic Press. 5th ed. New York: McGraw–Hill. A textbook on developmental neurobiology. A basic but comprehensive neuroscience textbook Shepherd GM 2003 The Synaptic Organization of the Brain. New York: Kempermann G 2011 Adult Neurogenesis 2. New York: Oxford University Oxford University Press. Press. A description of the circuitry of the brain. A summary of the knowledge on adult neurogenesis. Squire L, Kandel E 2008 Memory: From Mind to Molecules. Greenwood Kettenmann H, Ransom BR 2012 Neuroglia. New York: Oxford University Village, CO: Roberts. Press. A description of the mechanism of memory formation. A comprehensive textbook on glial cells. Squire L, Berg D, Bloom FE et al 2012 Fundamental Neuroscience, 4th ed. Levitan IB, Kaczmarek LK 2001 The Neuron: Cell and Molecular Biology. Oxford: Elsevier, Academic Press. New York: Oxford University Press. A basic but comprehensive neuroscience textbook. A basic neuroscience textbook with the focus on neurones. Nicholls JG, Martin AR, Fuchs PA et al 2011 From Neuron to Brain, 5th ed. Sunderland, MA: Sinauer Associates. A basic and comprehensive neuroscience textbook.

67 CHAPTER

Cerebellum 22

The cerebellum occupies the posterior cranial fossa, separated from the occipital lobes of the cerebral hemispheres by the tentorium cerebelli. 1 10 It is the largest part of the hindbrain; in adults, the weight ratio of 11 cerebellum to cerebrum is approximately 1 : 10, and in infants 1 : 20. The cerebellum lies dorsal to the pons and medulla, from which it is separated by the fourth ventricle. It is joined to the brainstem by three bilaterally paired cerebellar peduncles. 2 The basic internal organization of the cerebellum is of a superficial 12 cortex overlying a core of white matter. The cortex is highly convoluted, forming lobes and lobules that are further subdivided into folia (leaf- lets), separated by intervening transverse fissures. Aggregations of neu- 13 ronal cell bodies embedded within the white matter form the fastigial 3 (medial), globose (posterior interposed), emboliform (anterior inter- posed) and dentate (lateral) nuclei, which are collectively known as the (deep) cerebellar nuclei. 14 The cerebellum may be subdivided into a number of modules, each 4 consisting of a longitudinal cortical zone, a cerebellar or vestibular target nucleus, and a supporting olivocerebellar climbing fibre system. 15 Apart from their connections, the longitudinal cortical zones are char- 5 acterized by their immunohistochemical properties. The cerebellum receives input from peripheral receptors and from motor centres in the 6 spinal cord and brainstem and from large parts of the cerebral cortex through two different afferent systems: mossy and climbing fibres. It is located as a side path to the main ascending sensory and descending 7 motor systems, and it functions to coordinate movement. During move- 8 16 ment, the cerebellum provides corrections that are the basis for preci- sion and accuracy, and it is critically involved in motor learning and 17 9 reflex modification. Cerebellar output is directed to the thalamus and from there to the cerebral cortex, and also to brainstem centres such as the red nucleus, vestibular nuclei and reticular nuclei, which themselves give rise to descending spinal pathways. Fig. 22.1 A horizontal section through the cerebellum and brainstem. Ideas on the involvement of the cerebellum in motor functions were 1. Ethmoidal air cells. 2. Temporal lobe of brain. 3. Hypophysis. 4. Pons. derived mainly from movement disorders seen in experimental studies, 5. Cochlea. 6. Sigmoid sinus. 7. Fourth ventricle. 8. Vermis. 9. Diploë of summarized by Luciani (1891) in his triad of atonia, astasia and asthe- occipital bone. 10. Eyeball. 11. Optic nerve. 12. Internal carotid artery. nia, and in human patients with cerebellar lesions who displayed the 13. Middle cerebellar peduncle. 14. Petrous temporal bone. 15. Superior well-known symptoms of gait disturbances, limb ataxia, dysmetria, cerebellar peduncle. 16. Dentate nucleus. 17. Folia of cerebellar cortex. atonia and eye movement disorders (Glickstein et al 2009). Latterly, the (Courtesy of Dr GJA Maart.) observation that lesions of the cerebellar hemisphere not only resulted in minor and transient motor symptoms but also induced a cerebellar cognitive/affective syndrome (Schmahmann 2004) prompted the sug- nineteenth centuries from their shape, their position or their likeness gestion that the human cerebellum is also concerned with non-motor to anatomical structures in other body parts (Glickstein et al 2009). functions. These conceptual developments went hand in hand with the This classical nomenclature (see Fig. 22.4, right panel) was largely use of more sensitive experimental methods to trace cerebellar con- replaced in the early twentieth century by a nomenclature based on nectivity, mainly in subhuman primates, and the application of modern Bolk’s (1906) comparative anatomical investigations (see Fig. 22.4, imaging techniques to the human brain. Although it is now recognized left panel). that the cortex is more heterogeneous than previously supposed, and Bolk distinguished the relatively independent ‘folial chains’ of the despite our extensive knowledge of the sphere of influence of the cere- vermis and the hemispheres. In later studies, this relative independence bellum and the microcircuitry of its cortex and nuclei, we still do not was found to reflect the continuity or discontinuity of the cortex fully understand how it contributes to motor and non-motor systems. between the lobules within a folial chain, or between the folial chains The observation by Thomas Willis (1681) that ‘the Spirits inhabiting of the vermis and the hemispheres. Bolk used the cerebellum of a small the Cerebel perform unperceivedly and silently their Work of Nature lemur for his initial description (see Fig. 22.5F–G) and summarized the without our Knowledge or Care’ remains true today. configuration of the folial chains in a stick diagram (see Fig. 22.5H). His description proved to be applicable to the cerebella of all the mammals he studied, including the human cerebellum. EXTERNAL FEATURES AND RELATIONS Larsell (Larsell and Jansen 1972) based his subdivision of the cere- bellum on embryological studies of the emergence of the transverse The cerebellum consists of two large, laterally located hemispheres fissures with time. Contrary to Bolk, who emphasized the continuity that are united by a midline vermis (Figs 22.1–22.3). Numerous sulci within the folial chains, Larsell attached great importance to the medio- and fissures of varying depth subdivide it into lobes, lobules and folia lateral continuity of the lobules of the vermis and the hemispheres, and (leaflets) (Figs 22.4–22.5). The primary fissure, the deepest fissure on distinguished 10 lobules in the cerebellum, indicated using Roman a sagittal section, divides it into anterior and posterior lobes. Paramed- numerals I–X for the vermis and the prefix H for the hemisphere. The ian fissures, shallow in the anterior cerebellum but prominent more correspondence of Larsell’s lobules with the classical nomenclature is posteriorly, separate the vermis from the cerebellar hemispheres. shown in Figure 22.4. Both the anterior and posterior vermis and hemisphere are subdi- Lobules (H)I–V constitute the anterior lobe. Lobule I, the lingula, is vided into lobules that received their names in the eighteenth and conjoined with the superior medullary velum. Lobules VI (declive) and 331 CEREBELLUM

A Pons Corpus callosum Thalamus Occipital lobe Superior cerebellar peduncle Arbor vitae 3

CTION E S

Medulla oblongata Foramen magnum Tonsil of cerebellum

Fourth Internal jugular vein Medulla Tonsil Sigmoid sinus oblongata ventricle B

Falx cerebelli Confluence of sinuses Vermis

C

Transverse sinus

Vermis

Tonsil

Vallecula

Fig. 22.2 Magnetic resonance images of the cerebellum of a 16-year-old female. A, Sagittal view. B, Axial view. C, Coronal view. (Courtesy of Drs JP Finn and T Parrish, Northwestern University School of Medicine, Chicago.)

332 External features and relations

A B pcen 2a precul

1 prim 2 3a 2 psup

3 3 4a 4

5 hzl 6 4 5a 7 5 8 apm 22

9 ER preb

plat intb D CHAPT C sec 2 1a

1 2a 2 3

9 10a 1 4

8 9 5 9a

8 6 7 8a 7 6 7a

6a

Fig. 22.3 The terminology of the cerebellar lobes and fissures, using a schematic ‘unrolled’ diagram as a frame of reference. A, Unrolled cerebellar cortex. The lobules are labelled by numbers and the fissures between the lobules are listed. B, The cerebellum viewed from above. C, A median sagittal section of cerebellum. The lobules are numbered and listed. D, The cerebellum viewed from below. Key and abbreviations: Anterior lobe: 1, lingula; 2, central; 3, culmen, vermis. Posterior lobe: 4, declive; 5, folium; 6, tuber; 7, pyramis; 8, uvula; 9, nodule. Fissures: apm, ansoparamedian; hzl, horizontal; intb, intrabiventral; pcen, precentral; plat, posterolateral; preb, prebiventral; precul, preculminate; prim, primary; psup, posterior superior; sec, secondary. Hemisphere: 1a, wing of lingula; 2a, wing of central lobule; 3a, anterior quadrangular lobule; 4a, posterior quadrangular lobule; 5a, superior semilunar lobule; 6a, inferior semilunar lobule; 7a, gracile lobule; 8a, biventral lobule; 9a, tonsil of cerebellum, 10a, flocculus.

Hemisphere Vermis Hemisphere

Vincingulum lingulae Ala lobuli centralis

I Lingula Anterior quadrangular lobule Anterior lobe II Central Posterior quadrangular lobule III lobe Simplex lobule IV 1 Culmen V Superior semilunar lobule 1. Primary fissure 2. Posterior superior fissure Crus I 2 VI Declive 3. Intercrural fissure Horizontal fissure Ansiform lobule 3 Folium 4. Ansoparamedian fissure VIIA 5. Prepyramidal fissure Crus II 4 VIIB Tuber 6. Secondary fissure 5 Inferior semilunar lobule Paramedian lobe 7. Posterolateral fissure VIII Pyramis 6 Dorsal paraflocculus Gracile lobule IX Uvula

Nodulus Ventral paraflocculus X Biventral lobule

7 Accessory paraflocculus Flocculus Posterior medullary velum

Tonsil Flocculus Fig. 22.4 Cerebellar nomenclature. The left-hand panel illustrates the comparative anatomical nomenclature for the hemisphere and Larsell’s numbering system for the lobules of the vermis (Larsell and Jansen 1972). The right-hand panel shows the classical nomenclature. The homology of these lobules is indicated using the same colour. Asterisks denote areas devoid of cortex in the centre of the folial rosettes of the ansiform lobule and the paraflocculus. 333 CEREBELLUM

Horizontal fissure F/T Superior semilunar lobule Posterior superior fissure Semilunar lobules Posterior quadrangular lobule SI De Primary fissure Ansoparamedian Posterior fissure superior fissure Gracile lobule Cu Biventral lobule

Ce Prepyramidal fissure

Anterior lobe Vma Li FA Nod Vmp Posterolateral Uv Py Tonsil

3 A E fissure Primary fissure Anterior lobe Anterior lobe Primary fissure Simplex lobule CTION Posterior quadrangular lobule E Ansiform lobule S Posterior superior fissure

1 PFLD 1 Superior semilunar lobule

Horizontal fissure Caudal vermis B F Dorsal Paramedian lobe

Py F/T Inferior semilunar lobule Anterior lobe Superior semilunar lobule

1 Horizontal fissure Inferior semilunar lobules

Gracile lobule 2 2 Ansoparamedian fissure PFLV Caudal vermis PFLD

Prepyramidal fissure Paramedian lobe C Biventral lobule G Ventral Tonsil Anterior lobe Anterior lobe Scp Icp Mcp Simplex lobule

PFLV 2 1 Flocculus Ansiform lobule 2

PFLD Flocculus Nod Tonsil Paramedian lobule D Uv H Caudal vermis Fig. 22.5 A–D, Anterior, dorsal, posterior and ventral views of the human cerebellum. E, A sagittal section of the human cerebellum. F–G, Dorsal and ventral views of the cerebellum of Lemur albifrons, Bolk’s (1906) prototype for his ground plan of the mammalian cerebellum. Two loops are present in the folial chain of the hemisphere: (1) as the ansiform lobule, (2) as the paraflocculus. The course of the folial chains of the vermis and hemisphere in A–D and F–G is indicated with red lines. H, Bolk’s stick diagram of the folial chains of the vermis and hemisphere. Key and abbreviations: 1, 2, Ansiform and parafloccular loops of the folial chain of the hemisphere; Ce, central lobule; Cu, culmen; De, declive; FA, fastigium; F/T, folium and tuber; Icp, inferior cerebellar peduncle; Li, lingula; Mcp, middle cerebellar peduncle; Nod, nodulus; PFLD, dorsal paraflocculus; PFLV, ventral paraflocculus; Py, pyramis; Scp, superior cerebellar peduncle; SI, simplex (posterior quadrangular) lobule; Uv, uvula; Vma, anterior (superior) medullary velum; Vmp, posterior medullary velum.

HVI (posterior quadrangular lobule) are also known as Bolk’s simplex formed by the biventral lobule (HVIII), the hemisphere from the lobule. Behind the primary fissure, the folium (lobule VIIA) and tuber pyramis (VIII). Lobule VIII (the pyramis) is continuous with the biven- vermis (VIIB) are separated by the deep paramedian fissure from the tral lobule (HVIII) laterally. The gracile lobule corresponds to the rostral superior semilunar lobule (HVIIA), the inferior semilunar lobule and part of Bolk’s paramedian lobule; the biventral lobule corresponds to the gracile lobule (together corresponding to HVIIA). Superior and its caudal portion. The tonsil (HIX) corresponds to the dorsal parafloc- inferior semilunar lobules correspond to the crus I and II of Bolk’s culus in the monkey. In the human, the folial loop of the tonsil is ansiform lobule. Their folia fan out from the deep horizontal fissure directed medially, contrary to the situation in most mammals, where that represents the intercrural fissure. The gracile lobule corresponds to the paraflocculus arches laterally. The flocculus appears as a double 334 the rostral part of Bolk’s paramedian lobule. Its caudal portion is folial rosette; its dorsal leaf is known as the accessory paraflocculus of Internal organization

Cerebral peduncle Superior cerebellar peduncle characteristic branching pattern of nerve fibres (arbor vitae) extends towards the cortical surface (see Fig. 22.2). The white matter consists Trigeminal Inferior cerebellar of afferent and efferent fibres travelling to and from the cerebellar nerve peduncle cortex. Fibres cross the midline in the white core of the cerebellum Middle cerebellar and the superior medullary velum, effectively constituting a cerebellar peduncle ‘commissure’.

CEREBELLAR CORTEX

Although the human cerebellum makes up approximately one-tenth of Pyramid the entire brain by weight, the surface area of the cerebellar cortex, if unfolded, would be about half that of the cerebral cortex. The great Olive majority of cerebellar neurones are small granule cells, so densely packed that the cerebellar cortex contains many more neurones than the cerebral cortex. Unlike the cerebral cortex, where a large number of 22 diverse cell types are arranged differently in different regions, the cere-

bellar cortex contains a relatively small number of different cell types, ER which are interconnected in a highly stereotyped way. The elements of the cerebellar cortex possess a precise geometric Vestibulocochlear nerve Inferior cerebellar peduncle order, arrayed relative to the tangential, longitudinal and transverse planes in individual folia. Three layers are distinguished in the cerebel- CHAPT Fig. 22.6 Dissection of the left cerebellar hemisphere and its peduncles. lar cortex (Figs 22.7–22.8). A monolayer of large neurones with apical dendrites, first identified by Purkinje (Glickstein et al 2009), separates Henle, while its ventral leaf represents the true flocculus. The accessory a layer of small granule cells from the superficial, cell-poor molecular paraflocculus corresponds to the ventral paraflocculus in the monkey. layer. The Purkinje cell layer contains the large, pear-shaped somata of Both these lobules belong to the vestibulocerebellum. The cortex the Purkinje cells and the smaller somata of Bergmann glia. Clumps of between the tonsil and the accessory paraflocculus is interrupted. granule cells and occasional Golgi cells penetrate between the Purkinje Between lobule X (the nodulus) and the flocculus (HX) with the acces- cell somata. The granular layer consists of the somata of granule cells sory paraflocculus, the cortex is absent and the tissue is stretched out and the initial segments of their axons; dendrites of granule cells; as the inferior medullary velum. branching terminal axons of afferent mossy fibres; climbing fibres Two magnetic resonance imaging (MRI) atlases of the cerebellum passing through the granular layer en route to the molecular layer; and have been published to aid localization in functional MRI (fMRI) the somata, basal dendrites and complex axonal ramifications of Golgi (Schmahmann et al 1999, Diedrichsen 2006). The authors use Larsell’s neurones. numerals and retain Bolk’s terms crus I and II, but discard Larsell’s use The molecular layer contains a sparse population of neurones, den- of the prefix H for the lobules of the hemisphere. As a consequence, it dritic arborizations, unmyelinated axons and radial fibres of neuroglial is difficult to determine whether descriptions of lobules using these cells. criteria refer to the vermis or to the hemisphere. Purkinje cells

CEREBELLAR PEDUNCLES Purkinje cells are the only output neurones of the cortex. They are arranged in a single layer between the molecular and granular layers, Three pairs of peduncles connect the cerebellum with the brainstem and have a specific geometry that is conserved in all vertebrate classes (Fig. 22.6; see also Fig. 21.19). (Fig. 22.9). The middle cerebellar peduncle is the most lateral and by far the Their dendritic trees are flattened and orientated perpendicular to largest of the three. It passes obliquely from the basal pons to the cere- the parallel fibres in a plane transverse to the long axes of the folia (see bellum and contains the massive pontocerebellar mossy fibre pathway, Figs 22.7–22.8; see also Figs 3.3, 3.6). Large primary dendrites arise which is composed almost entirely of fibres that arise from the contra- from the outer pole of a Purkinje cell. The proximal dendritic branches lateral basal pontine nuclei, with a small addition from nuclei in the are smooth and are contacted by climbing fibres. The distal branches pontine tegmentum. carry a dense array of dendritic spines (spiny branchlets) that receive The inferior cerebellar peduncle is located medial to the middle synapses from the terminals of parallel fibres. Inhibitory synapses are peduncle. It consists of an outer, compact fibre tract – the restiform also received from basket and stellate cells, and from the recurrent col- body – and a medial, juxtarestiform body. The restiform body is laterals of Purkinje cell axons that contact the shafts of the proximal a purely afferent system; it receives spinocerebellar fibres and the dendrites. The total number of dendritic spines per Purkinje neurone trigeminocerebellar, cuneocerebellar, reticulocerebellar and olivocere- is in the order of 180,000. The axon of a Purkinje cell leaves the inner bellar tracts from the medulla oblongata (see Fig. 21.19). The juxtares- pole of the soma and crosses the granular layer to enter the subjacent tiform body is mainly an efferent system, made up almost entirely of white matter. The initial axon segment receives axo-axonal synaptic efferent Purkinje cell axons destined for the vestibular nuclei and contacts from distal branches of basket cell axons. Beyond the initial uncrossed fibres from the fastigial nucleus. It also contains primary segment, the axon enlarges, becomes myelinated and gives off collateral afferent mossy fibres from the vestibular nerve and secondary afferent branches. The main axon ultimately terminates in one of the cerebellar fibres from the vestibular nuclei. The crossed fibres from the fastigial or vestibular nuclei; recurrent axonal collaterals form a sagittally orien- nucleus pass dorsal to the superior cerebellar peduncle as the uncinate tated plexus with terminations on neighbouring Purkinje cells and tract, and enter the brainstem at the border of the juxtarestiform and Golgi cells. Purkinje cells are inhibitory and use -aminobutyric acid restiform bodies. γ (GABA) as their neurotransmitter. The superior cerebellar peduncle contains all of the efferent fibres from the dentate, emboliform and globose nuclei, and a small fascicle from the fastigial nucleus. Its fibres decussate in the caudal mesen- Cortical interneurones cephalon, and are destined to synapse in the contralateral red nucleus and thalamus. The ventral spinocerebellar tract reaches the upper part The cerebellar cortical interneurones were described by Ramón y Cajal of the pontine tegmentum, looping around the entrance of the trigemi- (1906) (see Fig. 22.8). They can be divided into the interneurones of nal nerve to join this peduncle and unite with the spinocerebellar fibres the molecular layer, the stellate and basket cells, and the Golgi cells of entering through the restiform body. the granular layer. All interneurones are inhibitory. Those of the mole- cular layer use GABA as their neurotransmitter. Most Golgi cells are glycinergic. Stellate cells are located in the upper molecular layer, their INTERNAL ORGANIZATION axons terminating on Purkinje cell dendrites. Basket cells occupy the deep molecular layer, their axons terminating on a series of Purkinje The vast majority of cerebellar neuronal cell bodies are located within cells with baskets surrounding their somata, ending in a plume around the outer, highly convoluted cortical layer. Beneath the cortex, the cere- their initial axon. The dendrites of these interneurones and their axons bellar white matter forms an extensive central core, from which a are oriented in the sagittal plane. Golgi cell dendrites are located in the 335 CEREBELLUM

Climbing fibre Purkinje cell with collateral plexus Fig. 22.7 The circuitry of the cerebellar cortex. Mossy fibre Glutamatergic neurones are shown in dark grey, GABAergic neurones in red and glycinergic elements in blue. A, A transparent dorsal view of the cortex showing the orientation of its major elements. Dendrites of Purkinje, stellate and basket Parallel fibre cells, the collateral plexus of Purkinje cells, the cell Granule cell P bodies of the Lugaro cells, and the climbing fibres Meningeal surface P and the axonal plexus of the Golgi cells are molecular layer orientated in the sagittal plane. B, A transverse P P P section. Parallel fibres and the axons of the Lugaro Golgi cell with cells are the only elements with a transverse P P axonal plexus orientation.

Sagittal plane A 3

Lugaro cell Basket cell Stellate cell CTION

E Parallel fibre S Molecular layer Lugaro cell

Golgi cell Purkinje cell layer Unipolar brush cell

Granular layer

B Climbing fibre Purkinje cell collateral Mossy fibre

Cerebellar nuclear Granule cell neurones

GABAergic nucleo-olivary pathway

Inferior olive neurone

Cerebellar output system

granular and molecular layers. Their axonal plexus occupies the granu- lar layer, where it terminates on the granule cell dendrites, and also has its greatest dimension in the sagittal plane (see Fig. 22.7). Golgi cells are innervated by collaterals of mossy fibres and Purkinje cell axons. The dendrites of interneurones in the molecular layer are contacted by e the parallel fibres. Synaptic contacts between climbing fibres and the dendrites or cell bodies of cerebellar interneurones in the molecular or a granular layers have not been observed. However, interneurones of the b n o d molecular layer can be activated by ‘spillover’ of glutamate from the climbing fibres (Galliano et al 2013, Szapiro and Barbour 2007). Golgi n b cells, therefore, provide feed-back inhibition to the granule cells. g Interneurones of the molecular layer provide feed-forward inhibition to the Purkinje cells. Stellate cells (Mann-Metzer and Yarom 2000) and f Golgi cells (Dugué et al 2009) are electrotonically coupled. The extent of this coupling is not known; it may be restricted to the sagittal com- partments that are one of the main features of the connectivity of the h cerebellum, discussed below. Two other types of interneurone exist. Lugaro cells are cigar-shaped neurones located at the level of the Purkinje cells (Lainé and Axelrad 1996) (see Fig. 22.7). These glycinergic neurones innervate the stellate p and basket cells, and provide a long, transversely orientated axon that h terminates on Purkinje cells. They receive a strong input from an extra- cerebellar serotoninergic system. Monopolar brush cells are excitatory j j m CB A neurones, mainly found in vestibular-dominated regions of the cerebel- Fig. 22.8 A sagittal section through a cerebellar folium showing the lum (Mugnaini et al 1997), where they are considered to be a ‘booster’ different cell types of the cerebellar cortex. Abbreviations: A, molecular system for vestibular mossy fibre input. Mossy fibres terminate with layer; a, Purkinje cell; B, granular layer; b, basket cells; C, white matter; extremely large synapses on the base (the ‘brush’) of these cells. Their d, baskets of basket cell; e, stellate cell; f, Golgi cell; g, granule cell with axons terminate as mossy fibres on the granule cells. ascending axon; h, mossy fibre; m, astrocyte; n, climbing fibre; o, Purkinje cell axon with collaterals; j, p, Bergmann glial cells. (Redrawn from Cajal SR y. 1906 Histologie du système nerveux de l’homme et des vertebrés. CEREBELLAR NUCLEI Maloine, Paris.) The four cerebellar nuclei were first described by Stilling (1864) as comprising (from medial to lateral) the fastigial nucleus, the emboli- 336 form and globose nuclei, and the dentate nucleus (Fig. 22.10). The Cerebellar circuitry

Dendritic tree of one Golgi cell Fig. 22.9 The general organization of the cerebellar cortex. A single folium has been Granule cells sectioned vertically, both in its longitudinal axis Parallel fibres (right side of diagram) and transversely.

Molecular layer

Basket cell

Granule cell Synaptic glomerulus Ramification of Dendritic Golgi cell axon

tree of one in granular layer 22

Purkinje cell Climbing fibres

* ER Axon of Mossy fibres basket cell * CHAPT Climbing fibre Axons of Purkinje cells * Recurrent collateral branches of Purkinje cell axons

Emboliform nucleus Fig. 22.10 The human cerebellar nuclei. A–E, Transverse sections through the cerebellar Dentate nucleus microgyric Fastigial nucleus nuclei, A being the most rostral level. The dentate nucleus can be subdivided into dorsomedial Globose nucleus microgyric and ventrocaudal macrogyric parts. F, A Weigert-stained section through the dentate A nucleus, showing its subdivision into micro- and macrogyric parts. (A–E Redrawn from Larsell O, Jansen J 1972 The comparative anatomy and histology of the cerebellum. III. The human cerebellum, cerebellar connections, and cerebellar cortex. Minneapolis, University of Minnesota D Press. F, Reproduced with permission from B Winkler C 1926 De bouw van het zenuwstelsel. Dentate nucleus Haarlem, de erven Bohn.) Dentate nucleus microgyric macrogyric

Globose nucleus

Emboliform nucleus F C E Dentate nucleus macrogyric emboliform and globose nuclei are also known as the anterior and 2011); as far as we know, all cell types receive an inhibitory input from posterior interposed nuclei. The nuclei form two interconnected groups: Purkinje cells and an excitatory input from mossy and climbing fibre a rostrolateral group consisting of the emboliform and dentate nuclei, collaterals. and a caudomedial group including the fastigial and globose nuclei. A collection of small, cholinergic neurones extends from the flocc- ulus to the nodulus in the roof of the fourth ventricle, invading the CEREBELLAR CIRCUITRY spaces between the nuclei. These cells are known as the basal interstitial nucleus (Langer 1985); their connections are not known. The main circuitry of the cerebellum was described by Ramón y Cajal The dentate nucleus is located most laterally and is by far the largest in the late nineteenth century and published in his Histologie du système of the group. It has the shape of a crumpled purse; the main efferent nerveux (1906). It involves two extracerebellar afferent systems (climb- pathway of the cerebellum, the brachium conjunctivum, emerges from ing fibres and mossy fibres), intrinsic cortical neurones, including its hilus. The convolutions of the dentate nucleus are narrow rostro- Purkinje, granule, stellate and basket cells, and neurones in the cerebel- medially and much wider ventrocaudally. Interestingly, these micro- lar nuclei (see Figs 22.7–22.9). The widely diverging mossy fibre-parallel and macrogyral characteristics of the human dentate were observed by fibre system terminates on Purkinje cells (the only output neurones of Vicq-d’Azir, who coined its name in the eighteenth century (Glickstein the cortex projecting to the cerebellar and vestibular nuclei); a climbing et al 2009). Recently, rostromedial motor and ventrocaudal non-motor fibre terminates on the proximal, smooth Purkinje cell dendrites; Golgi divisions have been distinguished in the human dentate nucleus using cells provide backward inhibition to granule cells; and stellate and fMRI (Küper et al 2012). Their significance and the possible correspond- basket cells provide forward inhibition to Purkinje cells. ence with the anatomical subdivisions of the dentate are considered Climbing fibres and most mossy fibres are excitatory and use gluta- below. mate as their neurotransmitter. All climbing fibres take their origin from The cerebellar nuclei contain cells of all sizes. Glutamatergic relay the contralateral inferior olivary nucleus in the medulla oblongata. In neurones provide the main output of the nuclei. Small GABAergic neu- the cerebellum, they split into several branches, each branch providing rones innervate the contralateral inferior olive. Both GABAergic and a climbing fibre to a single Purkinje cell. The branches of a single glycinergic interneurones have been identified (Uusisaari and Knöpfel olivocerebellar fibre innervate one or more sagittally orientated strips 337 CEREBELLUM

Molecular layer Fig. 22.11 The orientation and branching pattern Simplex lobule of mossy and climbing fibres. Left-hand panels: Simplex lobule mossy fibres are orientated transversely. They Granular layer distribute bilaterally and emit collaterals at Ansiform lobule specific, symmetrical locations. These collaterals terminate as sagittally orientated aggregates of mossy fibre terminals. Right-hand panels: VII olivocerebellar fibres branch in the sagittal plane. Each branch provides a Purkinje cell with a single Pyramis climbing fibre. These climbing fibres form narrow, longitudinally orientated strips that may correspond to the microzones; strips of Purkinje cells that share the same climbing fibre receptive Uvula fields. Abbreviations: Py, pyramis; Uv, uvula. (Reproduced with permission from Nieuwenhuys, Anterior lobe R, Voogd J, van Huijzen 2008 The Human Nervous 3

Nodulus Nodulus System. 4th Ed Springer Verlag.) Sagittal plane Transverse plane

CTION Anterior lobe E

S Simplex lobule

Ansiform VII lobules

Py Uv Paramedian lobe

Mossy fibre Climbing fibre

of Purkinje cells (Fig. 22.11). These strips probably correspond to a subdivision of the contralateral inferior olive terminate on the ‘microzones’ consisting of a narrow strip of Purkinje cells innervated by Purkinje cells of a particular zone and also send a collateral innervation climbing fibres sharing the same receptive field. Microzones, with their to the corresponding deep cerebellar nucleus. This collateral innerva- Purkinje cells, are considered to be the basic structural and functional tion is reciprocated by the, mainly crossed, nucleo-olivary pathway that unit of the cerebellar cortex (Andersson and Oscarsson 1978). originates from the small GABAergic neurones of the cerebellar nuclei. Mossy fibres take their origin from multiple sources in the spinal Modules can be visualized because their Purkinje cell axons and their cord and brainstem. Their myelinated axons terminate on the claw-like climbing fibre afferents collect in compartments in the cerebellar white dendrites of the granule cells and on Golgi cells. The granule cells give matter. The borders between these compartments, i.e. between the rise to an ascending axon that splits in the molecular layer into two modules, become visible when stained for acetylcholine esterase parallel fibres that run for some distance in the direction of the long (AChE) (Fig. 22.12B). The modular organization of the cerebellum has axis of the folia. Parallel fibres terminate on the spines of the spiny been studied in most detail in rodents and carnivores, and has been branches of the Purkinje cell dendritic tree and the dendrites of confirmed in non-human primates. For the human cerebellum, interneurones that they meet along their course (see Figs 22.7, 22.9). evidence for its presence is mainly embryological. The length of the parallel fibres in the human cerebellar cortex is not The modular organization of the cerebellum appears very early known but the two branches probably do not exceed 10 mm. Like the during its development, long before the emergence of any of its trans- climbing fibres, mossy fibres branch profusely in the cerebellar white verse fissures. Purkinje cells, born in the ventricular matrix of the cere- matter (see Fig. 22.11). The parent fibres enter the cerebellum laterally bellar anlage, migrate to the meningeal surface, where they form a series and run a transverse course to decussate in the cerebellar commissure. of mediolaterally arranged clusters (Korneliussen 1968, Kappel 1981). During their course, they emit thin collaterals that enter the white During the later increase of the cerebellar surface in the rostrocaudal matter of the lobules and terminate in multiple, longitudinally orien- dimension, reflecting the proliferation of millions of granule cells in tated and symmetrically distributed aggregates of mossy fibre terminals the transient external matrix (the external granular layer) (see Fig. in the granular layer (Wu et al 1999). 22.15), the Purkinje cell clusters increase in length and thus are trans- The climbing fibre microzones and the subjacent mossy fibre aggre- formed into Purkinje cell zones. The Purkinje cells become located in gates have been found to share the same peripheral receptive fields in a monolayer and the original borders between the clusters are no longer regions of the cerebellum receiving somatosensory information from visible. This mode of development has been studied in different species the periphery (Ekerot and Larson 1980, Ekerot and Jörntell 2003). A and can also be recognized in the human fetal cerebellum. Purkinje cell similar topographical relationship between microzones and mossy fibre clustering in the human does not differ from that in other species, with terminal aggregates exists in other parts of the cerebellar cortex; their the exception of the immense size of the most lateral cluster, which is common denominator remains unknown (Pijpers et al 2006). The sig- clearly related to the anlage of the dentate nucleus (Fig. 22.13). This nificance of such a topographical relationship is difficult to understand cluster develops into the D2 zone, the most lateral Purkinje cell zone, because the parallel fibres would disperse a localized mossy fibre input responsible for the large size of the human cerebellar hemisphere. over a wide, mediolateral region of the molecular layer. Different Eight or nine of the modules can be recognized in the cerebellum hypotheses to explain this topographical relationship have been pro- of subhuman primates and lower mammals (see Fig. 22.14A). Purkinje posed, some of the more recent ones involving the interneurones of the cell zones differ in their climbing fibre afferents and their cerebellar or cerebellar cortex, but the matter remains undecided (Ekerot and Jörntell vestibular target nucleus. Moreover, Purkinje cells of the different zones 2003, Barmack and Yakhnitsa 2011). differ in their immunohistochemical properties (Voogd and Ruigrok 2012). A Purkinje cell-specific antibody, known as ‘zebrin 2’, is distrib- uted in a pattern of zebrin-positive and zebrin-negative Purkinje cell THE MODULAR ORGANIZATION OF THE zones (Fig. 22.14D–E). This pattern has been shown to be congruent CEREBELLUM AND THE CORTICONUCLEAR AND with the olivocerebellar and corticonuclear projection zones (Voogd OLIVOCEREBELLAR PROJECTIONS et al 2003, Sugihara and Shinoda 2004). Many substances, such as the enzymes aldolase C, 5′ nucleotidase, protein kinase C and the The output of the cerebellum is organized as a series of parallel, sagittal metabotropic glutamate transporter 1A, co-localize with zebrin 2. Neu- modules (Voogd and Bigaré 1980, Voogd and Ruigrok 2012). Each rotransmission in different Purkinje cell zones may therefore differ: module consists of one or more longitudinal Purkinje cell zones that zebrin-positive Purkinje cells fire at a slower rate than the zebrin- project to one of the cerebellar or vestibular nuclei (Fig. 22.12A). Some negative cells (Zhou et al 2014). of these Purkinje cell zones are restricted to certain lobules; others span In the following, the olivocerebellar climbing fibre and the efferent 338 the entire rostrocaudal length of the cerebellum. Climbing fibres from corticonuclear projections of the Purkinje cell zones will be described. Cerebellar circuitry

zone cell je in rk u P Inferior olive with climbing fibres

C3 ABC1C2 D Posterior lobule Purkinje cell 22

EGL Primary fissure ER Cerebellar nucleus

A Reciprocal CHAPT nucleo-olivary Anterior lobule pathways

1mm Fig. 22.13 A transverse section through the cerebellum of a 65 mm human fetus, showing the Purkinje cell clusters that will develop into the m A, B, C1–C3 and D Purkinje cell zones. Note the large size of the D cluster. Abbreviations: EGL, external granular layer. (From the Schenk C1 BX A collection of the Dept. of Pathology of the Erasmus Medical Center C2 Rotterdam.) C3

anterior and posterior cerebellum (see Fig. 22.14E). The C1, C3 and Y zones project to the anterior interposed nucleus and receive their climb- ing fibre input from the rostral dorsal accessory olive (DAOr) (Fig. 22.16). This subnucleus receives an input from peripheral receptors through dorsal column and trigeminal pathways. The climbing fibre projections of the rostral dorsal accessory olive to the C1, C3 and Y B zones and the anterior interposed nucleus are somatotopically organ- ized. In each of the zones, the hindlimb is represented rostrally in the Fig. 22.12 A, A cerebellar module. Purkinje cell axons and climbing fibres are located in a white matter compartment, shown as a transparent anterior cerebellum and caudally in the posterior lobe; the forelimb structure in this diagram. B, An acetylcholinesterase-stained section and face occupy more central areas (Ekerot and Larson 1979) (see Fig. through the anterior lobe: macaque monkey. The borders of the white 22.16). This rostrocaudal distribution clearly differs from the medi- matter compartments of the modules A–C are heavily stained. olateral somatotopy in the vermal B zone. The somatotopical localiza- Abbreviations: m, midline. tion is an extremely detailed one, repeated in each of the zones. The C1, C3 and Y zones connect with motor centres in the brainstem and the cerebral cortex. The hemisphere of the anterior lobe and the simplex lobule, and the paramedian lobule (HVIIB – the gracile lobule, and Data are from experimental studies in the cat, the rat and subhuman HVIII – the biventral lobule) are considered as the motor regions of the primates (reviewed in Voogd and Ruigrok (2012)). The subdivision of cerebellum. the inferior olive, the sole source of the climbing fibres, is summarized The C2, D1 and D2 zones extend beyond the anterior and posterior in Figure 22.15. motor regions, where they interdigitate with the C1, C3 and Y zones, The A zone is located next to the midline and extends over the entire over most of the rostrocaudal length of the cerebellum. In rodents, these vermis (see Fig. 22.14A). It is composed of several zebrin-positive and zones are zebrin-positive. The C2 zone projects to the posterior inter- zebrin-negative subzones that may be present over limited segments of posed nucleus and receives its climbing fibre input from the rostral its extent. It projects to the fastigial and vestibular nuclei, and receives medial accessory olive (see Fig. 22.18). A somatotopical organization its climbing fibres from the caudal medial accessory olive. Whereas the is lacking in the C2 zone. The D1 and D2 zones project to the caudal A zone extends over the entire vermis, the X and B zones are restricted and rostral dentate and receive their climbing fibres from the ventral to the vermis of the anterior lobe, the simplex lobule (VI) and lobule and dorsal laminae of the principal olive, respectively. The main con- VIII (the pyramis). The narrow X zone separates the A zone from the B nections of the C2 and the D zones are with the cerebral cortex. The zone, which occupies the lateral vermis of these lobules. The X zone sections of the C2 and D2 zones located in the anterior and posterior projects to the interstitial cell groups, located between the fastigial and motor regions of the cerebellar hemisphere are connected with motor, posterior interposed nuclei and receives climbing fibres from the inter- premotor and parietal cortical areas; these sections of the D2 zone are mediate region of the medial accessory olive. The B zone projects to somatotopically organized. Sections of the C2 and the D zones located Deiters’ lateral vestibular nucleus and is innervated by climbing fibres in the ansiform lobule (HVII) and the paraflocculus (the tonsil, HIX) from the caudal part of the dorsal accessory olive. The dorsal accessory subserve visuomotor and non-motor functions. olive, the B zone and the lateral vestibular nucleus are somatotopically The modular organization of the vestibulocerebellum is fairly organized. In the B zone, the hindlimb is represented laterally and the complex; multiple Purkinje cell zones, innervated by climbing fibres forelimb is represented medially (Andersson and Oscarsson 1978). In from subnuclei in the inferior olive, transmit optokinetic and vestibular rodents, Purkinje cells of the X and B zones are zebrin-negative. information. The hemisphere is composed of the C1–C3 and the D1, Y and D2 Each lobule of the cerebellum contains a particular set of Purkinje zones. Like the X and B zones, C1, C3 and the Y are restricted to the cell zones. Apart from the parallel fibres, which may cross several anterior lobe, the simplex lobule (HVI) and the paramedian lobule Purkinje cell zones or microzones in their course through the molecular (HVIIB – the gracile lobule, and HVIII – the biventral lobule). Moreover, layer, there is no cross-talk between the modules. Parallel fibres are, the Purkinje cells of these zones are zebrin-negative and thus appear as therefore, a key element in the integrative function of the cerebellum. blank spaces in suitably immunostained histological sections of the The relative independence of the cerebellar modules is an important 339 CEREBELLUM

A X B C1 C2 C3 D1 Anterior lobe Y I-V D2 12 3 4 5 6

Simplex lobule VI Crus I

Crus II VII

Paramedian lobe 3

1234 5 6 7 VIII

CTION Dorsal

E paraflocculus S IX Ventral paraflocculus

X Flocculus

A D

Rostral dentate Emboliform nucleus Fast

ICG Globose nucleus Caudal dentate Cerebellar target M Lateral vestibular nucleus nuclei VII ANS

B Vestibular nucleus

DAOr Dmcc PY POvL

MAOr UV MAOint VLO POdL DAOc

Beta Dc 1mm PMD MAOc C Flattened right inferior olive E Fig. 22.14 The connections of the Purkinje cell zones of the mammalian cerebellum. A, The flattened cerebellar cortex. B, Target nuclei of Purkinje cells. C, Sources of climbing fibres associated with Purkinje cells, shown in the flattened contralateral inferior olive (see Fig. 22.15) and indicated in the same colour. D, Zebrin-positive and zebrin-negative Purkinje cell bands. The zebrin-positive bands are numbered from 1 to 7. A comparison with panel A shows that the A zone is a composite of zebrin-positive and zebrin-negative subzones; the X, B, C1, C3 and Y zones consist of zebrin-negative Purkinje cells. E, The zebrin-positive and zebrin-negative bands of the cerebellum of a rat. Abbreviations: A–D2, Purkinje cell zones A–D2; ANS, ansiform lobule; Beta, group beta; DAOc/r, caudal/rostral dorsal accessory olive; Dc, dorsal cap; Dmcc, dorsomedial cell column; Fast, fastigial nucleus; ICG, interstitial cell groups; M, midline, MAOc/int/r, caudal/intermediate/rostral medial accessory olive; PMD, paramedian lobule; POdL, dorsal lamina of the principal olive; POvL, ventral lamina of the inferior olive; PY, pyramis (lobule VIII); UV, uvula (lobule IX); VII, vermal lobule VII; VLO, ventrolateral outgrowth.

difference between the cerebellum and the cerebral cortex, where dif- fastigial nucleus, the interstitial cell groups and the lateral vestibular ferent functional areas are intimately interconnected. nucleus, respectively (see Fig. 22.14). In all mammals, the fastigial nucleus gives rise to the uncinate tract, which decussates in the cerebel- lar commissure, hooks around the brachium conjunctivum, and is CONNECTIONS OF THE CEREBELLAR NUCLEI: distributed to the vestibular nuclei and the medullary and pontine RECIPROCAL ORGANIZATION OF THE reticular formation. A branch of the uncinate tract ascends to the ipsi- CORTICO-OLIVARY SYSTEM lateral midbrain and thalamus. Projections to the cerebral cortex are bilateral because the crossed ascending fibres of the uncinate fasciculus The connections of the cerebellar nuclei with the brainstem, the thala- subsequently recross in the thalamus. Their projection to the cerebral mus and the spinal cord determine the sphere of influence of the cere- cortex is incompletely known. The uncrossed, direct fastigiobulbar tract 340 bellar modules. The A, X and B zones of the vermis project to the passes along the lateral margin of the fourth ventricle. It is distributed Cerebellar circuitry

DAOc Motor cortex POdL MAOc Dc

Beta VLO C1 Beta C3 Y

MAOr 1 2 POdL DAOr POdL Thalamus

Dmcc DAOr

Dmcc POvL 22

Emboliform nucleus ER

4 Magnocellular 3 red nucleus CHAPT Rostral C1, C3 and 4 Y zones POvL Dmcc DAOr POdL 3

Rostral DAO MAOr 2 VLO Beta DAOc Rubrospinal tract MAOc Dc 1 Pyramidal tract

Fig. 22.15 Transverse sections through the human inferior olive, section 1 being the most rostral. Lower panel: the flattened inferior olive showing the levels of sections 1–4 in the upper panel. Note the large size of the convoluted dorsal lamina of the principal olive (POdL), and the small Dorsal column ventral lamina (POvL). Other abbreviations: Beta, group beta; DAOc/r, nuclei caudal/rostral dorsal accessory olive; Dc, dorsal cap; Dmcc, dorsomedial Motor neurones cell column; MAOc/r, caudal/rostral medial accessory olive; VLO, ventrolateral outgrowth.

Fig. 22.16 The connections of the emboliform (anterior interposed) to the vestibular nuclei and the reticular formation in a symmetrical nucleus. The entire system is somatotopically organized: this organization manner that mirrors that of the uncinate tract (Batton et al 1977). is more detailed than indicated in the diagram. Abbreviations: rostral The direct fastigiobulbar tract is an inhibitory, glycinergic system DAO, rostral dorsal accessory olive. (Bagnall et al 2009). Small GABAergic neurones give rise to a nucleo- olivary pathway terminating in the contralateral caudal medial acces- sory olive. The caudal pole of the fastigial nucleus receives its Purkinje cell proximal musculature, and in the oculomotor centres in the brainstem. afferents from lobule VII (folium and tuber vermis). This lobule is also The skeletomotor and oculomotor functions are located in specific seg- known as the visual vermis because it is involved in the long-term ments of the vermis: skeletomotor functions in the anterior vermis and adaptation of saccades and, possibly, in other eye movements. The posterior lobule VIII (pyramis) (the X and B zones are restricted to these projections of the oculomotor region of the fastigial nucleus (Fig. lobules), and oculomotor functions in lobule VII. Caudalmost, lobule 22.17) are completely crossed. They terminate in the pontine para- X (nodulus) belongs to the vestibulocerebellum and is considered median reticular formation (the horizontal gaze centre), the superior below. Other functions, such as vegetative regulation, are subserved by colliculus, the rostral interstitial nucleus of the medial longitudinal the vermis but have not been studied in detail. fasciculus (the vertical gaze centre) and in the thalamic nuclei that may The anterior interposed (emboliform) nucleus is the target of the include the frontal and parietal eye fields as their targets (Noda et al C1, C3 and Y zones. The detailed somatotopical organization of these 1990). The fastigial nucleus influences visceromotor systems via pro- Purkinje cell zones is maintained in the anterior interposed nucleus, jections of the vestibular nuclei and connections with the catechol- where Purkinje cells of different zones, but with the same climbing fibre aminergic nuclei of the brainstem and the hypothalamus (Zhu et al input from a particular region of the body, project to a common set of 2006). neurones (see Fig. 22.16). Ascending axons from the anterior inter- The projections of the interstitial cell groups located between the posed nucleus enter the brachium conjunctivum. This tract decussates fastigial and posterior interposed nuclei, the target nucleus of the X at the border of the pons and the mesencephalon. The ascending module, have not been studied in primates. In lower mammals, these branch enters and surrounds the magnocellular red nucleus and pro- neurones provide collaterals to the superior colliculus, thalamus and ceeds to the thalamus, from where the anterior interposed nucleus is spinal cord (Bentivoglio and Kuypers 1982). connected with the contralateral primary motor cortex. The descending The lateral vestibular nucleus (Deiters’ nucleus) is the target nucleus branch of the brachium conjunctivum terminates in the nucleus reticu- of the lateral vermal B zone. This nucleus might better be considered laris tegmenti pontis (reticular tegmental nucleus of the pons). The as one of the cerebellar nuclei. It does not receive a primary input from entire system, including the magnocellular red nucleus and the primary the labyrinth and, contrary to the other vestibular nuclei, receives a motor cortex and their efferent tracts, is somatotopically organized. A collateral innervation from the climbing fibres innervating the B zone. nucleo-olivary pathway from the anterior interposed nucleus terminates It gives rise to the lateral vestibulospinal tract. Its nucleo-olivary pathway in the rostral dorsal accessory olive. targets the caudal dorsal accessory olive. The motor cortex and the magnocellular red nucleus give rise to the The zones of the cerebellar vermis are in a position to affect neuro- two main descending motor systems: the corticospinal (pyramidal) transmission in the vestibulospinal and reticulospinal systems, bilat- tract and the rubrospinal tract. Both of these tracts cross the midline, erally controlling postural and vestibular reflexes of the axial and the former at the bulbospinal junction and the latter at its level of origin 341 CEREBELLUM

Thalamus (Fig. 22.18A). The nuclei at the mesodiencephalic junction give rise to the ipsilaterally descending tegmental tracts that terminate in the infe- rior olive, forming reciprocally organized loops; the function of these Vertical gaze prominent recurrent loops has never been studied. centre riMLF The posterior interposed (globose) nucleus projects to the nucleus of Darkschewitsch and, via the thalamus, to most, if not all, cortical Superior colliculus areas (Fig. 22.18B). Reciprocal connections of the cerebral cortex to the nucleus of Darkschewitsch have been reported for most cortical areas. Caudal The nucleus of Darkschewitsch gives rise to a recurrent climbing fibre Horizontal gaze Lobule VII: visual vermis centre PPRF loop to the C2 zone that consists of the medial tegmental tract and the rostral medial accessory olive. Motor and visual divisions can be distin- Ascending branch guished in this system. The segments of the C2 zone located in the of uncinate tract anterior and posterior motor regions of the cerebellum and the rostro- medial posterior interposed nucleus receive input from the motor cortex. Visual and prefrontal input dominates in segments located in

3 the ansiform lobule (HVII), the paraflocculus (HIX) and the flocculus

(HX). The nucleo-olivary pathway from the posterior interposed nucleus innervates the contralateral rostral medial accessory olive. The rostral and caudal dentate nucleus give rise to different path- CTION ways. Neurones of the caudal pole of the dentate nucleus are known to E

S be activated by eye movements (van Kan et al 1993). The caudal dentate Fastigial projects to a dorsomedial subnucleus of the parvocellular red nucleus, nucleus located medial to the fasciculus retroflexus (Fig. 22.18C). Its thalamo- Dentate cortical projections include the frontal and parietal eye fields, which are nucleus reciprocally connected with the dorsomedial subnucleus. The latter projects to the ventral lamina of the principal olive, which innervates Bc the D1 zone. Although fairly prominent in lower mammals, the ventral lamina of the principal olive is represented by the narrow medial Uncinate lamina of the human olive (see Fig. 22.15). This module, presumably, tract is much reduced in the human cerebellum. Crossed nucleo-olivary pathways from the rostral and caudal dentate terminate in the dorsal and ventral laminae of the principal olivary nucleus, respectively. The rostral dentate includes the major part of the dentate nucleus. In monkeys, it has been divided into rostromedial motor and ventro- caudal non-motor portions (Strick et al 2009) (Fig. 22.18E). The motor division is somatotopically organized, with the hindlimb represented Vestibular rostrally and the face more caudally; it receives projections from motor nucleus Direct regions of the cerebellum. The caudal non-motor portion receives its Vestibulo- and fastigiobulbar corticonuclear projections from the ansiform lobule (HVII) and the Reticular formation reticulospinal tracts tract paraflocculus (the tonsil, HIX). A similar subdivision of the dentate has been proposed in humans (Küper et al 2012); it seems likely that these divisions correspond with the rostromedial microgyric and ventrocau- Fig. 22.17 A transverse section through the cerebellum and medulla dal macrogyric regions of the human dentate (see Fig. 22.10). oblongata, showing the symmetrical distribution of the crossed and The rostral dentate projects to the major, ventrolateral, portion of uncrossed connections of the fastigial nucleus. The inset depicts a the parvocellular red nucleus. Its thalamocortical projections target the sagittal section, showing the connections of the visual vermis (lobule VII) motor, premotor and posterior parietal cortices (Fig. 22.18D–E). Pro- with the caudal pole of the fastigial nucleus and its efferent pathways in jections from the caudal dentate include the dorsal prefrontal cortex. red. Abbreviations: Bc, brachium conjunctivum; PPRF, paramedian Reciprocal connections between these cortical areas and the parvocell- pontine reticular formation; riMLF, rostral interstitial nucleus of the medial ular red nucleus have been documented, mainly for the motor and longitudinal fasciculus. premotor areas. These projections are somatotopically organized; they occupy the lateral portion of the parvocellular red nucleus. Prefrontal projections are located more medially. The parvocellular red nucleus in the midbrain. The corticospinal tract provides the magnocellular red connects with the dorsal lamina of the principal olivary nucleus through nucleus with a collateral innervation. Both tracts influence distal move- the central tegmental tract. Motor input is transmitted by the dorsal ments of the limbs. During primate evolution, the corticospinal system lamina to segments of the D2 zone located in the motor regions of the increases in prominence at the cost of the rubrospinal system, which cerebellum; non-motor input is transmitted to the ansiform lobule comes to occupy a subsidiary position in the human brain. (HVII) and the tonsil (HIX). Climbing fibres innervating the C1, C3 and Y zones and the anterior In humans, the D2 zone accounts for most of the cerebellar hemi- interposed nucleus take their origin from the rostral dorsal accessory sphere. This is exemplified by its development (see Fig. 22.13) and by olive, which receives a somatotopically organized cutaneous input, the size of the different components of its circuitry. In Figure 22.19, mainly through the dorsal column and trigeminal nuclei, and contains the first ever published lithograph of a section through the pontine a refined cutaneous map of the entire contralateral body surface tegmentum (Stilling 1846), the central tegmental tract can be recog- (Gellman et al 1983). The corticospinal and rubrospinal tracts provide nized immediately as one of the largest fibre systems in the brainstem. the dorsal column nuclei with a collateral innervation. Several explanations have been offered for the size of the dentate and It should be emphasized that the concept of the cerebellum as a its connections. They include the complexity of the cortical motor motor system is closely allied to the circuitry of the C1, C3 and Y zones, system, which is a major target of the dentate nucleus. (Multiple, and to the anterior interposed nucleus and its output systems. The interconnected premotor and posterior parietal areas involved in the double decussation of the brachium conjunctivum and the rubrospinal preparation of movement converge on the primary motor cortex; the and corticospinal tracts is responsible for the clinical observation that precise contribution of the cerebellum to these processes is not lesions of the cerebellum affect the ipsilateral half of the body. For most known.) Other possible explanations are the increased connectivity of of the other modules with predominantly cerebral cortical connections, the dentate with the prefrontal cortex subserving its non-motor func- the functional relations are much less clear. tions (Stoodley and Schmahmann 2009), and an increase in the The connections of the posterior interposed (globose) and dentate dentate-parietal projection, given that non-motor functions also nuclei are arranged according to the same plan. They ascend and decus- involve the parietal cortex. sate in the brachium conjunctivum, and terminate in a group of nuclei During evolution, the shapes of the dentate and the principal olivary at the mesodiencephalic junction that includes the parvocellular red nucleus change from compact nuclei to intricately folded sheets. This nucleus and the nucleus of Darkschewitsch in the central grey, and in may indicate the presence of a detailed topical localization in the cor- the thalamic nuclei that project to motor, premotor, prefrontal and ticonuclear and climbing fibre afferent connections in the D2 zone, but 342 posterior parietal cortical areas and the frontal and parietal eye fields almost nothing is known about its intrinsic organization. Cerebellar circuitry

Premotor cortex

Accessory eye field Parietal eye field M1 Prefrontal cortex Posterior parietal cortex Frontal eye field Premotor cortex

Parietal eye field Thalamus M1 S1 Accessory eye field Superior colliculus

Prefrontal cortex Globose nucleus

Darkschewitsch 22

nucleus

Frontal eye field C2 zone ER

Medial tegmental tract CHAPT

Posterior parietal cortex Rostral Dorsal A B MAO M1 face Inferior olive Motor M1 AIP M1 arm PMV Accessory eye field Posterior M1 leg arm Parietal eye field SMA Leg Pre parietal Prefrontal cortex hand SMA Arm AIP cortex SMA hand 7b FEF 9I 46d Premotor cortex Face FEF PMV 7b 46d Frontal eye field arm Pre SMA

9I Ventral Dorsomedial Non-motor subnucleus Thalamus Thalamus Rostral Caudal E Superior colliculus

C2 Rostral D1 Caudal dentate Lateral dentate D2 subnucleus Parvocellular Parvocellular red nucleus red nucleus D1 zone D2 zone

Medial tegmental tract Central tegmental tract

Ventral lamina PO Dorsal lamina PO

Inferior olive Inferior olive C D F Fig. 22.18 A, Cortical areas targeted by the cerebellothalamic pathways of the posterior interposed and dentate nuclei. The primary motor area (M1) with the primary sensory area (S1), the premotor cortex with the posterior parietal areas and the frontal and parietal eye fields constitute interconnected networks. B, The connections of the globose (posterior interposed) nucleus. C, The connections of the caudal pole of the dentate nucleus. D, The connections of the rostral dentate nucleus. E, The subdivision of the rostral dentate into rostral motor and caudal non-motor divisions, showing the location of neurones retrogradely labelled from injection sites indicated in the diagram of the cerebral cortex in D. F, The flattened cerebellar cortex showing localization of the C2, D1 and D2 zones. Abbreviations: AIP, anterior intraparietal area; FEF, frontal eye field; PMV, ventral premotor cortex; PO, principal olive; rostral MAO, rostral medial accessory olive; SMA, supplementary motor area; 7b, 46d, 9l, cortical areas. (D, Modified with permission from Strick PL, Dum RP, Fiez JA 2009 Cerebellum and nonmotor function. Annu Rev Neurosci 32:413–434.)

Mossy fibre systems share several common features. Individual AFFERENT MOSSY FIBRE CONNECTIONS mossy fibres distribute bilaterally and give off collaterals at specific OF THE CEREBELLUM mediolateral positions that terminate in longitudinal aggregates of mossy fibre rosettes (see Fig. 22.11). Entire mossy fibre systems termi- Mossy fibre systems take their origin from multiple sites in the spinal nate as multiple, bilaterally distributed bands of mossy fibre terminals cord and the brainstem. The pontocerebellar pathway is the major (Fig. 22.20A). These bands are not continuous, but are often restricted mossy fibre system in primates. Although mossy fibre systems have to either the apices or the bases of the folia. Exteroceptive components rarely been traced with experimental methods in primates, fMRI has of mossy fibre systems terminate superficially, whereas proprioceptive provided information on their organization in the human cerebellum. systems terminate in the bases of the folia (Ekerot and Larson 1972) 343 CEREBELLUM

Abducens nerve (Fig. 22.20B). The mossy fibre aggregates are not as distinct as the Central tegmental tract Genu of facial nerve climbing fibre zones and often merge in the bases of the fissures. Mossy fibre aggregates of different systems interdigitate or overlap; precise information is lacking. The termination of the spinocerebellar, reticulocerebellar, cuneocere- bellar and trigeminocerebellar tracts is restricted to the anterior and posterior motor regions of the cerebellum, i.e. to the anterior lobe, the simplex lobule (VI and HVI), lobule VIII and the paramedian lobule (gracile HVIIB and biventral HVIII lobules). These lobules also receive primary and secondary vestibulocerebellar inputs and pontocerebellar mossy fibres relaying information from cortical motor areas. Many of these mossy fibre systems terminate according to a somatotopical pattern (Fig. 22.20C). A very similar somatotopical organization occurs in the C1, C3 and Y climbing fibre zones that are restricted to the hemisphere of the same lobules (see Fig. 22.16). 3

Pontine nuclei Spinocerebellar, trigeminocerebellar, NRTP reticulocerebellar and

CTION Descending tracts vestibulocerebellar fibres E (corticospinal etc.) S Medial lemniscus The spinal cord is connected to the cerebellum through the spinocere- Fig. 22.19 A lithograph of a transverse section through the pons, showing bellar and cuneocerebellar tracts, and through indirect mossy fibre the localization of the central tegmental tract in the pontine tegmentum. pathways relayed by the lateral reticular nucleus in the medulla oblon- Abbreviation: NRTP, nucleus reticularis tegmenti pontis. (Reproduced gata. These pathways are all excitatory in nature. Some of them give from Stilling B 1846 Untersuchungen über den Bau und die Verrichtungen collaterals to the cerebellar nuclei before ending on cortical granule des Gehirns. I. Über den Bau des Hirnknotens oder der Varolischen Brücke. Jena, Druck und Verlag von Friedrich Make.) cells.

Trigeminocerebellar tract Cuneocerebellar tract Sensory nuclei of trigeminal nerve Reticulocerebellar tract

Central cervical spinocerebellar tract

G CI CE Dorsal column nuclei A

DV Exteroceptive Proprioceptive Lateral reticular nucleus

Anterior lobe Simplex lobule NRL

Central cervical nucleus C1 Central cervical Rostral nucleus spinocerebellar tract VIII

Dorsal C4-T1 spinocerebellar B tract Anterior lobe Intermediate grey Ventral Dorsal horn spinocerebellar tract Posterior thoracic Simplex lobule T2-12 nucleus column

Paramedian lobule

L1-S2

Spinal border cells C D Fig. 22.20 A, The termination of spinocerebellar fibres as multiple sagittal bands in the anterior lobe of Tupaia glis. B, A sagittal section through the cerebellum showing the termination of exteroceptive mossy fibre systems in the apices of the lobules of the anterior lobe, the simplex lobule and lobule VIII (pyramis), and of proprioceptive systems in the bases of the fissures. C, The somatotopic organization of the termination of the exteroceptive components of the spinocerebellar, cuneocerebellar and trigeminocerebellar tracts in the hemisphere of the anterior lobe, the simplex lobule and the paramedian (biventral) lobule. D, The origin of the spinocerebellar, cuneocerebellar and reticulocerebellar tracts. Abbreviations: CE, external cuneate 344 nucleus; CI, internal cuneate nucleus; DV, nucleus of the spinal tract of the trigeminal nerve; G, gracile nucleus; NRL, lateral reticular nucleus. Cerebellar circuitry

The dorsal spinocerebellar tract transmits information from the ipsi- Trigeminocerebellar mossy fibres stem from the ipsilateral principal lateral lower limb (Ch. 20). It contains proprioceptive fibres that arise sensory nucleus and the nucleus of the spinal tract of the trigeminal from neurones in the posterior thoracic nucleus (Clarke’s column) in nerve, and terminate in the hemisphere in the anterior and posterior the thoracic and upper lumbar spinal cord, and exteroceptive fibres face regions (simplex lobule – HVI – and gracile lobule – HVIIB). from the thoracic and lumbar dorsal horns. It enters the cerebellum in The distinct somatotopic organization of the anterior and posterior the inferior cerebellar peduncle to terminate bilaterally in the vermis motor regions of the hemispheres is reflected in the termination of the and hemisphere of the anterior and posterior lower limb regions. exteroceptive components of the dorsal spinocerebellar, cuneocerebel- The cuneocerebellar tract is considered as the upper limb equivalent lar and trigeminocerebellar tracts. It is much less distinct for propriocep- of the dorsal spinocerebellar tract (Ch. 20). It takes its origin from the tive systems, such as the central cervical spinocerebellar tract. dorsal column nuclei, the exteroceptive component from the internal Reticulocerebellar mossy fibres stem from the lateral and paramed- cuneate and gracile nuclei, and the proprioceptive component from the ian reticular nuclei of the medulla oblongata (Ch. 21). The lateral external cuneate nucleus. Both components terminate in the anterior reticular nucleus supplies major collateral projections to the cerebellar and posterior upper limb regions: the proprioceptive component bilat- nuclei. Spinoreticular fibres terminate in a somatotopical pattern within erally in the bases of the fissures, and the exteroceptive component the ventral lateral reticular nucleus, which projects bilaterally, mainly ipsilaterally in the apices of the lobules of the hemisphere. The extero- to the vermis. Spinoreticular fibres from the cervical cord overlap with ceptive component has been shown to terminate in multiple longitu- collaterals from the rubrospinal tract and a projection from the cerebral 22 dinal zones congruent with the climbing fibre zones of this region; cortex, and all terminate in the dorsal part of the nucleus, which projects

these zones share the same detailed somatotopical organization as the to forelimb regions of the ipsilateral hemisphere. The cerebellar cortical ER C1, C3 and the Y climbing fibre zones (Ekerot and Larson 1980). projection of the paramedian reticular nucleus is very similar to that of The ventral spinocerebellar tract is a composite pathway. It informs the ventral lateral reticular nucleus. the cerebellum about the state of activity of spinal reflex arcs related to Primary vestibulocerebellar mossy fibres enter the cerebellum with the lower limb and lower trunk. Its fibres originate in the intermediate the ascending branch of the vestibular nerve, pass through the superior CHAPT grey matter and the spinal border cells of the lumbar and sacral seg- vestibular nucleus and juxtarestiform body, and terminate, mainly ipsi- ments of the spinal cord, cross near their origin, and ascend close to laterally, in the granular layer of the nodule, caudal part of the uvula, the surface as far as the lower midbrain before looping around the ventral part of the anterior lobe and bases of the deep fissures of the entrance of the trigeminal nerve to join the superior cerebellar pedun- vermis (Fig. 22.21A). Secondary vestibulocerebellar mossy fibres arise cle. Most of these fibres cross again in the cerebellar white matter. from the superior vestibular nucleus and the caudal portions of the The rostral spinocerebellar tract originates from cell groups of the medial and inferior vestibular nuclei, and terminate bilaterally, intermediate zone and horn of the contralateral cervical enlargement. not only in the same regions that receive primary vestibulocerebellar Although considered to be the upper limb and upper trunk equivalent fibres, but also in the flocculus and the adjacent ventral paraflocculus of the ventral spinocerebellar tract, most of its fibres remain ipsilateral (the access ory paraflocculus of the human cerebellum), which lack throughout their course. They enter the cerebellum through both the a primary vestibulocerebellar projection (Fig. 22.21B). Some of the superior and the inferior cerebellar peduncles and terminate in the mossy fibres from the medial and inferior vestibular nuclei are anterior and posterior vermis. cholinergic. An upper (central) cervical spinocerebellar tract originates from a central cervical nucleus at high cervical levels (C1–C4). The tract termi- nates bilaterally in the bases of the fissures of the entire cerebellum, CORTICOPONTOCEREBELLAR PROJECTION lacks a somatotopical organization and conveys labyrinthine informa- tion and proprioception from neck muscles (Matsushita and Tanami The cerebral cortex is the largest single source of fibres that project 1987). to the pontine nuclei (Fig. 22.22). The fibres traverse the cerebral

Hemisphere Vermis

Primary fissure Primary fissure

Anterior lobe Simple

Folium Culmen

Simple Folium Tuber Tuber

Pyramis Central lobule Pyramis Biventral Uvula Lingula lobule Nodule Uvula Tonsil C Nodule Flocculus

Superior

Vestibular nerve Lateral Vestibular nuclei Medial

Inferior Vestibular ganglion

A B Fig. 22.21 Vestibulocerebellar mossy fibre projections. A, Primary vestibulocerebellar projections from the bipolar neurones of the vestibular ganglion. B, Secondary vestibulocerebellar projections from the vestibular nuclei. C, A sagittal section showing the distribution of both sets of afferents. 345 CEREBELLUM

peduncle: those from the frontal lobe occupy the medial part of the via the crossed descending branch of the superior cerebellar peduncle. peduncle; corticonuclear and corticospinal fibres occupy its central part; The medial, visuomotor, division of the nucleus reticularis tegmenti and fibres from the parietal, occipital and temporal lobes occupy its pontis receives visuomotor afferents from the frontal eye fields, the lateral part. The mediolateral sequence of the fibres in the cerebral contralateral superior colliculus (the tectopontine tract) and other peduncle is approximately maintained in their termination in the visuomotor centres in the brainstem, and targets lobule VII, the visual pontine nuclei. Prefronto-pontine fibres and the frontal eye fields vermis and the adjacent crus I, the flocculus and the adjacent ventral project medially and rostrally; motor and premotor projections termi- paraflocculus. The bilateral projection of its lateral, motor, portion nate centrally and caudally; and parietal, occipital and temporal fibres overlaps with similar projections from the pontine nuclei. Mossy fibres terminate in the lateral pontine nuclei (Schmahmann and Pandya from the nucleus reticularis tegmenti pontis provide the cerebellar 1997). Motor and premotor projections are somatotopically organized, nuclei with a collateral innervation complementary to that of the lateral such that the face is represented rostrally and the hindlimb caudally in reticular nucleus. An uncrossed component of the tectopontine tract the nuclei. In monkeys, the majority of the corticopontine fibres stem terminates in the dorsolateral corner of the pontine nuclei, where it from motor, premotor and parietal areas. The prefrontal, general overlaps extrastriatal visual afferents. sensory and visual projections are relatively minor (Glickstein et al 1985). A prefrontal projection from the dorsal prefrontal cortex has

3 been confirmed for humans (Beck 1950). Many corticopontine fibres ‘OCULOMOTOR CEREBELLUM’ are collaterals of axons that project to other targets in the brainstem and spinal cord (Ugolini and Kuypers 1986). The pontocerebellar pro- Traditionally, the flocculus and the nodulus are known as the ‘vestib- jection is almost completely crossed. Fibres from the pontine nuclei ulocerebellum’ because they maintain afferent and efferent connec- CTION access the cerebellum via the middle cerebellar peduncle and terminate tions with the vestibular system. They also belong to the functionally E

S throughout the entire cerebellar cortex, with the exception of lobule X more comprehensive oculomotor division of the cerebellum, which (nodulus). Visual cortical mossy fibre input is found in the paraflocc- includes lobule VII (visual vermis), the adjacent ansiform lobule, ulus (tonsil, HIX). The pontocerebellar projection is still incompletely dorsal lobule IX (uvula), the ventral paraflocculus (the human acces- known; the relevant literature has been reviewed by Nieuwenhuys et al sory paraflocculus) and the dorsal paraflocculus (the human tonsil). (2008) and by Voogd and Ruigrok (2012). Figure 22.22C is a simplified The mossy fibre projection of the nucleus prepositus hypoglossi, a key version of this projection (Glickstein et al 1985). element in the saccade-producing system (Ch. 41), outlines the entire The nucleus reticularis tegmenti pontis (tegmental reticular nucleus oculomotor cerebellum, with the exception of the dorsal paraflocculus of the pons) is located along the midline, dorsal to the pontine nuclei (Belknap and McCrea 1988) (Fig. 22.23B). The function of lobule X (see Fig. 22.22C). It gives rise to bilateral components of the middle (nodulus) is not an exclusive oculomotor one because it also influences cerebellar peduncle and receives a projection from the cerebellar nuclei labyrinthine and postural reflexes and vegetative systems.

Cingulate premotor areas 20 18 Extrastriatal cortex 2 Prefrontal 16 Frontal eye fields 14 12 Premotor 10 Sensory–motor 8 Posterior parietal 6 25 19 Extrastriatal 9 24 23 4 6 Vestibular/optokinetic 5 2 M1 S1 Number of labelled cells per mm 11 10 9 8 6 25 23 34 5 7 19 18 17 13 22 Areas Prefrontal cortex Frontal Premotor cortex Posterior parietal cortex B 24 2 14 eye fields 1 M1 S1 5

1,2,3 7 9 8 22 10 4 19 Anterior lobe 11 17 6 18 Simplex lobule

I-V Crus I A Extrastriatal cortex

Crus II VI

VII NRTP VIII

Ventral paraflocculus IX

Flocculus X Dorsal paraflocculus Cerebral peduncle Paramedian lobe C D Fig. 22.22 The corticopontocerebellar system. A, The origin of corticopontine fibres from the cerebral cortex in the monkey (macaque). B, The relative proportions of corticopontine neurones in different areas of the cerebral cortex of the monkey, indicated in panel A. C, A transverse section through the pons showing the distribution of corticopontine fibres in the pontine nuclei and the nucleus reticularis tegmenti pontis (NRTP). D, The flattened cortex of the monkey cerebellum showing the distribution of pontocerebellar mossy fibres. (B, Modified from Glickstein M, May JG, 3rd, Mercier BE 1985 Corticopontine projection in the macaque: the distribution of labelled cortical cells after large injections of horseradish peroxidase in the pontine nuclei. 346 J Comp Neurol 235:343–359.) Cerebellar circuitry

Lobules I and II

Anterior lobe SI Simplex lobule

Ansiform lobule VII ANS

Paramedian lobe VII VIII PETR Dorsal paraflocculus PMD

IX IX 22

Ventral paraflocculus X ER X PFLV Flocculus PFLD FL

A B Contralateral Ipsilateral CHAPT Fig. 22.23 A, A flattened map of the cerebellar cortex of the mammalian cerebellum showing the distribution of vestibulocerebellar mossy fibres in orange. The hatched lobules belong to the oculomotor cerebellum. B, The distribution of mossy fibres originating from the nucleus prepositus hypoglossi outlines the oculomotor cerebellum of the squirrel monkey, with the exception of the dorsal paraflocculus (PFLD). Other abbreviations: ANS, ansiform lobule; FL, flocculus; PETR, petrosal lobule; PFLV, ventral paraflocculus; PMD, paramedian lobule; SI, simplex lobule; VII–X, lobules VII–X. (Reproduced with permission from Belknap DB, McCrea RA 1988 Anatomical connections of the prepositus and abducens nuclei in the squirrel monkey. J Comp Neurol 268:13–28.)

The oculomotor cerebellum is involved in long-term adaptation of nerve, known as the accessory optic tract. These nuclei project to the saccades, ocular stabilization reflexes and smooth pursuit (reviewed in ventrolateral outgrowth of the inferior olive, located immediately rostral Voogd et al (2012)). The role of the flocculus and the adjacent ventral to the dorsal cap (see Fig. 22.15). The ventrolateral outgrowth innervates paraflocculus in long-term adaptation of the vestibulo-ocular reflex the F1 and F3 zones. Repeated simultaneous activation of the vestibular (VOR) has been extensively studied. It is one of the few instances where mossy fibre-parallel fibre input and the climbing fibres, relaying retinal the function of the cerebellum is clearly understood. slip, induces plastic changes in the Purkinje cell output that compen- sates for the retinal slip. Combinations of the horizontal and anterior Vestibulo-ocular reflex canal systems ensure compensation of retinal slip in all possible planes. Knowledge of this system has been instrumental in the concept that The VOR is an ancient reflex, being present in agnatha and fishes. It climbing fibres are carriers of error signals, used in cerebellar learning stabilizes the position of the retina in space, during movements of the (Marr 1969, Ito 1982). head, by rotating the eyeball in the opposite direction. The VOR is an open reflex; there is no time for a feed-back connection that would compensate for inaccuracies in the execution of the reflex. This function NEUROIMAGING AND THE FUNCTIONAL DIVISIONS is taken over by the long-term adaptation of the reflex by the flocculus. OF THE CEREBELLUM The circuitry of the flocculus, similar to the VOR, is organized on the coordinate system of the semicircular canals (Simpson and Graf 1981, Mossy fibre projections have been studied in the human brain using van der Steen et al 1994). The VOR consists of different components. fMRI. In the cerebellum, activity in climbing fibres and Purkinje cells One component connects the lateral (horizontal) semicircular canal, is overwhelmed by the massive activity of the mossy fibres (Diedrich- via oculomotor neurones in the vestibular nuclei, with the oculogyric sen et al 2010), which means that the modular organization of the muscles that move the eyes in a plane co-linear with the plane of the cerebellum therefore cannot be visualized with this method. The divi- lateral canal (Fig. 22.24). The anterior semicircular canal influences the sion of the human cerebellum into anterior and posterior motor and ipsilateral superior oblique and the contralateral inferior oblique intermed iate non-motor portions has been observed in numerous muscles that move the eye in the plane of this canal. (For further details, neuroimaging studies (reviewed by Stoodley and Schmahmann see Chs 38 and 41.) (2009)). The somatotopical localization in each hemisphere of the Five Purkinje cell zones are present in the cortex of the flocculus and anterior lobe and the simplex and biventral lobules in the posterior the adjacent ventral paraflocculus. Apart from the C2 zone, located most lobe has been confirmed with fMRI (Grodd et al 2001, Buckner et al medially, two pairs of zones occupy its lateral portion. Zones F1 and 2011, Yeo et al 2011). A systematic somatotopical gradient has been F3 connect with the oculomotor neurones in the vestibular nuclei, reported for the digits of the hand in the hemisphere of lobule V subserving the anterior canal VOR. The F2 and F4 zones connect with (Wiestler et al 2011). oculomotor neurones of the horizontal canal VOR. The flocculus and The crura of the ansiform lobule (HVII) are activated during the ventral paraflocculus receive vestibular mossy fibre input, relaying an execution of cognitive tasks. More recently, resting-state functional con- efferent copy of the output of the vestibulo-oculomotor neurones. They nectivity fMRI has been used to map topographical correlations between also receive climbing fibre input, signalling retinal slip that occurs when remote, functionally coupled regions in the cerebral cortex and the the stabilization of the retina by the VOR is incomplete. Retinal slip is cerebellum. Several functional networks in the cerebral cortex have been perceived by two groups of neurones in the mesencephalon. In the identified with this method (Yeo et al 2011) (Fig. 22.25). However, it horizontal plane, it is relayed by the nucleus of the optic tract. This does not provide information on the anatomical connections or the nucleus, located in the pretectum, receives fibres of the contralateral excitatory or inhibitory nature of the constituent areas of each of these optic nerve via the optic tract and projects to the dorsal cap of the systems; connections between the cerebrum and the cerebellum could inferior olive, located dorsomedial to the caudal medial accessory olive be indirect, e.g. through cortical association systems or brainstem nuclei (see Fig. 22.15). The dorsal cap provides the F2 and F4 zones with other than the pontine nuclei. The networks are distributed in a mir- climbing fibres. Retinal slip in the plane of the anterior canal is conveyed rored fashion in the anterior and posterior cerebellum. The default by the lateral and medial nuclei of the accessory system, which belong mode network, a network of brain regions that are active when an to a group of nuclei located on the periphery of the rostral mesen- individual is not focused on the outside world (Buckner et al 2008; cephalon, receiving optic nerve fibres from an offshoot of the optic Commentary 3.1), occupies a central position.

347 CEREBELLUM

Left eye Right eye Fig. 22.24 The circuitry used by the flocculus in long-term adaptation of the Retinal slip in the vestibulo-ocular reflex (VOR). The horizontal and system is organized in the planes of the vertical planes semicircular canals. For an explanation, see the text. Abbreviations: C2, C2 Purkinje cell zone; DC, dorsal cap; F1–F4, floccular Purkinje cell zones F1–F4; VLO, ventrolateral outgrowth. (Modified with permission from Voogd J Medial rectus and contralateral et al 2012 Visuomotor cerebellum in lateral rectus human and nonhuman primates. Superior rectus Cerebellum 11:392–410.) and contralateral inferior oblique 3

Optic tract Vestibular nuclei nucleus and group Y Accessory CTION Anterior semicircular canal optic system E C2 F1 F2 F3 F4 S

DC VLO

Left labyrinth

Lateral semicircular canal

FEF S1 M1 Parietal

Frontal

Primary fissure V Posterior superior IV Temporal fissure A Occipital B VI

Posterior superior fissure VI

IV Primary fissure VII Intercrural fissure V Posterior VI superior VIII fissure VII

C D Ansoparamedian fissure

Tonsil Prebiventral fissure

Somatomotor Dorsal association Default Ventral association Frontoparietal Visual

Fig. 22.25 A map of the topographical correlations between remote, functionally coupled regions in the human cerebral cortex and the cerebellum using resting state functional connectivity fMRI. A, Networks distinguished in the cerebral cortex. B, An anterior view of the human cerebellum showing regions that are functionally coupled to the different cerebral networks. C, A dorsal view of the human cerebellum. D, A caudal view of the human cerebellum. Abbreviation: FEF, frontal eye field. (A, Redrawn from Yeo BTT, Krienen FM, Sepulere J 2011 The organization of the human cerebral cortex estimated by intrinsic functional connectivity. J Neurophysiol 106:1125–1165; B–D, Reconstructions based on the transverse sections illustrated in Buckner RJ, 348 Krienen FM, Castellanos A 2011 The organization of the human cerebellum estimated by functional connectivity. J Neurophysiol 106:2322–2345.) Key references

KEY REFERENCES

Bolk L 1906 Das Cerebellum der Säugetiere. Haarlem: Fischer. Olivary Nucleus. Anatomy and Physiology. New York: Raven, pp. A classic text on the subdivision and the comparative anatomy of the 207–34. mammalian cerebellum. A description of the zonal organization of the corticonuclear and olivocerebellar climbing fibre projections. Glickstein M, Strata P, Voogd J 2009 Cerebellum: history. Neurosci 162:549–59. Voogd J, Ruigrok TJH 2012 Cerebellum and precerebellar nuclei. In: May J, The historical basis of the cerebellar nomenclature, anatomy and physiology. Paxinos G (eds) The Human Nervous System. Amsterdam: Elsevier, pp. 471–547. Nieuwenhuys R, Voogd J, van Huijzen C 2008 The Human Nervous System, A recent and extensive review of the anatomy of the cerebellum and the 4th ed. Berlin: Springer. precerebellar nuclei. A treatise on the anatomy of the central nervous system. Places the anatomy of the cerebellum in a wider context. Voogd J, Schraa-Tam CK, van der Geest JN et al 2012 Visuomotor cerebellum in human and nonhuman primates. Cerebellum 11:392–410. Ramón y Cajal S 1906 Histologie du système nerveux de l’homme et des A review of the anatomy, physiology and functional magnetic resonance vertebrés. Paris: Maloine.

imaging of the visuomotor cerebellum. 22

A classic text on the histology of the cerebellar cortex. Willis T 1681 On the Anatomy of the Brain. Englished by Samuel Pordage Strick PL, Dum RP, Fiez JA 2009 Cerebellum and nonmotor function. Annu Esquire. London: Harper, Leigh & Martin. ER Rev Neurosci 32:413–34. A classic text on the anatomy of the brain, including the cerebellum, written A review of cerebello-cortical pathways and their division into motor and by the ‘father of modern neurology’. non-motor paths. CHAPT Voogd J, Bigaré F 1980 Topographical distribution of olivary and cortico- nuclear fibres in the cerebellum: a review. In: Courville J (ed.) The

349 CHAPTER

24 Basal ganglia

The term basal ganglia is used to denote a number of subcortical therapeutic strategies for both medical and neurosurgical treatments of nuclear masses that lie in the inferior part of the cerebral hemisphere, movement disorders. in close relationship with the internal capsule (Fig. 24.1; see Fig. 25.43). The traditional definition of the basal ganglia included the corpus striatum, claustrum and amygdaloid complex. The term has now been CORPUS STRIATUM restricted to the corpus striatum and, according to some authorities, other nuclei in the diencephalon and midbrain (principally the subtha- The corpus striatum consists of the caudate nucleus, putamen and lamic nucleus, substantia nigra, pedunculopontine nucleus) that col- globus pallidus (Fig. 24.2). Because of their close proximity, the lectively form a functional complex involved in the control of movement putamen and globus pallidus were once considered as an entity, the and motivational aspects of behaviour (Jankovic 2012). The function lentiform (lenticular) complex or nucleus. However, although the name of the claustrum is unknown; the amygdala is more closely related to has been retained in gross anatomical terminology and in some com- the limbic system and is, therefore, described in that context. pound names (e.g. sublenticular, retrolenticular), the putamen and Disorders of the basal ganglia are principally characterized by abnor- globus pallidus have quite different connections. Rather, it is now malities of movement, muscle tone and posture. There is a wide spec- known that the putamen and caudate nucleus share a common chemo- trum of clinical presentations ranging from poverty of movement and cytoarchitecture and connections, and they are referred to jointly as the hypertonia at one extreme (typified by Parkinson’s disease) to abnormal neostriatum, or simply the striatum. involuntary movements (dyskinesias) at the other. The underlying The striatum is considered to be the principal ‘input’ structure of the pathophysiological mechanisms that mediate these disorders have been basal ganglia since it receives the majority of afferents from other parts much studied in recent years and are better understood than for any of the neuraxis. Its principal efferent connections are to the globus pal- other type of complex neurological dysfunction (Penney and Young lidus and pars reticularis of the substantia nigra. The globus pallidus 1986, Jankovic 2012). This has led to the introduction of new rational and, more particularly, its medial segment, together with the pars reticu- laris of the substantia nigra, is regarded as the main ‘output’ structure because it is the source of basal ganglia efferent fibre projections, mostly directed to the thalamus.

LENTIFORM COMPLEX

Head of The lentiform complex (see Figs 16.7, 24.1; Fig. 24.3) lies deep to the External caudate nucleus insular cortex, with which it is roughly coextensive, although they are capsule Anterior limb of separated by a thin layer of white matter and by the claustrum. The Putamen internal capsule latter splits the insular subcortical white matter to create the extreme

Globus pallidus Thalamus Basal ganglia

Insula

Corpus striatum Amygdala A

Anterior horn of the lateral Neostriatum (striatum) Paleostriatum ventricle Head of Anterior limb of caudate the internal nucleus capsule Globus pallidus Claustrum Caudate nucleus Putamen Globus pallidus Putamen Insula

B Lentiform nucleus Fig. 24.1 Axial (A) and coronal (B) magnetic resonance images of the brain showing the basal ganglia, thalamus and internal capsule. (Courtesy Fig. 24.2 Terminology and relationships of structures forming the basal 364 of Alan Jackson.) ganglia. Striatum and external capsules; the external capsule separates the claustrum from dorsal striatum. A smaller inferomedial part of the rostral striatum is the putamen. The internal capsule separates the lentiform complex referred to as the ventral striatum, and includes the nucleus accumbens. from the caudate nucleus. The caudate and putamen are traversed by numerous small bundles of The lentiform complex consists of the laterally placed putamen and thinly myelinated, or unmyelinated, small-diameter axons, which are the more medially placed globus pallidus (pallidum), which are sepa- mostly striatal afferents and efferents. They radiate through the striatal rated by a thin layer of fibres, the external medullary lamina. The globus tissue as though converging on, or radiating from, the globus pallidus. pallidus is itself divided into two segments, a lateral (external) segment The bundles are occasionally referred to by the archaic term ‘Wilson’s and a medial (internal) segment, separated by an internal medullary pencils’ and they account for the striated appearance of the corpus lamina. The two segments have distinct afferent and efferent connec- striatum. tions. Inferiorly, a little behind the fundus striati, the lentiform complex is grooved by the anterior commissure, which connects inferior parts of the temporal lobes and the anterior olfactory cortex of the two sides CAUDATE NUCLEUS (see Fig. 24.6). The area above the commissure is referred to as the dorsal pallidum, and that below it as the ventral pallidum. The caudate nucleus is a curved, tadpole-shaped mass. It has a large anterior head, which tapers to a body, and a down-curving tail (Fig. 24.4). The head is covered with ependyma and lies in the floor and 24 STRIATUM lateral wall of the anterior horn of the lateral ventricle, in front of the interventricular foramen. The tapering body is in the floor of the body The striatum consists of the caudate nucleus, putamen and ventral of the ventricle, and the narrow tail follows the curve of the inferior striatum, which are all highly cellular and well vascularized. The vast horn, and so lies in the ventricular roof, in the temporal lobe. Medially, bulk of the caudate nucleus and putamen are often referred to as the the greater part of the caudate nucleus abuts the thalamus, along a junction that is marked by a groove, the sulcus terminalis. The sulcus CHAPTER contains the stria terminalis, lying deep to the ependyma (Fig. 24.5).

Anterior limb of Body of caudate internal capsule nucleus Posterior limb of internal capsule

Head of caudate Caudate nucleus nucleus Thalamostriate vein External capsule Thalamus

Fornix Extreme capsule Thalamus, anterior part Thalamus, lateral part Putamen Thalamus, medial part Internal capsule ANTERIOR POSTERIOR Internal medullary lamina Globus pallidus Subthalamic Optic tract nucleus Substantia nigra Pes hippocampi Putamen Tail of caudate Crus cerebri nucleus Collateral sulcus

Amygdala Fig. 24.4 The striatum within the left cerebral hemisphere. (With Fig. 24.3 The anterior aspect of a coronal section through the left permission from Crossman AR, Neary D 2010 Neuroanatomy, 4th edn. cerebral hemisphere. Edinburgh: Churchill Livingstone.)

Dorsomedial Choroid Corpus Cingulate Thalamostriate vein Fig. 24.5 An oblique section through the Body of lateral ventricle Subcallosal thalamic plexus collosum gyrus and stria terminalis fasciculus diencephalon and basal ganglia. Abbreviations: nucleus Fronto-occipital Ventral nucleus of thalamus ICV, internal cerebral veins; H, H1, H2, subthalamic fasciculus Body of fields of Forel. Posterior limb of caudate nucleus internal capsule Superior External capsule longitudinal fasciculus Putamen Thalamic reticular Insula nucleus External ICV medullary Globus pallidus lamina (lateral segment) Claustrum Globus pallidus (medial segment) Extreme capsule H Internal 1 medullary H lamina H2 Subthalamic nucleus Third ventricle

Zona incerta Centromedian nucleus of thalamus Substantia pars reticularis nigra pars compacta (A9) Crus cerebri Red nucleus Retrorubral area (A8) Paranigral nucleus (A10) 365 BASAL GANGLIA

A Corpus callosum B Fig. 24.6 Coronal sections through the corpus Anterior horn of striatum and anterior perforated substance. A is Subcallosal Anterior horn of Subcallosal lateral ventricle fasciculus lateral ventricle fasciculus anterior to B. Fronto-occipital Head of Corpus bundle caudate nucleus callosum Head of caudate Anterior limb nucleus of internal Anterior limb of capsule internal capsule External Septal nuclei External capsule capsule Septal Bed nucleus of Extreme nuclei Extreme stria terminalis capsule capsule Anterior Diagonal Claustum Claustrum band (Broca) commissure Preoptic Nucleus accumbens hypothalamus Putamen Paraolfactory Optic 3 Lateral olfactory stria (tract) Putamen area chiasma N Olfactory tubercle of Fundus striati Olfactory tubercle

O Nucleus basalis (of Meynert) I anterior perforated substance Globus pallidus, lateral segment Globus pallidus, medial segment Ventral pallidum SECT

Anterior horn of Septum nucleus accumbens (see Fig. 24.6), which abuts the nuclei of the lateral ventricle pellucidum Corpus callosum Caudate nucleus septum, close by the paraolfactory area, diagonal band of Broca and the fornix. The nucleus accumbens receives a dopaminergic innervation from the midbrain ventral tegmental area (cell group A10). It is believed to represent the neural substrate for the rewarding effects of several classes of drugs of abuse and is, therefore, a major determinant of their addic- tive potential. The experimental observation that the locomotor activat- ing effects of psychomotor stimulant drugs such as amphetamine and cocaine (which act presynaptically on dopaminergic neurones to enhance dopamine release or block its reuptake, respectively) are dependent on dopamine transmission in the nucleus accumbens, led to the hypothesis that the reinforcing or rewarding properties of these Anterior drugs are mediated by the mesolimbic dopamine system. limb of internal capsule STRIATAL CONNECTIVITY

Neurones of both dorsal and ventral striatum are mainly medium-sized multipolar cells. They have round, triangular or fusiform somata, mixed with a smaller number of large multipolar cells. The ratio of medium to large cells is at least 20 : 1. The large neurones have extensive spherical or ovoid dendritic trees up to 60 μm across. The medium-sized neu- Olfactory tract Putamen Claustrum Insula rones also have spherical dendritic trees, approximately 20 μm across, Fig. 24.7 The posterior aspect of a coronal section through the anterior which receive the synaptic terminals of many striatal afferents. The horn of the lateral ventricles. dendrites of both medium and large striatal cells may be either spiny or non-spiny. The most common neurone (approximately 75% of the total) is a medium-sized cell with spiny dendrites (so-called medium The sulcus terminalis is especially prominent anterosuperiorly (because spiny neurones). These cells utilize γ-aminobutyric acid (GABA) as their of the large size of the head and body of the caudate nucleus relative transmitter and also express the genes coding for either enkephalin or to the tail), and here the stria terminalis is accompanied by the thala- substance P/dynorphin, depending on their efferent connections. mostriate vein. Enkephalinergic neurones express D2 dopamine receptors. Substance The corpus callosum lies above the head and body of the caudate P/dynorphin neurones express D1 receptors. These neurones are the nucleus. The two are separated laterally by the fronto-occipital fascicu- major, and perhaps exclusive, source of striatal efferents to the pallidum lus, and medially by the subcallosal fasciculus, which caps the nucleus and substantia nigra pars reticularis. The other medium-sized striatal (see Fig. 24.5; Fig. 24.6). The caudate nucleus is largely separated from neurones are aspiny, and are intrinsic cells that contain acetylcholinester- the lentiform complex by the anterior limb of the internal capsule (see ase (AChE), choline acetyltransferase (CAT) and somatostatin. Large Figs 24.1, 24.6; Fig. 24.7). However, the inferior part of the head of the neurones with spiny dendrites contain AChE and CAT; most, perhaps caudate becomes continuous with the most inferior part of the putamen all, are intrinsic neurones. Aspiny large neurones are all intrinsic immediately above the anterior perforated substance; this junctional neurones. region is sometimes known as the fundus striati (see Fig. 24.6). Variable Intrinsic synapses are probably largely asymmetric (type II), while bridges of cells connect the putamen to the caudate nucleus for most those derived from external sources are symmetric (type I). The amin- of its length. They are most prominent anteriorly, in the region of the ergic afferents from the substantia nigra, raphe and locus coeruleus all fundus striati and the head and body of the caudate nucleus, where end as profusely branching axons with varicosities, which contain they penetrate the anterior limb of the internal capsule (see Fig. 24.7). dense-core vesicles (the presumed store of amine transmitters). Many In the temporal lobe, the anterior part of the tail of the caudate nucleus of these varicosities have no conventional synaptic membrane speciali- becomes continuous with the posteroinferior part of the putamen. zations, and may release transmitter in a way analogous to that found in peripheral postsynaptic sympathetic axons. Neuroactive chemicals, whether intrinsic or derived from afferents, VENTRAL STRIATUM are not distributed uniformly in the striatum. For example, 5-HT (5-hydroxytryptamine, serotonin) and glutamic acid decarboxylase The ventral striatum consists of the nucleus accumbens and the olfac- (GAD) concentrations are highest caudally, while substance P, acetyl- tory tubercle. In front of the anterior commissure, much of the choline (ACh) and dopamine are highest rostrally. However, there is a grey matter of the anterior perforated substance, and especially the finer-grain neurochemical organization showing that the striatum con- olfactory tubercle, is indistinguishable from, and continuous with, sists of a mosaic of islands or striosomes (sometimes referred to as the fundus striati, in terms of cellular composition, histochemistry and patches), each 0.5–1.5 mm across, packed into a background matrix. 366 interconnections. The caudate nucleus is continuous medially with the Striosomes contain substance P and enkephalin. During development, Striatum the first dopamine terminals from the substantia nigra are found in suggested that some of the cells of origin lie in the supragranular ‘corti- striosomes. Although this exclusivity does not persist after birth, strio- cal association’ layers II and III. The projection is organized topographi- somes in the adult caudate nucleus still contain a higher concentration cally. The greater part of the input from the cerebral cortex to the dorsal of dopamine than the matrix. The latter contains ACh and somatosta- striatum is derived from the frontal and parietal lobes, with that from tin, and is the target of thalamostriatal axons. Receptors for at least some the occipitotemporal cortex being relatively small. Thus, the orbitofron- neurotransmitters are also differentially distributed. For example, opiate tal association cortex projects to the inferior part of the head of the receptors are found almost exclusively within striosomes, and mus- caudate nucleus, which lies next to the ventral striatum. The dorsola- carinic receptors predominantly so. Moreover, the distribution of neu- teral frontal association cortex and frontal eye fields project to the rest roactive substances within the striosomes is not uniform. In humans, of the head of the caudate nucleus, and much of the parietal lobe the striosome/matrix patchwork is more evident in the caudate than the projects to the body of the nucleus. The somatosensory and motor putamen, the latter consisting predominantly of matrix. cortices project predominantly to the putamen. Their terminals estab- All afferents to the striatum terminate in a mosaic manner. The size lish a somatotopic pattern, in which the lower body is represented later- of a cluster of terminals is usually 100–200 μm across. Some afferent ally and the upper body is represented medially. The motor cortex is terminal clusters are not arranged in register with the clear striosome/ unique in also sending axons through the corpus callosum to the oppo- matrix distributions seen in nigrostriatal and thalamostriatal axons. In site putamen, where they end with the same spatial ordering. The general, afferents from the neocortex end in striatal matrix and those occipital and temporal cortices project to the tail of the caudate nucleus 24 from the allocortex end in striosomes. However, the distinction is not and to the inferior putamen. absolute: although afferents from the neocortex arise in layers V and VI, The striatum receives afferents from the polysensory intralaminar those from the superficial part of layer V end predominantly in striatal thalamus and these are more crudely organized spatially. The nucleus matrix, whereas those from deeper neocortex project to striosomes. centralis lateralis, which receives a cerebellar input, projects to the Striatal cell bodies that are the sources of efferents also form clusters anterior striatum (especially the caudate nucleus), while the centrome- but, again, are not uniformly related to striosomes. For example, the dian nucleus, which receives input from both the cerebellum and inter- CHAPTER cell bodies of some striatopallidal and striatonigral axons lie clustered nal (medial) globus pallidus, projects to the putamen. within striosomes, but others lie outside them, still in clusters. The The aminergic inputs to the caudate and putamen are derived from neurones and neuropil of the ventral striatum are essentially similar to the substantia nigra pars compacta (dopaminergic group A9; see Fig. those of the dorsal striatum, but the striosomal/matrix organization is 21.17), the retrorubral nucleus (dopaminergic group A8), the dorsal less well defined and seems to consist predominantly of striosomes. raphe nucleus (serotoninergic group B7) and the locus coeruleus The major connections of the striatum are summarized in Figure (noradrenergic group A6). The nigrostriatal input is sometimes referred 24.8. Although the connections of the dorsal and ventral divisions to as the ‘mesostriatal’ dopamine pathway. It reaches the striatum by overlap, the generalization can be made that the dorsal striatum is traversing the H fields (of Forel) in the subthalamus and running in predominantly connected with motor and associative areas of the cer- the medial forebrain bundle. These aminergic inputs appear to modu- ebral cortex, whilst the ventral striatum is connected with the limbic late the responses of the striatum to cortical and thalamic afferent system and orbitofrontal and temporal cortices. For both dorsal and influences. ventral striatum, the pallidum and substantia nigra pars reticularis are Efferents from the striatum pass to both segments of the globus pal- key efferent structures. The fundamental arrangement is the same for lidus and to the substantia nigra pars reticularis, where they end in a both divisions. The cerebral cortex projects to the striatum, which in topically ordered fashion. Fibres ending in either the external or inter- turn projects to the pallidum and substantia nigra pars reticularis. Effer- nal pallidum originate from different striatal cells (see Fig. 24.8). Those ents from the pallidum and substantia nigra pars reticularis influence projecting to the external pallidum come from neurones that co-localize the cerebral cortex (either the supplementary motor area or prefrontal GABA and encephalin, and give rise to the so-called ‘indirect pathway’. and cingulate cortices via the thalamus) and the superior colliculus (see This name refers to the fact that these striatal neurones influence the below). activity of basal ganglia output neurones in the internal pallidum via The entire neocortex sends glutamatergic axons to the ipsilateral the intermediary of the subthalamic nucleus. Other striatal neurones, striatum. Previously, these axons were thought to be collaterals of corti- which co-localize GABA and substance P/dynorphin, project directly to cal efferents to other regions, but it is now known that they arise exclu- the internal pallidum and are, therefore, described as the ‘direct sively from small pyramidal cells in layers V and VI. It has also been pathway’. The striatal projection to the pars reticularis of the substantia nigra also has both direct and indirect components, via the external pallidum Centromedian nucleus and subthalamic nucleus (Figs 24.9–24.10). The axons of the direct Corticostriatal fibres Caudate nucleus striatonigral projection constitute the laterally placed ‘comb’ system, which is spatially quite distinct from the ascending dopaminergic nigrostriatal pathway. Striatonigral fibres end in a spatially ordered way in the pars reticularis. Putamen The ventral striatum is the primary target of fibres from limbic cor- tices, including allocortex, and from limbic associated regions (see Fig. 24.9). Thus, the hippocampus (through the fornix) and orbitofrontal cortex (through the internal capsule) project to the nucleus accumbens, and the olfactory, entorhinal, anterior cingulate and temporal visual cortices project to both the nucleus accumbens and olfactory tubercle. The olfactory tubercle also receives afferents from the amygdala. The contiguity of the cortical areas that project to the ventral striatum and neighbouring dorsal striatum emphasizes the imprecise nature of the boundaries between these two divisions. All the cortical regions abut and overlap with neighbouring areas and they project to neighbouring parts of the dorsal striatum as well as to the ventral striatum. The fundus striati and ventromedial caudate nucleus abut the olfactory tubercle and nucleus accumbens (see Fig. 24.6) and receive connections from the orbitofrontal cortex and, to a lesser extent, from the lateral prefrontal and anterior cingulate cortices (which also project to the contiguous head of the caudate nucleus). Striatopallidal fibres Globus pallidus (lateral This continuity of the ventral and dorsal striata, as revealed by the ‘Indirect pathway’ segment) arrangements of corticostriatal projections, is reinforced by considera- Striatopallidal fibres Globus pallidus (medial tion of the aminergic inputs to the ventral striatum. They are derived ‘Direct pathway’ segment) from the dorsal raphe (serotoninergic group B7), the locus coeruleus Substantia nigra Thalamostriatal fibres (noradrenergic group A6) and the ventral tegmental area (dopamine group A10), as well as the most medial part of the substantia nigra pars Striatonigral fibres Nigrostriatal fibres compacta (A9) (see Fig. 21.17). The dopamine projections constitute Fig. 24.8 Connections of the striatum. The major afferent projections to the so-called mesolimbic dopamine pathway, which also projects to the striatum are shown on the right and major efferent projections from the septal nuclei, hippocampus and amygdala, and prefrontal and cin- the striatum on the left. gulate cortices through the medial forebrain bundle. The lateromedial 367 BASAL GANGLIA

A Thalamic nuclei: Cerebral cortex (including motor, Ventralis lateralis, pars anterioris Supplementary motor cortex prefrontal and Dorsal striatum Dorsal pallidum cingulate areas) Centromedian Somato-sensory-motor cortices Intralaminar thalamus Nucleus tegmentalis pedunculopontinus Medullary reticular formation DA: Substantia nigra pars compacta (A9); Retrorubral nucleus (A8) Ventralis lateralis, pars medialis Prefrontal and cingulate cortices 5-HT: Raphe (B7) Substantia nigra NA: Locus coeruleus (A6) pars reticularis Superior colliculus (deep layers) Medullary reticular formation; spinal cord

3 B Thalamic nuclei: N

O Limbic, prefrontal and I temporal lobe cortices Mediodorsalis Prefrontal and cingulate cortices Ventral striatum Ventral pallidum Olfactory cortex Midline Hippocampus

SECT and amygdala Habenular nuclei ‘Limbic’ midbrain DA: Paranigral nucleus (A10) 5-HT: Raphe (B7) NA: Locus coeruleus (A6) Ventralis lateralis, pars medialis Prefrontal and cingulate cortices Substantia nigra pars reticularis Superior colliculus (deep layers) Medullary reticular formation; spinal cord

Fig. 24.9 The principal output connections of the basal ganglia derived from dorsal (A) and ventral (B) divisions of the striatum. In each case, pathways established through the pallidum are distinguished from those passing through the substantia nigra pars reticularis. Abbreviations: DA, dopamine; NA, noradrenaline (norepinephrine); 5-HT, 5-hydroxytryptamine (serotonin).

Cerebral cortex

Ventral anterior and ventral lateral nuclei of thalamus

Caudate nucleus

Habenula

Centromedian nucleus of thalamus

Putamen Subthalamic nucleus Superior colliculus

Globus pallidus (lateral segment) Globus pallidus (medial segment) Substantia nigra, pars compacta

Substantia nigra, pars reticularis

Glutamatergic neurones Crus cerebri GABAergic neurones Dopaminergic neurones Pedunculopontine nucleus

Brachium conjunctivum

Fig. 24.10 The principal connections of the basal ganglia with the diencephalon and brainstem.

contin uity of cell groups A9 and 10 is thus reflected in the relative posi- As with the dorsal striatum, efferents from the ventral striatum tions of their ascending fibres in the subthalamus and hypothalamus project to the pallidum (in this case, the ventral pallidum) and the (the H fields and medial forebrain bundle, respectively), as well as in substantia nigra pars reticularis (see Figs 24.9–24.10). In the latter the lateromedial topography of the dorsal and ventral striata (see Fig. case, the connection is both direct and indirect, via the subthalamic 24.6), which in turn have contiguous and overlapping sources of corti- nucleus. The projections from the pars reticularis are as described for 368 cal afferents. the dorsal system, but axons from the ventral pallidum reach Substantia nigra the thalamic mediodorsal nucleus (which projects to cingulate and capsule, both pathways unite in the subthalamic region, where they prefrontal association cortex) and midline nuclei (which project to the follow a horizontal hairpin trajectory, and turn upwards to enter the hippocampus). Ventral pallidal axons also reach the habenular complex thalamus as the thalamic fasciculus. The trajectory circumnavigates the of the limbic system. zona incerta and creates the so-called ‘H’ fields of Forel (see Figs 24.5, The brain areas beyond the basal ganglia, substantia nigra and sub- 24.11). Within the thalamus, pallidothalamic fibres end in the ventral thalamic nucleus to which both ventral and dorsal systems appear to anterior and ventral lateral nuclei and in the intralaminar centromedian project are, therefore, the prefrontal association and cingulate cortices nucleus. These in turn project excitatory (presumed glutamatergic) and the deep superior colliculus. fibres primarily to the frontal cortex, including the primary and sup- plementary motor areas. The internal pallidum also projects fibres caudally to the pedunculopontine nucleus, as described below (see GLOBUS PALLIDUS Fig. 24.10).

The globus pallidus (pallidum) lies medial to the putamen and lateral to the internal capsule. It consists of external (lateral) and internal SUBTHALAMIC NUCLEUS (medial) segments separated by an internal medullary lamina, which have substantially different connections. Both segments receive large The subthalamic nucleus is a biconvex, lens-shaped nucleus in the 24 numbers of fibres from the striatum and subthalamic nucleus. The subthalamus of the diencephalon. It lies medial to the internal capsule, external segment projects to the subthalamic nucleus as part of the immediately rostral to the level at which the latter becomes continuous ‘indirect pathway’. The internal segment is considered to be a homo- with the crus cerebri of the midbrain (see Figs 24.5, 24.3). Within the logue of the pars reticularis of the substantia nigra, with which it nucleus, small interneurones intermingle with large multipolar cells shares similar cellular and connectional properties. Together, therefore, with dendrites that extend for about one-tenth the diameter of the these structures constitute the main output of the basal ganglia to nucleus. Dorsally, the nucleus is encapsulated by axons, many of which CHAPTER other levels of the neuraxis, principally to the thalamus and superior are derived from the subthalamic fasciculus; these carry a major colliculus. GABAergic projection from the external segment of the globus pallidus The cell density of the globus pallidus is less than one-twentieth that as part of the indirect pathway. The nucleus also receives afferents from of the striatum. The morphology of the majority of cells is identical in the cerebral cortex. It is unique in the intrinsic circuitry of the basal the two segments. They are large multipolar GABAergic neurones that ganglia in that its cells are glutamatergic and project excitatory axons closely resemble those of the substantia nigra pars reticularis. The den- to both the globus pallidus and substantia nigra pars reticularis. Within dritic fields are discoid, with planes at right angles to incoming striato- the pallidum, subthalamic efferent fibres end predominantly in the pallidal axons, each of which, therefore, potentially contacts many internal segment but many also end in the external segment. pallidal dendrites en passant. This arrangement, coupled with the diam- The subthalamic nucleus plays a central role in the normal function eters of the dendritic fields (500 μm), suggests that a precise topo- of the basal ganglia and in the pathophysiology of basal ganglia-related graphical organization is unlikely within the pallidum. disorders. Destruction of the nucleus, which occurs rarely as a result of Striatopallidal fibres are of two main types. They project to either stroke, results in the appearance of violent, uncontrolled involuntary the external or the internal pallidum. Those projecting to the external movements of the contralateral side of the body, known as ballism segment constitute the beginning of the so-called ‘indirect pathway’. (hemiballismus). The subthalamic nucleus is crucially involved in the They utilize GABA as their primary transmitter and also contain pathophysiology of Parkinson’s disease and is a target for functional enkephalin. Efferent axons from neurones in the external segment pass neurosurgical therapy of the condition. (For a detailed description of through the internal capsule in the subthalamic fasciculus and travel to the anatomical structure and variability of the subthalamic nucleus seen the subthalamic nucleus (Fig. 24.11). using high-field magnetic resonance imaging (MRI) with histological Striatopallidal axons destined for the internal pallidum constitute validation, see Massey et al (2012) and Massey and Yousry (2010).) the so-called ‘direct pathway’. Like the indirect projection, these utilize GABA as their primary transmitter but they also contain substance P and dynorphin. Efferent axons from the internal pallidum project SUBSTANTIA NIGRA through the ansa lenticularis and fasciculus lenticularis (see Fig. 24.11). The former runs round the anterior border of the internal capsule and The substantia nigra is a nuclear complex deep to the crus cerebri the latter penetrates the capsule directly. Having traversed the internal in each cerebral peduncle of the midbrain (see Fig. 21.16); its

Fibres of internal capsule Anterior nuclear group of thalamus GABAergic neurones Lateral nuclear group Glutamatergic neurones of thalamus

Thalamic fasciculus (field H1 of Forel) Field H2 of Forel (continuation of fasciculus lenticularis) Third ventricle

External capsule Internal capsule Dentatothalamic and rubrothalamic fibres Claustrum Lenticular fasciculus (fasciculus lenticularis) Zona incerta Putamen Prerubral field (field H of Forel) Subthalamic fasciculus Prerubral nucleus Globus pallidus, of tegmental field lateral segment Subthalamic nucleus Globus pallidus, Column of fornix medial segment Endopeduncular nucleus (nucleus of ansa lenticularis) Ansa lenticularis Fig. 24.11 A coronal section of the brain showing the major connections of the basal ganglia with the diencephalon. 369 BASAL GANGLIA

cytoarchitecture and connections are described in Chapter 21. Briefly, it A B consists of a pars compacta, a pars reticularis and a smaller pars lateralis. Thalamus The pars compacta and pars lateralis correspond to the dopaminergic Caudate nucleus cell group A9. The pars compacta projects heavily to the caudate nucleus and putamen; lesser projections end in the globus pallidus and the subthalamic nucleus. The neurones of the pars reticularis and the inter- nal pallidum collectively constitute the output neurones of the basal ganglia system. In Parkinson’s disease, the levels of dopamine in the Putamen substantia nigra and striatum decrease dramatically as a result of the degeneration of pars compacta neurones and their terminals in Globus pallidus the neostriatum. (lateral segment) Globus pallidus (medial segment) PEDUNCULOPONTINE NUCLEUS

3 Subthalamic The pedunculopontine nucleus (nucleus tegmentalis pedunculoponti- nucleus N nus) lies in the dorsolateral part of the pontomesencephalic tegmen- O Substantia I tum. Anterograde tract-tracing studies in non-human primates and nigra rodents have revealed multiple afferent and efferent connections between the pedunculopontine nucleus and the basal ganglia, cerebel- Pedunculopontine SECT lum, substantia nigra, thalamus, cerebral cortex and spinal cord. nucleus The pedunculopontine nucleus receives GABAergic efferent fibres from the globus pallidus and substantia nigra. In animal models of Glutamatergic neurones Parkinson’s disease, these projections are overactive and the peduncu- lopontine nucleus is inhibited. Taken in conjunction with other experi- GABAergic neurones mental evidence, this suggests that the pedunculopontine nucleus is involved in the pathophysiology of disturbances of locomotion, gait and posture in Parkinson’s disease (Mena-Segovia et al 2004). That Dopaminergic neurones said, the variation in nuclear connectivity between quadripedal and Bold lines indicate overactivity bipedal animals must be considered when evaluating evidence from Interrupted lines indicate underactivity any non-primate animal model (Alam et al 2011, Stein and Aziz 2012).) Fig. 24.12 The pathophysiology of Parkinson’s disease (A) and dyskinesias (B). (Adapted with permission from Crossman AR, Neary D 2010 Neuroanatomy, 4th edition. Edinburgh: Churchill Livingstone.) PATHOPHYSIOLOGY OF BASAL GANGLIA DISORDERS treatment of Parkinson’s disease resemble those seen in Huntington’s The basal ganglia might be said to guide intention into action. As far disease, tardive dyskinesia and ballism. Experimental evidence suggests as their role in movement control is concerned, they appear to promote that these may share a common neural mechanism (see Fig. 24.12). and support patterns of behaviour that are appropriate in the prevail- Thus, the indirect pathway becomes underactive, e.g. due to the effects ing circumstances and to inhibit unwanted or inappropriate move- of dopaminergic drugs in Parkinson’s disease, or the degeneration of ments. This is exemplified by disorders of the basal ganglia, which are the striatopallidal projection to the external pallidum in Huntington’s characterized, depending on the underlying pathology, by an inability disease. This leads to physiological inhibition of the subthalamic to initiate and execute wanted movements (as in Parkinson’s disease) nucleus by overactive pallidosubthalamic neurones. The involvement or an inability to prevent unwanted movements (as in Huntington’s of the subthalamic nucleus explains why the dyskinetic movements of disease). levodopa-induced dyskinesia and Huntington’s disease resemble those Parkinson’s disease is the most common pathological condition of ballism produced by lesion of the subthalamic nucleus. Underactiv- affecting the basal ganglia. It is characterized by akinesia, muscular ity of the subthalamic nucleus removes the excitatory drive from inter- rigidity and tremor due to degeneration of the dopaminergic neurones nal pallidal neurones, which are known to be underactive in dyskinesias of the substantia nigra pars compacta (which project to the striatum in (Crossman 1990). Once again, this anatomical model of basal ganglia the nigrostriatal pathway). As a consequence, dopamine terminals are function is an oversimplification. Whilst it is true that underactivity of lost in the striatum and dopamine levels are severely depleted. the internal globus pallidus is associated with dyskinesias, it is also Dopamine receptors, which are located on medium spiny neurones and known that lesions of the globus pallidus alleviate them. This so-called are the target of the nigrostriatal pathway, are spared. ‘pallidotomy paradox’ suggests that the dynamic aspects of pallidal and Dopamine appears to have a dual action on medium spiny striatal nigral efferent activity are important factors in the generation of dyski- neurones. It inhibits those of the indirect pathway and excites those of nesia (Marsden and Obeso 1994). A more recent ‘rate model’ of basal the direct pathway. Consequently, when dopamine is lost from the ganglia function hypothesizes that specific components oscillate in neu- striatum, the indirect pathway becomes overactive and the direct ronal synchrony at different frequencies to select specific voluntary pathway becomes underactive (Fig. 24.12). Overactivity of the striatal motor patterns (Little and Brown 2014). Both rodent electrophysiology projection to the external pallidum results in inhibition of pallidosub- and recording from deep-brain electrodes implanted in patients with thalamic neurones and, consequently, overactivity of the subthalamic Parkinson’s disease suggest that neuronal activity suppression in the nucleus. Subthalamic efferents mediate excessive excitatory drive to the 8–30 Hz beta frequency band correlates with relief of parkinsonian internal globus pallidus and substantia nigra pars reticularis. This is akinesia and rigidity by both dopamine and subthalamic nucleus deep- exacerbated by underactivity of the GABAergic, inhibitory direct brain stimulation. pathway. Overactivity of basal ganglia output then inhibits the motor There is evidence that dysfunction of the basal ganglia is also thalamus and its excitatory thalamocortical connections. While this involved in other complex, less well understood, behavioural disorders. description is little more than a first approximation of the underlying In animal experiments, lesions of the basal ganglia, especially of the pathophysiology, this model of the basis of parkinsonian symptoms caudate nucleus, induce uncontrollable hyperactivity (e.g. obstinate has led to the introduction of new neurosurgical approaches to the progression, incessant pacing and other constantly repeated behav- treatment of Parkinson’s disease, based upon lesioning and deep-brain iours). This and other evidence has led to the notion that the corpus stimulation of the internal globus pallidus and subthalamic nucleus striatum enables the individual to make motor choices and to avoid (see below). ‘stimulus-bound’ behaviour. Positron emission tomography (PET) The current medical treatment for Parkinson’s disease largely relies studies in humans have shown that sufferers from obsessive–compulsive on levodopa (L-DOPA; L-dihydroxyphenylalanine), the immediate disorder (OCD), which is characterized by repeated ritualistic motor metabolic precursor of dopamine, or on dopamine agonists. Whilst behaviour and intrusive thoughts, exhibit abnormal activity in the pre- these usually provide good symptomatic relief for many years, eventu- frontal cortex and caudate nuclei. There are similar suggestive observa- ally they lead to the development of side-effects, including dyskinesias. tions in childhood attention deficit hyperactivity disorder (ADHD) and 370 The involuntary movements that occur as a consequence of long-term Gilles de la Tourette’s syndrome. In this respect, it may be significant Pathophysiology of basal ganglia disorders that the basal ganglia, besides receiving connections from the frontal lobe and limbic cortices, also have an ascending influence on the pre- frontal and cingulate cortices through the substantia nigra pars reticu- laris and dorsomedial and ventromedial thalamus in parallel, functionally segregated, corticostriatothalamic circuits subserving loco- motor, oculomotor, cognitive and affective behaviour (Alexander et al 1986; see Fig. 24.9). Before the advent of levodopa therapy, neurosurgery for Parkinson’s disease was commonplace. The globus pallidus and thalamus were favoured targets for chemical or thermal lesions. Pallidotomy and tha- lamotomy often improved rigidity and tremor, but they produced little consistent beneficial effect on akinesia. With the arrival of levodopa therapy, which very effectively alleviates akinesia, the surgical treatment of Parkinson’s disease underwent a progressive decline. It soon became clear, however, that long-term use of levodopa was associated with a number of side-effects such as dyskinesias, ‘wearing-off’ and the ‘on–off’ 24 phenomenon. More recent advances in understanding the pathophysio- logy of movement disorders, and of Parkinson’s disease in particular, have stimulated a renaissance in the use of neurosurgery to treat move- ment disorders. Lesioning the subthalamic nucleus in experimental primates that had been made parkinsonian with the neurotoxin MPTP dramatically CHAPTER alleviated the parkinsonian symptoms, suggesting that that the subtha- lamic nucleus might be an appropriate clinical target (Pereira and Aziz 2006). While therapeutic surgical lesions of the subthalamic nucleus can alleviate tremor, rigidity and bradykinesia in patients with Parkin- son’s disease, the risk of side-effects is not trivial: the subthalamic nucleus is a small structure wrapped by fibres of passage and close to the hypothalamus and internal capsule. In 1992, Laitinen et al reintroduced pallidotomy for the treatment of end-stage Parkinson’s disease, but confined the lesions to the postero- ventral part of the internal pallidum. These lesions were found to be extremely reliable in abolishing contralateral rigidity and drug-induced dyskinesias, with slightly less efficacy on tremor and bradykinesia Fig. 24.13 A magnetic resonance image showing the placement of (Laitinen et al 1992). deep-brain stimulating electrodes (arrows) bilaterally in the globus pallidus Implantation of deep-brain electrodes, through which high- of a patient with Parkinson’s disease. (Courtesy of Mr L Zrinzo and frequency pulses generated by a pacemaker could inhibit cells in the Professor M Hariz, National Hospital of Neurology and Neurosurgery, vicinity, has been a concept since the early 1970s but did not become London, UK.) a widespread reality until the late 1980s, as a result of technological advances. The introduction of the technique of deep-brain stimulation (DBS), which avoids making permanent lesions, made bilateral surgery safer. There have been numerous reports of the effectiveness of both bilateral pallidal and subthalamic nucleus stimulation in Parkinson’s disease (Rodriguez-Oroz et al 2012; Figs 24.13–24.14). Subthalamic nucleus stimulation is favoured by most for relieving akinesia and rigid- ity, with pallidal stimulation considered to ameliorate dyskinesias. Sub- thalamic stimulation is more effective than pallidal stimulation in allowing patients to reduce their anti-parkinsonian medication. Tremor is best relieved by stimulation of either the contralateral ventral inter- mediate thalamic nucleus or the zona incerta. Another manifestation of basal ganglia dysfunction is dystonia, which is characterized by increased muscle tone and abnormal pos- tures. Dystonia may occur as a consequence of levodopa treatment in Parkinson’s disease or inherited disease (e.g. early-onset torsion, or Oppenheim’s dystonia, an autosomal dominant disorder most com- monly associated with a mutation in the DYT1 gene that encodes torsin A). Although the pathophysiological basis of dystonia is unclear (Hallett 2006), it is probably caused by underactivity of basal ganglia output. The observation that painful dystonic posturing of the limbs in parkin- sonian patients responds dramatically to bilateral pallidal stimulation led to the development of bilateral pallidal stimulation for dystonia. Intriguingly, in dystonia the pallidal neurones are held to fire at rates below normal, and so it is open to question how this stimulation works. Moreover, while the effect of stimulation is immediate in the case of Parkinson’s disease, in dystonia the improvement may take weeks to emerge, suggesting that the neural mechanism(s) underlying the thera- peutic effect of stimulation for these conditions is/are different and implicating a role for neuroplasticity in dystonia. An even more recent neurosurgical development in the therapy of Parkinson’s disease has involved targeting the pedunculopontine nucleus, which both receives fibres from and sends fibres to basal ganglia and related nuclei. Recently, DBS of the pedunculopontine nucleus through implanted electrodes has been applied in drug-resistant Fig. 24.14 A magnetic resonance image showing the placement of akinetic parkinsonian patients (Fig. 24.15). Low-frequency stimulation deep-brain stimulating electrodes (arrows) bilaterally in the subthalamic alleviates postural instability and on-medication gait freezing and nucleus of a patient with Parkinson’s disease. (Courtesy of Mr L Zrinzo falling, symptoms that conventional medication and surgery fail to and Professor M Hariz, National Hospital of Neurology and Neurosurgery, improve. London, UK.) 371 BASAL GANGLIA

PM line

SC IC B

Floor fourth ventricle 3

N A O I Fig. 24.15 Stimulation locations represented in Montreal Neurological Institute space in pedunculopontine nucleus deep-brain stimulation for Parkinson’s disease. The relative extent of the pedunculopontine nucleus has been outlined based on choline acetyltransferase immunohistochemistry. A, Coronal SECT view. B, Sagittal view. Abbreviations: IC, inferior colliculus; PM line, pontomesencephalic line connecting the pontomesencephalic junction to the caudal end of the inferior colliculi; SC, superior colliculus. (Courtesy of Mr J A Hyam, Departments of Neurosurgery, Oxford University Hospitals and adapted from Thevathasan et al., Alpha oscillations in the pedunculopontine nucleus correlate with gait performance in parkinsonism. Brain. 2012 Jan;135(Pt 1):148–60.)

KEY REFERENCES

Alexander GE, DeLong MR, Strick PL 1986 Parallel organization of function- A review that highlights some limitations of the anatomical model of basal ally segregated circuits linking basal ganglia and cortex. Ann Rev Neu- ganglia function. rosci 9:357–82. A landmark publication setting out a conceptual framework for the way in Mena-Segovia J, Bolam JP, Magill PJ 2004 Pedunculopontine nucleus and which the basal ganglia and cerebral cortex process different types of basal ganglia: distant relatives or part of the same family? Trends Neu- information through largely distinct parallel circuits based on known rosci 27:585–8. anatomical connectivity. A review presenting a persuasive argument for functional similarities and intimate reciprocal connections between pedunculopontine nucleus and other Crossman AR 1990 A hypothesis on the pathophysiological mechanisms basal ganglia structures. that underlie levodopa- or dopamine agonist-induced dyskinesia in Parkinson’s disease: implications for future strategies in treatment. Mov Penney JB Jr, Young AB 1986 Striatal inhomogeneities and basal ganglia Disord 5:100–8. function. Mov Disord 1: 3–15. A seminal paper outlining the limitations of dopaminergic treatments of A landmark publication, introducing some of the basic concepts behind Parkinson’s disease and translating an anatomical model of basal ganglia current models of the pathophysiology of Parkinson’s disease and function into potential neurosurgical treatment targets. Huntington’s disease. Hallett M 2006 Pathophysiology of dystonia. Neural Transm Suppl 70: Pereira EA, Aziz TZ 2006 Parkinson’s disease and primate research: past, 485–8. present, and future. Postgrad Med J 82:293–9. An explanation of the abnormalities in basal ganglia function involved in A review of the non-human primate experiments that led to contemporary dystonia. functional neurosurgery for Parkinson’s disease that discusses the anatomical model of basal ganglia function. Laitinen LV, Bergenheim AT, Hariz MI 1992 Ventroposterolateral pallidot- omy can abolish all Parkinsonian symptoms. Stereotact Funct Neuro- Rodriguez-Oroz MC, Moro E, Krack P 2012 Long-term outcomes of surgical surg 58:14–21. therapies for Parkinson’s disease. Mov Disord 27:1718–28. A key paper that ignited widespread interest in functional neurosurgery for A contemporary review and results of five-year, long-term follow-up of Parkinson’s disease. patients receiving deep-brain stimulation for Parkinson’s disease. Marsden CD, Obeso JA 1994 The functions of the basal ganglia and the paradox of stereotaxic surgery in Parkinson’s disease. Brain 117:877–97.

372 CHAPTER

Inner ear 38

The inner ear contains the organ of hearing and the organs of balance. A Ampulla of anterior All are located within the labyrinth, a series of interlinked cavities in semicircular canal the petrous temporal bone containing interconnected membranous Ampulla of lateral sacs and ducts. All spaces within the labyrinth are filled with fluid. The semicircular canal Crus commune different sacs contain sensory epithelia consisting of supporting cells Cupula of cochlea (common limb) and mechanosensory cells, the hair cells that underlie acoustico-lateralis sensory systems in all vertebrates. In humans, there are six such mech- anosensory epithelia: the organ of Corti within the cochlea (the hearing organ); the utricle and saccule (static balance organs); and the cristae of the semicircular canals (dynamic balance organs). Whilst sharing the same basic structure, hair cells and the accessory systems that sur- round them show specific adaptations to each of the different sensory modalities. The disarticulated temporal bone is described in detail in Chapter 37. The internal acoustic meatus (internal auditory canal) and bony and membranous labyrinths are described here.

OSSEOUS (BONY) LABYRINTH Vestibule Oval window Round window Ampulla of posterior semicircular canal The bony labyrinth consists of the vestibule (sacculus and utriculus/ B Anterior semicircular canal saccule and utricle), semicircular canals and cochlea, which are all cavi- Opening of crus commune Crus commune ties lined by periosteum and which contain the membranous labyrinth Elliptical recess (Fig. 38.1). The bone is denser and harder than that of the other parts of the petrous bone, and it is therefore possible, particularly in young Vestibular crest Posterior skulls, to dissect the bony labyrinth out from the petrous temporal semicircular canal bone. Spherical recess The osseous and membranous labyrinths are filled with fluid (Fig. 38.2). The gap between the internal wall of the osseous labyrinth and the external surface of the membranous labyrinth is filled with peril- ymph, a clear fluid with an ionic composition similar to that of other extracellular fluids, i.e. low in potassium ions and high in sodium and calcium. The membranous labyrinth contains endolymph, a fluid with an ionic composition more like that of cytosol, i.e. high in potassium ions and low in sodium and calcium. Moreover, the endolymphatic compartment has an electrical potential that is approximately 80 mV more positive than the perilymphatic compartment (the endolym- phatic potential). These differences in ionic composition and potential, maintained by homeostatic tissues in the walls of the labyrinth, are Lateral semicircular canal essential to maximize the sensitivity of the mechanosensory hair cells Osseous Scala vestibuli Round window Orifice of vestibular aqueduct that convert the vibrations set up in the inner ear fluids by head or spiral lamina Scala tympani Orifice of Cochlear recess sound movements into electrical signals that are transmitted via the cochlear recess vestibulocochlear nerve to the vestibular and cochlear nuclei, respec- Anterior tively, in the brainstem. semicircular duct C Helicotrema Ampulla of anterior semicircular duct Crus commune Utricle Ampulla of lateral VESTIBULE Hamulus semicircular duct Posterior The vestibule is the central part of the bony labyrinth and lies medial Saccule semicircular to the tympanic cavity, posterior to the cochlea and anterior to the duct semicircular canals (see Fig. 38.1). It is somewhat ovoid in shape but flattened transversely, and (on average) measures 5 mm from front to back and vertically, and 3 mm across. In its lateral wall is the opening of the oval window (fenestra vestibuli), into which the base of the Cochlear duct stapes inserts, and to which the base of the stapes is attached by an anular ligament. Anteriorly, on the medial wall, is a small spherical Vestibule recess that contains the saccule; it is perforated by several minute holes, Oval window the macula cribrosa media, which transmit fine branches of the vestibu- Lateral lar nerve to the saccule. Behind the recess is an oblique vestibular crest, Ductus reuniens semicircular duct the anterior end of which forms the vestibular pyramid. This crest Round window Ampulla of posterior divides below to enclose a small depression, the cochlear recess, which semicircular duct is perforated by vestibulocochlear fascicles as they pass to the vestibular Endolymphatic duct end of the cochlear duct. Posterosuperior to the vestibular crest, in the Endolymphatic sac Utriculosaccular duct roof and medial wall of the vestibule, is the elliptical recess (see Fig. Fig. 38.1 The left bony labyrinth. A, Lateral aspect. B, Interior. C, The 38.1B), which contains the utricle. The pyramid and adjoining part of membranous labyrinth (blue) projected on to the bony labyrinth. 641 INNER EAR

Interglobular spaces Semicircular duct Tunica propria canals in 10 human skulls led Blanks et al (1975) to suggest that containing cartilage cells (containing endolymph) the planes of the three ipsilateral canals are not completely perpen- dicular to each other. The angles were measured as: lateral/anterior 111.76 ± 7.55°, anterior/posterior 86.16 ± 4.72°, posterior/lateral 95.75 ± 4.66°. The planes of similarly orientated canals of the two sides also showed some departure from being parallel: left anterior/right posterior 24.50 ± 7.19°, left posterior/right anterior 23.73 ± 6.71°, left lateral/right lateral 19.82 ± 14.93°. The same observers (Curthoys et al 1977) also measured the dimensions of the canals. The mean radii of the osseous canals were found to be as follows: lateral 3.25 mm, ante- rior 3.74 mm, posterior 3.79 mm. The diameters of the osseous canals are 1 mm (minor axis) and 1.4 mm (major axis). The membranous ducts within them are much smaller, but are also elliptical in transverse section, and have major and minor axes of 0.23 and 0.46 mm (see Fig. 38.2). Representative means for ampullary dimensions are as follows: length 1.94 mm, height 1.55 mm. Phylogenetic studies suggest that the arc sizes of the semicircular canals in humans and other primates may be functionally linked to sensory control of body movements. The angulation and dimensions of the canals may be related to locomotor behaviour and possibly to agility, or more specifically to the frequency spectra of natural head movements (see review by Spoor and Zonneveld (1998)). 4

COCHLEA N Semicircular canal The cochlea (from the Greek cochlos for snail) is the most anterior part (containing perilymph) Endosteum Blood vessels of the labyrinth, lying in front of the vestibule (see Figs 38.1 and 38.9A). CTIO Fig. 38.2 A transverse section through the left posterior semicircular It is 5 mm from base to apex, and 9 mm across its base. Its apex, or E

S canal and duct. cupula, points towards the anterosuperior area of the medial wall of the tympanic cavity (see Fig. 38.9A). Its base faces the bottom of the internal acoustic meatus and is perforated by numerous apertures for the cochlear nerve. The cochlea has a conical central bony core, the the elliptical recess are perforated by a number of holes, the macula modiolus, and a spiral canal runs around it. A delicate osseous spiral cribrosa superior; those in the former transmit nerves to the utricle and lamina (or ledge) projects from the modiolus, partially dividing the those in the latter transmit nerves to the ampullae of the anterior and canal (see Fig. 38.9B). Within this bony spiral lies the membranous lateral semicircular canals (see Fig. 38.1B). The region of the pyramid cochlear duct, attached to the modiolus at one edge and to the outer and elliptical recess corresponds to the superior vestibular area in the cochlear wall by its other edge. There are therefore three longitudinal internal acoustic meatus (see Fig. 38.3). The vestibular aqueduct opens channels within the cochlea. The middle canal (the cochlear duct or below the elliptical recess. It reaches the posterior surface of the petrous scala media) is blind and ends at the apex of the cochlea; its flanking bone and contains one or more small veins and part of the membra- channels communicate with each other at the modiolar apex at a nous labyrinth, the endolymphatic duct (see Fig. 38.1C). In the poste- narrow slit, the helicotrema (see Fig. 38.1C). Two elastic membranes rior part of the vestibule are the five openings of the semicircular canals; form the upper and lower bounds of the scala media. One is Reissner’s in its anterior wall is an elliptical opening that leads into the scala membrane, the thin vestibular membrane that separates the scala media vestibuli of the cochlea. from the scala vestibuli. The other is the basilar membrane, which forms the partition between the scala media and the scala tympani. The organ of Corti, the sensory epithelium of hearing, sits on the inner SEMICIRCULAR CANALS surface of the basilar membrane. At the base of the scala vestibuli is the oval window (fenestra vestibuli), which leads on to the vestibular cavity The three semicircular canals – anterior (superior), posterior and lateral but is sealed by the footplate of the stapes. The scala tympani is sepa- (horizontal) – are located posterosuperior to the vestibule (see Fig. rated from the tympanic cavity by the secondary tympanic membrane 38.1). They are compressed from side to side and each forms approxi- at the round window (fenestra cochleae). The central cochlear core, the mately two-thirds of a circle. They are unequal in length but similar in modiolus, has a broad base near the lateral end of the internal acoustic diameter along their lengths, except where they bear a terminal swell- meatus, where it corresponds to the spiral tract (tractus spiralis forami- ing, an ampulla, which is almost twice the diameter of the canal. nosus). There are several openings in this area for the fascicles of the 1 The anterior semicircular canal is 15–20 mm long. It is vertical in cochlear nerve: those for the first 1 2 turns run through the small holes orientation and lies transverse to the long axis of the petrous temporal of the spiral tract, and those for the apical turn run through the hole bone under the anterior surface of its arcuate eminence. The eminence that forms the centre of the tract. Canals from the spiral tract go through may not accurately coincide with this semicircular canal but may the modiolus and open in a spiral sequence into the base of the osseous instead be adapted to the occipitotemporal sulcus on the inferior spiral lamina. Here the small canals enlarge and fuse to form Rosenthal’s surface of the temporal lobe of the brain. The ampulla at the anterior canal, a spiral canal in the modiolus that follows the course of the end of the canal opens into the upper and lateral part of the vestibule. osseous spiral lamina and contains the spiral ganglion (see Fig. 38.9B). Its other end unites with the upper end of the posterior canal to form The main tract continues through the centre of the modiolus to the the crus commune (common limb), which is 4 mm long, and opens cochlear apex. 3 into the medial part of the vestibule. The osseous cochlear canal spirals for about 2 4 turns around the The posterior semicircular canal is also vertical but curves backwards modiolus and is 35 mm long. At its first turn, the canal bulges towards almost parallel with the posterior surface of the petrous bone. It is the tympanic cavity, where it underlies the promontory. At the base of 18–22 mm long and its ampulla opens low in the vestibule, below the the cochlea, the canal is 3 mm in diameter but it becomes progressively cochlear recess where the macula cribrosa inferior transmits nerves to reduced in diameter as it spirals apically to end at the cupula. In addi- it. Its upper end joins the crus commune. tion to the round and oval windows, which are the two main openings The lateral canal is 12–15 mm long and its arch runs horizontally at its base, the canal has a third, smaller opening for the cochlear aque- backwards and laterally. Its anterior ampulla opens into the upper and duct or canaliculus. The latter is a minute funnel-shaped canal that runs lateral angle of the vestibule, above the oval window and just below the to the inferior surface of the petrous temporal bone; it transmits a small ampulla of the anterior canal; its posterior end opens below the opening vein to the inferior petrosal sinus (see p. 437) and connects the sub- of the crus commune. arachnoid space to the scala tympani. The two lateral semicircular canals of the two ears are often described The osseous or primary spiral lamina is a ledge that projects from as being in the same plane and the anterior canal of one side as being the modiolus into the osseous canal like the thread of a screw (see Fig. almost parallel with the opposite posterior canal. However, measure- 38.9B). It is attached to the inner edge of the basilar membrane and 642 ments of the angular relations of the planes of the semicircular osseous ends in a hook-shaped hamulus at the cochlear apex, partly bounding Membranous labyrinth the helicotrema (see Fig. 38.1C). From Rosenthal’s canal, many tiny canals, the habenula perforata, radiate through the osseous lamina to Facial nerve area its rim, where they each carry a fascicle of the cochlear nerve through the foramen nervosum to the organ of Corti. A secondary spiral lamina Superior projects inwards from the outer cochlear wall towards the osseous spiral vestibular area lamina and is attached to the outer edge of the basilar membrane. It is Transverse crest most prominent in the lower part of the first turn; the gap between the Inferior two laminae increases progressively towards the cochlear apex, which vestibular area means that the basilar membrane is wider at the apex of the cochlea than at the base. Tractus spiralis Foramen singulare foraminosus cut obliquely MICROSTRUCTURE OF THE BONY LABYRINTH Fig. 38.3 The fundus of the left internal acoustic meatus, exposed by a The wall of the bony labyrinth is lined by fibroblast-like perilymphatic section through the petrous part of the left temporal bone nearly parallel to the line of its superior border. cells and extracellular matrix fibres (see Fig. 38.2). The morphology of the cells varies in different parts of the labyrinth. Where the perilym- phatic space is narrow, as in the cochlear aqueduct, the cells are reticular or stellate in form; they give off sheet-like cytoplasmic extensions that cross the extracellular space. Where the space is wider, as in the scalae INTERNAL ACOUSTIC MEATUS vestibuli and tympani of the cochlea and much of the vestibule, the perilymphatic cells on the periosteum and the external surface of the The internal acoustic meatus (internal acoustic/auditory canal) is sepa- membranous labyrinth are extremely flat and resemble a squamous rated from the internal ear at its lateral fundus by a vertical plate divided epithelium. Elsewhere, on parts of the perilymphatic surface of the unequally by a transverse (falciform) crest (Fig. 38.3). Five nerves – facial, nervus intermedius, cochlear, superior and inferior vestibular –

basilar membrane, the cells are cuboidal. 38 Recent evidence suggests that micropores or canaliculi (canaliculi pass through openings in the vertical plate, above and below the transverse crest. The facial and superior vestibular nerves enter canals perforantes) (0.2–23.0 μm diameter) are more widely distributed ER within the bony surfaces lining the perilymphatic space than was previ- that are superior to the crest. The facial nerve is anterior to the superior vestibular nerve, from which it is separated at the lateral end of the PT ously suspected; they are numerous in the peripheral and modiolar A portions of the osseous spiral lamina and the floor of the scala tympani, meatus by a vertical ridge of bone (Bill’s bar). The nervus intermedius but sparse in the osseous wall of the scala vestibuli. The proposal that lies between the facial motor root and the superior vestibular nerve, to CH these canaliculi normally provide an extensive fluid communication which it may be adherent. The superior vestibular area contains open- channel between the scala tympani and the spiral canal of the cochlea ings for nerves to the utricle and anterior and lateral semicircular ducts. could have implications not only for novel drug-based cochlear thera- Below the crest, an anterior cochlear area contains a spiral of small pies delivered via the scala tympani and the delivery of stem cells or holes, the tractus spiralis foraminosus, which encircles the central coch- appropriate cell lines into the deafened cochlea, but also for the design lear canal. Behind this, the inferior vestibular area contains openings of implanted perimodiolar electrode arrays (Shepherd and Colreavy for saccular nerves, and most posteroinferiorly, a single hole (foramen 2004). (For further reading about the changes in the inner ear that are singulare) admits the nerve to the posterior semicircular duct. It has induced by implanted cochlear electrodes, both acute and long-term, been suggested that vascular loops in the internal acoustic meatus see Keifer et al (2006).) (from the anterior inferior cerebellar artery) might generate pulsatile tinnitus. Composition of inner ear fluids MEMBRANOUS LABYRINTH Perilymph resembles cerebrospinal fluid in ionic composition, partic- ularly in the scala tympani. Its composition differs a little between The membranous labyrinth is separated from the periosteum by a space the two cochlear scalae: concentrations of potassium, glucose, amino that contains perilymph and a web-like network of fine blood vessels acids and proteins are greater in the scala vestibuli. This has led to (see Figs 38.1C, 38.2). It can be divided into two major regions: the the suggestion that perilymph in the scala vestibuli is derived from vestibular apparatus and the cochlear duct. plasma via the endothelial boundary of the cochlear blood vessels, whereas the perilymph in the scala tympani contains some cerebros- pinal fluid derived from the subarachnoid spaces via the cochlear VESTIBULAR APPARATUS canaliculus. However, the lack of significant bulk flow suggests that perilymph homeostasis is predominantly locally regulated. Perilymph The vestibular apparatus consists of three membranous semicircular contains approximately 5 mM K+, 150 mM Na+, 120 mM Cl– and canals that communicate with the utricle, a membranous sac leading 1.5 mM Ca2+. into a smaller chamber, the saccule, via the utriculosaccular duct. This The membranous labyrinth is filled with endolymph, a fluid pro- Y-shaped duct has a side branch to the endolymphatic duct, which duced by the marginal cells of the stria vascularis and the dark cells of passes to the endolymphatic sac, a small but functionally important the vestibule (see review by Wangemann and Schacht (1996)) (see Fig. expansion situated under the dura of the petrous temporal bone. From 38.2B). Whatever their relative contributions, endolymph probably cir- the saccule, a narrow canal, the ductus reuniens, leads to the base of culates in the labyrinth; it enters the endolymphatic sac, where it is the cochlear duct. These various ducts and sacs form a closed system transferred into the adjacent vascular plexus via the specialized epithe- of intercommunicating channels. Endolymph is resorbed into the cere- lium of the sac. Pinocytotic removal of fluid may also occur in other brospinal fluid from the endolymphatic sac, which therefore provides labyrinthine regions. the site for the drainage of endolymph for the entire membranous Endolymph contains greater K+ (150 mM) and Cl– (130 mM) con- labyrinth. centrations and lower Na+ (2 mM) and Ca2+ (20 μM) concentrations The terminal fibres of the vestibular nerve are connected to the five than perilymph. The high potassium concentration is important for the specialized sensory epithelia (two maculae and three cristae) in the function of the mechanosensory hair cells and is maintained by the walls of the membranous labyrinth. Maculae are flat plaques of sensory actions of the lateral wall, which contains two tissues, namely: the spiral hair cells surrounded by supporting cells, and are found in the utricle ligament and the stria vascularis. Together, these tissues promote the and saccule. The cristae (crests) are ridges bearing sensory hair cells and recirculation of potassium from perilymph back to endolymph by supporting cells. They are found in the walls of the ampullae near the uptake via potassium channels and gap junctional communication. utricular openings of the three semicircular canals, one for each canal. Gap junctions are formed from connexins; their importance to this process is emphasized by the fact that mutations in connexins are sig- Utricle nificant causes of hearing loss (Duman and Tekin 2013). The vestibular regions may not have an endolymphatic potential, as their lateral wall The utricle is the larger of the two major vestibular sacs. It is an irregular, structure is simplified compared to that of the cochlea, although the oblong, dilated sac that occupies the posterosuperior region of the difference in potassium concentration between endolymph and peril- vestibule (see Fig. 38.1C), and contacts the elliptical recess (where it is ymph remains important. a blind-ended pouch) and the area inferior to it. 643 INNER EAR

The macula of the utricle (or utriculus) is a specialized neurosensory midline of the striola (see Fig. 38.5). The macula in each utricle is epithelium lining the membranous wall, and is the largest of the ves- approximately horizontal when the head is in its normal position. tibular sensory areas (Fig. 38.4). It is triangular or heart-shaped in Linear acceleration of the head in any horizontal plane will result in surface view and lies horizontally with its long axis orientated antero- the otolithic membrane lagging behind the movement of the membra- posteriorly and its sharp angle pointing posteriorly (Fig. 38.5). It is flat nous labyrinth as a result of the inertia produced by its mass. The except at the anterior edge, where it is gently folded in on itself, and it membrane thus maximally stimulates one group of hair cells by deflect- measures 2.8 mm long by 2.2 mm wide. The mature form of the macula ing their bundles towards the striola whilst inhibiting others by deflect- is reached early in development, but in the adult a bulge is often present ing their bundles away from it. Hence each horizontal movement of on the anterolateral border; there is sometimes an indentation at the the head will produce a specific pattern of firing in utricular afferent anteromedial border. The epithelial surface is covered by the otolithic nerve fibres. membrane (statoconial membrane), a gelatinous structure in which many small crystals, the otoconia (otoliths, statoliths), are embedded. Saccule A curved ridge, the ‘snowdrift line’, runs along the length of the otolithic membrane. It corresponds to a narrow crescent of underlying sensory The saccule (or sacculus) is a slightly elongated, globular sac lying in epithelium termed the striola, 0.13 mm wide. The density of sensory the spherical recess near the opening of the scala vestibuli of the cochlea hair cells in this strip of epithelium is 20% less than in the rest of (see Fig. 38.1C). The saccular macula is an almost elliptical structure, the macula. The striola is convex laterally and runs from the medial 2.6 mm long and 1.2 mm at its widest point. Its long axis is orientated aspect of the anterior margin in a posterior direction towards, but not anteroposteriorly but, in contrast to the utricular macula, the saccular reaching, the posterior pole. The part of the macula medial to the striola macula lies in a vertical plane on the wall of the saccule. Its elliptical is called the pars interna and is slightly larger than the pars externa, shape is very slightly distorted by a small anterosuperior bulge. Like the which is lateral to it. The significance of this area is that the sensory utricular macula, it is covered by an otolithic (statoconial) membrane cells are functionally and anatomically polarized with respect to the and possesses a striola similar to that of the utricle, 0.13 mm wide, which extends along its long axis as an S-shaped strip about which the sensory cells are functionally and anatomically polarized (see Fig. 4

38.5). The part of the macula above the striola is termed the pars Hair cells Supporting cell nuclei N interna, and that below it, the pars externa. The operation of the saccule is similar to that of the utricle. However, because of its vertical orienta- tion, the saccule is particularly sensitive to linear acceleration of the CTIO

E head in the vertical plane and is, therefore, a major gravitational sensor S when the head is in an upright position. It is also particularly sensitive to movement along the anteroposterior axis. Vestibular nerve fibres Semicircular canals

Fig. 38.4 A section of the utricular macula from a guinea pig, showing the The lateral, anterior and posterior semicircular ducts follow the course relative positions of the hair cells and supporting cell nuclei. Semi-thin of their osseous canals. Throughout most of their length they are resin section, toluidine blue stain. (The inner ear is extremely vulnerable to securely attached, by much of their circumference, to the osseous walls. hypoxia and situated in one of the hardest bones in the body, which They are approximately one-quarter of the diameter of their osseous means that well-fixed human tissue is rarely obtained for histology. canals (see Fig. 38.2). The medial ends of the anterior and posterior Guinea pigs are one of the most frequently used animal models of human canals fuse to form a single common duct, the crus commune, before hearing and their inner ear ultrastructure is very similar.) (Courtesy of RM entering the utricle. The lateral end of each canal is dilated to form an Walsh, DN Furness and CM Hackney, The Institute of Science and ampulla, within the ampulla of the osseous canal. The short segment Technology in Medicine, School of Life Sciences, Keele University.) of duct between the ampullae and utricle is the crus ampullaris.

A B C Otoliths

Saccule Otolithic membrane

Kinocilium Stereocilia

Cuticular plate

Afferent nerve calyx Type II hair cell Utricle Type I hair cell Efferent Synaptic bar nerve and vesicles ending Otolithic membrane Efferent Afferent nerve ending nerve ending

Macula Sensory epithelium, Supporting hair cells (blue) and cells hair bundles (black)

Fig. 38.5 A, The morphological organization of the saccular and utricular maculae and the relationship of their hair cells to the otolithic membrane. The utricular macula has been tilted in the plane of the page to emphasize that it lies horizontally, whereas the saccular macula lies vertically when the head is in an upright position. Note the different shapes of the maculae, the position of the striola as indicated by a curved line in each case, and the different orientations of their stereociliary bundles. The arrows indicate the excitatory direction of deflection. B, A scanning electron micrograph of a fracture of a utricular macula (guinea pig) showing a type I hair cell (left) and a type II hair cell (right). C, The differing innervation patterns of the two types of hair cell. 644 (B, Courtesy of DN Furness, The Institute of Science and Technology in Medicine, School of Life Sciences, Keele University.) Membranous labyrinth

Fig. 38.7 A scanning electron micrograph of a stereociliary bundle from the utricle (guinea pig). The stereocilia are arranged in rows of increasing height towards the tallest Cupula element, the kinocilium. Deflection in the direction of the kinocilium results in depolarization of the hair cell. The inset shows a tip link connecting a short stereocilium tip to the Cilium tall stereocilium side behind. (Courtesy of DN Stereocilia Furness, The Institute of Science and Receptor cells Technology in Medicine, School of Life Supporting Sciences, Keele cell University.) 38

ER PT A CH

Nerve fibre Fig. 38.6 A section of an ampullary crest.

The membranous wall of each ampulla contains a transverse eleva- Close to the inner surface of their basal two-thirds, every cell con- tion (septum transversum), on the central region of which is a saddle- tains numerous synaptic ribbons with associated synaptic vesicles. The shaped sensory ridge, the ampullary crest, containing hair cells and postsynaptic surface of an afferent nerve ending encloses the greater part supporting cells. It is broadly concave on its free edge along most of its of the sensory cell body in the form of a cup (chalice or calyx). Efferent length and has a concave gutter (planum semilunatum) at either end nerve fibres make synapses with the external surface of the calyx, rather between the ridge and the duct wall. Sectioned across the ridge, the than directly with the sensory cell. crests of the lateral and anterior semicircular canals have smoothly The kinocilium confers structural polarity on the bundle, which rounded corners; the posterior crest is more angular. A vertical plate of relates to functional polarity. The stereocilia and kinocilium are all gelatinous extracellular material, the cupula, is attached along the free interconnected by fine extracellular filaments of various types, called edge of the crest (Fig. 38.6). It projects far into the lumen of the cross links. One in particular, the tip link, connects the shorter stereo- ampulla so that it is readily deflected by movements of endolymph cilia in each row with adjacent stereocilia in the taller row next to it derived from head rotations within the duct, by means of which stimuli (see Fig. 38.7). The tip link is common to all types of hair cell and is are delivered to the sensory hair cells. The three semicircular canals thus thought to play a central role in transduction; mutations in the proteins detect angular accelerations during tilting or turning movements of the that comprise the tip link are significant in Usher syndrome, which is head in all three different planes of three-dimensional space. characterized by auditory and visual abnormalities. Deflection of the bundle towards the kinocilium results in depolarization of the hair cell and increases the rate of neurotransmitter release from its base. Deflec- Microstructure of the vestibular system tion away from the kinocilium hyperpolarizes the hair cell and reduces the release of neurotransmitter. How deflections produce these responses The maculae and crests detect the orientation of the head with respect will be considered in more detail later. to gravity and changes in head movement by means of the mechano- There is much greater variation in the sizes of type II sensory cells sensitive hair cells. These hair cells are in synaptic contact with afferent (see Fig. 38.5B,C; Fig. 38.8). Some are up to 45 μm long and almost and efferent endings of the vestibular nerve on their basolateral aspect. span the entire thickness of the sensory epithelium, whereas others are The entire epithelium lies on a bed of thick, fibrous connective tissue shorter than type I cells. They are mostly cylindrical, but otherwise containing myelinated vestibular nerve fibres and blood vessels. The resemble type I cells in their contents and the presence of an apical axons lose their myelin sheaths as they perforate the basal lamina of kinocilium and stereocilia. However, their kinocilia and stereocilia tend the sensory epithelium. There are two types of sensory hair cell in the to be shorter and less variable in length. The most striking difference vestibular system, type I and type II (see Figs 38.5, 38.8). between type I and II cells is their efferent nerve terminals: type II cells Type I vestibular sensory cells measure 25 μm in length, with a free receive several efferent nerve boutons containing a mixture of small surface of 6–7 μm in diameter. The basal part of the cell does not reach clear and dense-core vesicles around their bases, and afferent endings the basal lamina of the epithelium. Each cell is typically bottle-shaped, are small expansions rather than chalices. with a narrow neck and a rather broad, rounded basal portion contain- Polarization allows the hair cells to have specific orientations that ing the nucleus (see Fig. 38.5). The apical surface is characterized by optimize their function within each sensory organ. In the maculae, they 30–50 stereocilia (large, regularly arranged, modified microvilli about are arranged symmetrically on either side of the striola. In the utricle, 0.25 μm across) and a single kinocilium (with the typical ‘9 + 2’ the kinocilia are positioned on the side of the sensory cell nearest to arrangement of microtubules characteristic of true cilia). The kinocil- the striola so that the excitatory direction is towards the midline. In the ium is considerably longer than the stereocilia, and may attain 40 μm, saccule, the structural and functional polarity is the opposite, i.e. away whereas the stereocilia are of graded lengths. They are characteristically from it. In the ampullary crests, the cells are orientated with their rows arranged in regular rows behind the kinocilium in descending order of of stereocilia at right angles to the long axis of the semicircular duct. In height, the longest being next to the kinocilium (Fig. 38.7). The kino- the lateral crest, the kinocilia are on the side towards the utricle, whereas cilium emerges basally from a typical basal body, with a centriole in the anterior and posterior crests they are away from it. These different immediately beneath it. arrangements are important functionally because any given acceleration 645 INNER EAR

ST Fig. 38.8 Human vestibular hair cells (transmission electron micrographs). A, Type I cell (VR) bearing an apical group of stereocilia (ST) seen in a vertical section through the macula. Note that the hair cell is bottle-shaped, and that much of it is enclosed in the calyceal ending (C) of an afferent nerve terminal. Other abbreviations: SC, supporting cells. B, Human type II vestibular hair cell. A bouton-type afferent nerve terminal is in contact with the basal part. (Courtesy of H Felix, SC M Gleeson and L-G Johnsson, ENT Department, University of Zurich and GKT School of Medicine, London.)

VR

C A B 4

N of the head maximally depolarizes one group of hair cells and maxi- epithelium. The epithelial lining and subepithelial connective tissue mally inhibits a complementary set, thus providing a unique represen- become more complex where the duct dilates to form the endolym-

CTIO tation of the magnitude and orientation of any movement (for further phatic sac. An intermediate or rugose segment and a distal sac can be E

S details, see Furness (2002)). distinguished. In the intermediate segment, the epithelium consists of The type I and II sensory cells are set within a matrix of supporting light and dark cylindrical cells. Light cells are regular in form and have cells that reach from the base of the epithelium to its surface and form numerous long surface microvilli with endocytic invaginations between rosettes round the sensory cells, as seen in surface view. Although their them and large clear vesicles in their apical region. In contrast, dark form is irregular, they can easily be recognized by the position of their cells are wedge-shaped and have a narrow base, few apical microvilli nuclei, which tend to lie below the level of sensory cell nuclei and just and dense, fibrillar cytoplasm. above the basal lamina (see Fig. 38.4). The apices of the supporting The endolymphatic sac has important roles in the maintenance of cells are attached by tight junctions to neighbouring supporting cells vestibular function. Endolymph produced elsewhere in the labyrinth is and to the hair cells to produce the reticular lamina, a composite layer absorbed in this region, probably mainly by the light cells. Damage to that forms a plate that is relatively impermeable to cations other than the sac, or blockage of its connection to the rest of the labyrinth, causes via the mechanosensitive transduction channels of the hair cells. endolymph to accumulate; this produces hydrops, which affects both The otolithic membrane is a layer of extracellular material divided vestibular and cochlear function. The epithelium is also permeable to into two strata. The external layer is composed of otoliths or otoconia, leukocytes, including macrophages, which can remove cellular debris which are barrel-shaped crystals of calcium carbonate with angular from the endolymph, and to various cells of the immune system that ends, up to 30 μm long, and heterogeneous in distribution. They are contribute antibodies to this fluid. attached to a more basal gelatinous layer into which the stereocilia and kinocilia of the sensory cells are inserted (see Fig. 38.5). The gelatinous material consists largely of glycosaminoglycans associated with fibrous COCHLEAR DUCT protein. The cochlear duct is a spiral tube that runs within the bony cochlea (see Epley’s manœuvre Figs 38.1C, Fig. 38.9). The osseous spiral lamina projects for part of the Benign paroxysmal positional vertigo is a condition in which a sensa- distance between the modiolus and the outer wall of the cochlea and tion of rotation with associated nystagmus is induced by adopting a is attached to the inner edge of the basilar membrane. Above it is a particular position (with the abnormal ear dependent). It is believed thickened ridge of endosteum called the spiral limbus. The endosteum that calcium carbonate crystals from the otoliths become freed from the of the outer wall is thickened to form a spiral cochlear ligament that otolithic membrane and, in certain positions, drop into the ampulla of projects inwards as a triangular basilar crest attached to the outer rim the posterior semicircular canal, possibly becoming adherent to the of the basilar membrane. Immediately above this is a concavity, the cupula and rendering it gravity-sensitive. In certain positions, the align- external spiral sulcus (sulcus spiralis externus), above which the thick, ment of the axis of the posterior semicircular canal with gravity results highly vascular periosteum projects as a spiral prominence. Above the in the displacement of the cupula and the activation of the vestibulo- prominence is a specialized, thick epithelial layer, the stria vascularis. ocular reflex, leading to compensatory nystagmoid eye movements in A second, thinner vestibular membrane, Reissner’s membrane, extends response to apparent head movements. from the spiral limbus to the outer wall of the cochlea, where it is Epley’s canalith repositioning procedure relies on the adoption of a attached above the stria. Reissner’s membrane consists of two layers of series of body postures designed to allow the aberrant crystals (or cana- squamous epithelial cells separated by a basal lamina. The side facing liths) to float out of the posterior semicircular canal and to stick to the the scala vestibuli bears flattened perilymphatic cells, with tight junc- wall of the vestibule. Cure rates in excess of 80% have been recorded tions between them, creating a diffusion barrier. The endolymphatic and the procedures have largely superseded surgical procedures designed side is lined by squamous epithelial cells with many microvilli; these to denervate the ampulla of the posterior semicircular canal (singular are also joined by tight junctions and are involved in ion transport. The neurectomy) or obliterate the canal completely. canal thus enclosed between the scala tympani and the scala vestibuli is the cochlear duct (see Fig. 38.9B). It is triangular in cross-section Endolymphatic duct and sac throughout the length of the cochlea. The closed upper end, the lagena, is attached to the cupula. The lower end of the duct turns medially, The endolymphatic duct runs in the osseous vestibular aqueduct and narrowing into the ductus reuniens, and connects with the saccule (see becomes dilated distally to form the endolymphatic sac. This is a struc- Fig. 38.1C). ture of variable size, which may extend through an aperture on the The organ of Corti, the sensory epithelium of the cochlea, sits on posterior surface of the petrous bone to end between the two layers of the basilar membrane. The apices of the sensory hair cells and the sup- the dura on the posterior surface of the petrous temporal bone near the porting cells it contains are joined by tight junctions to form the reticu- sigmoid sinus (see Fig. 38.1C). The surface cells throughout the entire lar lamina. The diffusion barriers that line the cochlear duct ensure that endolymphatic duct resemble those lining the non-specialized parts of the apices of the sensory hair cells are bathed in endolymph, whereas 646 the membranous labyrinth and consist of squamous or low cuboidal their lateral and basal regions are bathed in perilymph. CHAPTER

Development of the eye 40

The development of the eye involves a series of inductive interactions pigmented epithelium). The area of surface ectoderm that is closely between neighbouring tissues in the embryonic head. These are the apposed to the distal optic vesicle thickens to form the lens placode, neurectoderm of the forebrain (which forms the sensory retina and and the mesenchymal sheath of the vesicle begins to show signs accessory pigmented structures), the surface ectoderm (which forms the of angiogenesis. Between 33 and 35 days post ovulation, the lens lens and the corneal epithelium) and the intervening neural crest mes- placode and optic vesicle undergo coordinated morphogenesis. The enchyme (which contributes to the fibrous coats of the eye and to lens placode invaginates, forming a pit that pinches off from the surface tissues of the anterior segment of the eye). A broad anterior domain of ectoderm to form the lens vesicle (Fig. 40.1). The surface ectoderm neurectoderm, characterized by the activation of several homeobox- reforms a continuous layer that will become the corneal epithelium. containing transcriptional regulators, including PAX6, RX, SIX3 and The lateral part of the optic vesicle invaginates to form a cup; the inner OTX2, develops the potential to form optic vesicles. Subsequent interac- layer (facing the lens vesicle) will become the sensory (neural) retina, tions between mesenchyme and neurectoderm, involving expression of and the outer layer, influenced by signals from the surrounding extra- the secreted protein sonic hedgehog (SHH) at the midline, subdivides ocular mesenchyme, becomes the retinal pigmented epithelium. As a this eye-field region into bilateral domains at the future sites of the eyes result of these folding movements, what were the apical (luminal) (Chow and Lang 2001). Loss of SHH function causes holoprosencephaly surfaces of the two layers of the cup now face one another across a and a range of malformations that can include cyclopia, due to incom- much-reduced lumen, the intraretinal space. The pigmented layer plete separation of the prosencephalon (Dubourg et al 2004). becomes attached to the mesenchymal sheath, but the junction between In three-dimensional culture of murine embryonic stem cells sup- the pigmented and sensory layers is less firm and is the site of patho- plemented with the correct growth factors and Matrigel® (to promote logical detachment of the retina. The two layers are continuous at the basement membrane formation), but in the absence of surface ecto- lip of the cup (Fig. 40.2). The narrow part of the optic vesicle between derm or lens epithelium, an epithelial vesicle develops and undergoes the base of the cup and the brain forms the optic stalk. As well as the dynamic shape change to form a two-layered optic cup (Eiraku and invagination of the lateral part of the optic vesicle, the ventral surface Sasai 2012). The tissue so formed demonstrated interkinetic nuclear of the vesicle and distal part of the stalk similarly invaginate, forming migration and a fully stratified architecture similar to that of postnatal a wide groove, the choroid (optic) fissure, through which mesenchyme eyes, including appropriate synapses. Similar results, i.e. production of and the hyaloid artery extend. These infoldings involve differential retinal architecture and retinal pigment epithelial cells, have been growth and cell movement, and high levels of proliferation in the inner reported using human embryonic stem cells (Nakano et al 2012, Zhu neuroepithelial layer. et al 2013). As growth proceeds, the fissure closes and the artery is included in The parallel process of lens determination appears to depend on a the distal part of the stalk. The fusion process is characterized by apop- brief period of inductive influence that spreads through the surface tosis at the margins of the fissure. Failure of the optic fissure to close is ectoderm from the rostral neural plate and elicits a lens-forming area a rare anomaly that is accompanied by a corresponding deficiency in of the head. Reciprocal interactions that are necessary for the complete the choroid and iris (congenital coloboma) and is often associated with development of both tissues take place as the optic vesicle forms and microphthalmia (small eyes). Reduced growth of the optic cup caused contacts the potential lens ectoderm (Saha et al 1992, Nakano et al by mutation of the homeobox gene CHX10, important for specification 2012, Fuhrmann 2010). The vascular tissue of the developing eye forms and growth of the neural retina, is one known cause of microphthalmia by local angiogenesis or vasculogenesis of angiogenic mesenchyme (Graw 2003). Anophthalmia, microphthalmia and coloboma are also (Hughes et al 2000). (Accounts of the development of the eye are given associated with mutation of the SOX2 gene. in O’Rahilly (1966, 1983).)

EMBRYONIC COMPONENTS OF THE EYE DIFFERENTIATION OF THE FUNCTIONAL COMPONENTS OF THE EYE The first morphological sign of eye development is a thickening of the diencephalic neural folds at 29 days post ovulation, when the embryo The developments just described bring the embryonic components of has seven to eight somites. This optic primordium (eye field) extends the eye into the spatial relationships necessary for the passage, focusing on both sides of the neural plate and crosses the midline at the primor- and sensing of light. The next phase of development involves further dium chiasmatis. A slight transverse indentation, the optic sulcus, patterning and phenotypic differentiation in order to develop the spe- appears in the inner surface of the optic primordium on each side of cialized structures of the adult organ. the brain. During the period when the rostral neuropore closes, at about The optic cup becomes patterned, from the base to the rim, into 30 days (stage 11), the walls of the neuromere diencephalon 1 (see Fig. regions with distinct functions. Several secreted factors, including bone 17.2) begin to evaginate at the optic sulcus, projecting laterally towards morphogenetic proteins (BMP), retinoic acid and SHH, and transcrip- the surface ectoderm; by 32 days, the optic vesicles are formed. Failure tional regulators, including PAX6 and PAX2, are important for specify- of the specification and development of the optic vesicle is associated ing each region (Chow and Lang 2001, Sinn and Wittbrodt 2013). The with mutation of several transcriptional regulator genes expressed in outer layer of the optic cup remains as a thin layer of cells, which begin the eye field and leads to anophthalmia (absence of the eye) (Graw to acquire pigmented melanosomes and form the pigmented epithe- 2003). Each optic vesicle is surrounded by a sheath of mesenchymal lium of the retina at around 36 days. In a parallel process, which begins cells derived from the head mesenchyme and neural crest; its lumen is before invagination, the cells of the inner layer of the cup proliferate to continuous with that of diencephalon 1. By 31 days, regional differen- form a thick pseudostratified neuroepithelium, the future neural retina, tiation is apparent in each of the source tissues of the eye. The optic over the base and sides of the cup. The peripheral region around the vesicle is visibly differentiated into its three primary parts: at the junc- lip of the cup extends, and is further differentiated into the components tion with the diencephalon, a thick-walled region marks the future optic of the prospective iris at the rim, and the ciliary body a little further stalk; laterally, the tissue that will become the sensory (neural) retina back, adjacent to the neural retina (see Fig. 40.2). The development of forms a flat disc of thickened epithelium in close contact with the this pattern is reflected in regional differences in the expression of surface ectoderm; and the thin-walled part that lies between these various genes that encode transcriptional regulators and which are regions will later form the pigmented layer of the retina (retinal therefore likely to play key roles in controlling and coordinating 661 DEVELOPMENT OF THE EYE

A B C D

E F G H

Retinal pigmented Optic Retinal pigmented Optic Early space between Position epithelium stalk epithelium stalk neural retina and Retinal of lens retinal pigmented pigmented Retinal epithelium epithelium pigmented epithelium 4

ON I T C E S Choroid fissure Optic Fusing Hyaloid stalk choroid Lens Lens Neural Lens Neural artery fissure Optic cup vesicle retina vesicle retina stalk Fig. 40.1 Development and morphogenesis of the optic cup. A–D Scanning electron micrographs of early eye development, A and C showing external view, B and D showing longitudinal sections through the eye. B and E show the early invagination of the lens placode and the modification of the optic vesicle to an optic cup. F and D show the formation of the lens vesicle and the two layers of the retina (nervous and pigmented). G and H show three dimensional images of the outer aspect of the optic cup. G. Early formation of the choroid fissure along the side of the optic cup and optic stalk; the layers of the retina are shown. H The fused choroid fissure; the position of the lens within the optic cup is shown in dotted outline. (A, B, C, D Courtesy of Kathleen Sulik PhD, Professor, University of North Carolina.)

A B C Mesenchyme condensing to Pigmented layer form sclera of retina and choroid Trabecular Peripheral meshwork retina Pigmented Superficial layer of retina epithelium of lens Corneal Cavity of Mesenchyme epithelium Lens vesicle optic cup invading interval Corneal Lens Neural between lens Optic stroma layer of retina and ectoderm fissure Corneal Cavity of endothelium Ectoderm Developing optic stalk lens fibres

D E Neural layer of retina Choroidal extension Scleral spur Pigmented layer of retina forming ciliary Ciliary processes body and iris Scleral venous sinus Developing Anterior chamber aqueous Posterior chamber chamber Fused eyelids Cornea Developing Iris stroma vitreous body Iris epithelium Inferior conjunctival fornix Fig. 40.2 Sections through the developing eyes of human embryos. A, Equivalent to 8 mm crown–rump length (CRL); stage 16. The thick nervous and the thinner pigmented layers of the developing retina and the lens are shown. The two layers of the embryonic optic cup are separated by the intraretinal space. B, Equivalent to 13.2 mm CRL; stage 17. The surface ectoderm anterior to the lens forms the corneal epithelium, whereas the corneal stroma and endothelium will differentiate from the invading mesenchyme (of neural crest and mesodermal origin). C, Equivalent to 40 mm CRL. The development of the anterior aqueous chamber is apparent with choroidal extensions and iris visible. The eyelids have developed and are fused; the extent of the conjunctival fornices can be seen. D, Anterior growth of the peripheral retina, pigmented layer of the retina and mesenchymal proliferation at the anterior part of the retina will give rise to the ciliary body and iris. The surface ectoderm anterior to the lens forms the corneal epithelium, whereas the corneal 662 stroma and endothelium will differentiate from invading mesenchyme (of neural crest and mesodermal origin). E, Details of the developing uveal tract. Note the development of the anterior and posterior aqueous chambers, separated by the iris, and the attachment of the lens to the ciliary body. Differentiation of the functional components of the eye development. Distinct sets of genes are expressed prior to and during Ciliary body overt cell-type differentiation. For example, PAX6 is expressed in the prospective ciliary and iris regions of the optic cup; individuals hetero- The ciliary body is a compound structure. Its epithelial components are zygous for mutations in PAX6 lack an iris (aniridia), which suggests a derived from the region of the inner layer of the retina, between the iris causal role for this gene in the development of the iris. The genes and the neural retina, and the adjacent outer layer of pigmented epi- expressed in the eye are also often active at a variety of other specific thelium. The cells here differentiate in close association with the sur- sites in the embryo, which may, in part, account for the co-involvement rounding mesenchyme to form highly vascularized folds that secrete of the eye and other organs in syndromes that result from single genetic aqueous fluid into the globe of the eye. lesions, e.g. PAX2 mutation causes coloboma and kidney defects, reflect- The inner surface of the ciliary body forms the site of attachment of ing the sites of expression of the gene (Graw 2003). the lens. The outer layer is associated with smooth muscle derived from mesenchymal cells in the choroid that lie between the anterior scleral Developing neural retina condensation and the pigmented ciliary epithelium (p. 691).

The developing neural retina consists of an outer nuclear zone, which Iris contains dividing neuroepithelial retinal progenitor cells, and an inner marginal zone, which is initially devoid of nuclei. At around 37 days, The iris develops from the tip of the optic cup, where the two neuroepi- the cells of the nuclear zone invade the marginal zone, and by stage 18 thelial layers remain thin and are associated with vascularized, muscu- (44 days), the nervous stratum of the retina consists of inner and outer lar connective tissue. The muscles of the sphincter and dilator pupillae neuroblastic layers. Cell lineage analyses have shown that seven retinal are unusual in that they are of neurectodermal origin, and develop as cell types are all derived from a common multipotential retinal progeni- a result of further growth and differentiation of the two layers of the tor cell. Different types of retinal cells are born (cease dividing) in a optic cup. Mesenchymal cells, largely composed of neural crest cells conserved sequence during development: ganglion cells, amacrine cells, that have migrated anterior to the lens, form the collagen-rich iris cone photoreceptors and horizontal cells develop early, whereas bipolar stroma; they overlay the pigmented epithelium of the iris, which is cells, rod photoreceptors and Müller glial cells develop later (Cepko continuous with the ciliary body and neural retina, and is of neurecto- 40 et al 1996). Newly born cells migrate from the apical (ventricular) dermal origin. The mature colour of the iris develops after birth and is surface to the appropriate cell layer in the developing retina, establish- dependent on the relative contributions made by the pigmented epi- R ing its characteristic laminar structure. The developing ganglion cell thelium on the posterior surface of the iris and the neural crest-derived layer first separates from the neuroblastic layers by formation of the melanocytes in the mesenchymal stroma of the iris. If only epithelial PTE inner plexiform layer. The inner nuclear layer, containing developing A

pigment is present, the eye appears blue, whereas if there is an addi- H

amacrine, horizontal, bipolar and Müller glial cells, then separates from tional contribution from the melanocytes, the eye appears brown. Ani- C the outer nuclear layer, containing the developing rod and cone photo- ridia, the absence of the iris, is commonly caused by heterozygous receptors by formation of the outer plexiform layer. Mature retinal mutations of PAX6. neurones first appear in the central part of the retina. By the eighth month, all the named layers of the retina can be identified. However, Lens the photoreceptor cells continue to differentiate after birth, generating an array of increasing resolution and sensitivity; the macula does not The lens develops from the lens vesicle (see Figs 40.1F, 40.2A). Ini- reach maturity until 15–45 months after birth (Hendrickson and Yuo- tially, this is a ball of actively proliferating epithelium, but by stage 16, delis 1984). there is a discernible difference between the thin anterior (i.e. outward- The divergent differentiation of the pigmented and sensory layers of facing) epithelium and the thickened posterior epithelium. Cells of the retina from the initially bipotential neuroepithelium of the optic the posterior wall lengthen and fill the vesicle (see Fig. 40.2B,C), vesicle involves activation of region-specific regulatory genes, e.g. reducing the original cavity to a slit by about 44 days. The posterior CHX10 in the presumptive neural retina and MITF in the presumptive cells become filled with a very high concentration of proteins (crystal- pigmented epithelium (Bharti et al 2006). Patterning by gene expres- lins), which render them transparent; they also become densely sion is an important aspect of establishing regional identity of the optic packed within the lens as primary lens fibres. Cells at the equatorial cup and the subsequent maturation of these respective tissues. Soluble region of the lens elongate and contribute secondary lens fibres to the factors from the retina elicit the polarized distribution of plasma mem- body of the lens in a process that continues into adult life, sustained brane proteins and the formation of tight junctions in the pigmented by continued proliferation of cells in the anterior epithelium (p. 697). epithelium. Neural retinal differentiation is mediated by several growth The polarity and growth of the lens appear to depend on the differen- factors, including fibroblast growth factors, SHH and retinoic acid. Basic tial distribution of soluble factors that promote either cell division or helix-loop-helix proneural transcriptional regulatory genes also play a lens fibre differentiation, and are present in the anterior chamber and central role in regulating retinal cell fate. However, the pigmented epi- vitreous humour, respectively. Congenital cataracts can be associated thelium initially retains the potential to become neural retina and will with mutations in genes that encode structural lens proteins, particu- do so if the embryonic retina is wounded, demonstrating the plasticity larly crystallin proteins, as well as genes that encode transcriptional of the early commitment to pigment epithelium or neural retinal fate. regulatory factors expressed specifically in the lens, such as MAF and The retinal vasculature forms by the aggregation of spindle-shaped PITX3, which are needed for normal lens development and which also cells (mesenchymal cells) that emanate from the optic disc by week influence normal growth of the globe of the eye (Graw 2003, Graw 15 and form vascular cords, consistent with vessel formation by vas- 2010). culogenesis, which give rise to the inner plexus of the retina. Vessel The developing lens is surrounded by a vascular mesenchymal con- formation in the temporal and peripheral retina occurs by angiogen- densation, the vascular capsule, the anterior part of which is named the esis. New vessel segments sprout from pre-existing vessels and grow pupillary membrane. The posterior part of the capsule is supplied by tangentially by angiogenesis into the neuroepithelium (Hughes et al branches from the hyaloid artery, and the anterior part is supplied by 2000). branches from the anterior ciliary arteries. During the fourth month, the hyaloid artery gives off retinal branches. By the sixth month, all of Optic nerve the vessels have atrophied, except the hyaloid artery, which becomes occluded during the eighth month of intrauterine life, although its The optic nerve develops from the optic stalk. The centre of the optic proximal part persists in the adult as the central artery of the retina. cup, where the optic fissure is deepest, will later form the optic disc, Atrophy of the hyaloid vasculature and of the pupillary membrane where the neural retina is continuous with the corresponding invagi- appears to be an active process of programmed tissue remodelling that nated cell layer of the optic stalk; the developing axons of the ganglion is macrophage-dependent; visual impairment occurs in persistent cells, therefore, pass directly into the wall of the stalk and convert it hyperplastic primary vitreous, a condition in which normal regression into the optic nerve. Myelination of the axons within the optic nerve of the hyaloid vasculature fails to occur. The hyaloid canal, which carries begins shortly before birth but the process is not completed until some the vessels through the vitreous, persists after the vessels have become time later. The optic chiasma is formed by the meeting and partial occluded. In the neonate, it extends more or less horizontally from the decussation of the axons within the two optic nerves in the ventral part optic disc to the posterior aspect of the lens, but when the adult eye is of the lamina terminalis (at the junction of the telencephalon with the examined with a slit-lamp, it can be seen to follow an undulating diencephalon in the floor of the third ventricle). Beyond the chiasma, course, sagging downwards as it passes forwards to the lens. With the the axons continue as the optic tracts, and pass principally to the lateral loss of its blood vessels, the vascular capsule disappears and the lens geniculate bodies and to the superior tectum of the midbrain. becomes dependent for its nutrition on diffusion via the aqueous and 663 DEVELOPMENT OF THE EYE

vitreous humours. The lens remains enclosed in the lens capsule, a Choroid and sclera thickened basal lamina derived from the lens epithelium. Sometimes, the pupillary membrane persists at birth, giving rise to congenital The choroid and sclera differentiate as inner vascular and outer fibrous atresia of the pupil. layers, respectively, from the neural crest mesenchyme that surrounds the optic cup; the choroid is continuous with the leptomeningeal inter- Vitreous body nal sheath of the optic nerve, and the sclera is continuous with the outer dural sheath of the optic nerve. The blood vessels of the choroid develop The vitreous body develops between the lens and the optic cup as a from the fifteenth week and include the vasculature of the ciliary body. transparent, avascular gel of extracellular substance. The precise deriv- ation of the vitreous remains controversial. The lens rudiment and the optic vesicle are, at first, in contact, but they draw apart after closure of DIFFERENTIATION OF STRUCTURES the lens vesicle and formation of the optic cup, and remain connected AROUND THE EYE by a network of delicate cytoplasmic processes. This network, derived partly from cells of the lens and partly from those of the retina, is the primitive vitreous body. At first, these cytoplasmic processes are con- Extraocular muscles nected to the whole of the neuroretinal area of the cup but, later, they become limited to the ciliary region, where, by a process of condensa- Development of the extrinsic ocular muscles is complex, involving tion, they form the basis of the suspensory ligaments of the ciliary coordinated juxtaposition of specific muscle precursors, cranial motor zonule. The vascular mesenchyme, which enters the cup through the nerve outgrowth and neural crest mesenchyme. During gastrulation, choroidal fissure and around the equator of the lens, associates locally when the earliest cells are migrating through the primitive node to form with this reticular tissue and thus contributes to the formation of the the prechordal plate and notochordal process, they transitorily express vitreous body. myogenic markers. In stages 9 and 10 a population of prechordal mes- enchyme cells migrate laterally from the lateral edge of the prechordal plate toward the unsegemental paraxial mesenchyme each side of the 4

Anterior segment notochord. After neurulation is complete, bilateral premandibular, intermediate and caudal cavities develop adjacent to the neural tube. ON

I Mesenchymal cells of neural crest origin migrate anteriorly around the The walls of these ‘head cavities’ are lined by flat or cylindrical cells that T optic cup and between the surface ectoderm and the lens to contribute do not exhibit the characteristics of a germinal epithelium; they were C

E to the development of anterior segment structures, including the ciliary previously termed preotic somites. S body, the iris, the cornea and the iridocorneal angle tissues (see Fig. As the oculomotor nerve grows towards the developing eye, at the 40.2C–E) (Gould et al 2004). The anterior chamber initially appears as level of the premandibular head cavity, the prechordal mesenchyme a cleft in this mesenchymal tissue. The mesenchyme superficial to the becomes apparent as a condensation of premuscle cells at its ventrola- cleft forms the stroma (substantia propria) and endothelium of the teral side. This later subdivides into the blastema of superior, inferior, cornea, and that deep to the cleft forms the stroma of the iris and medial and lateral recti and inferior oblique. Similar events occur in the the pupillary membrane. Tangentially, this early cleft extends as far as intermediate head cavity which is associated with the trochlear nerve the iridocorneal angle, where communications are established with the and premuscle cells forming superior oblique, and the caudal head scleral venous sinus (canal of Schlemm). Mesenchymal cells of neural cavity (abducens nerve and lateral rectus) (see Figs 12.4 and 35.7). crest origin lying at the angle of the anterior chamber differentiate to The early myogenic properties of the prechordal mesenchyme have form a specialized meshwork of trabecular beams (collagen fibrils been demonstrated experimentally; if transplanted into limb buds, the covered by cells); the open spaces of the meshwork become open to the cells are able to develop into muscle tissue (Wachtler and Jacob 1986), anterior chamber as the beams develop. The canal of Schlemm devel- however, the timing of expression of myosin heavy chain isoforms is ops deep to the trabecular meshwork and is derived from mesodermal different from that of limb myoblasts. Myotube formation starts later mesenchyme. Initially a vascular structure lined by endothelial cells, and progresses at a slower pace than in the limb, and coexpression of the canal acts as an aqueous sinus from the fifth month of gestation MyHCI/1st and MyHCI/2nd are both seen from the earliest stages of (McMenamin 1989). The forward-growing optic cup rim differentiates development in contrast to only MyHCI/1st in the limb. These early into the ciliary epithelium and the iris; the posterior chamber is formed myoblasts retain their distinct differences from other skeletal muscles between the iris, the lens capsule, the zonular suspensory fibres and the as they mature: they are smaller and loosely arranged, and belong to ciliary processes. The ciliary processes produce the aqueous humour very small motor units (Porter and Baker 1996, Pedrosa-Domellöf et al that flows through the pupil and is drained in the iridocorneal angle, 2000). Their final attachment to the eye is specified by neural crest mainly by the trabecular meshwork and the canal of Schlemm. In this formation of the sclera, extraocular muscle tendons and the orbit. In way, the walls of the anterior and posterior segment chambers furnish cases of anophthalmia the relative location and structure of extraocular both the sites of production, and the channels for circulation and re- muscles can be identified (Pedrosa-Domellöf et al 2000, Bohnsack et al absorption, of the aqueous humour (p. 687). The FOXC1 and PITX2 2011). transcriptional regulatory genes are expressed in the neural crest cells migrating into the presumptive anterior segment, and regulate differen- tiation of the anterior segment tissues. Anterior segment dysgenesis, Eyelids involving malformation of the iris, cornea and angle, occurs when these genes are mutated (e.g. Axenfeld–Rieger syndrome) and is often The eyelids are formed as small cutaneous folds of surface ectoderm associated with raised intraocular pressure and glaucoma (Gould et al with a core of neural crest mesenchyme (see Fig. 40.2D). During the 2004). middle of the third month, their edges come together and unite over the cornea to enclose the conjunctival sac; they usually remain united until about the end of the sixth month. When the eyelids open, the Cornea conjunctiva lining their inner surfaces and covering the scleral region of the eye fuses with the corneal epithelium. Transforming growth The cornea is induced in front of the anterior chamber by the lens and factor alpha (TGF-α) and several other growth factors regulate the optic cup. The corneal epithelium is formed from surface ectoderm. The mesenchyme–epithelium interactions and cell migration that are primordial corneal endothelium lining the front of the anterior chamber required for eyelid formation; keratinization is thought to play an is formed from mesenchymal cells derived from the neural crest (see important role in lid separation. The eyelashes and lid glands, i.e. seba- Fig. 40.2C,D,E). Mesenchymal cells migrate between these layers and ceous glands associated with the eyelashes and tarsal (Meibomian) differentiate to form specialized fibroblasts (keratocytes) that secrete glands, develop from ectoderm, as do the lacrimal and accessory lac- the extracellular matrix of the corneal stroma. A regular array of collagen rimal glands. Orbicularis oculi, which closes the eyelids, develops on fibres (lamellae) is established between these two layers and serves to each side from skeletal myoblasts from the second pharyngeal arch that reduce scattering of light entering the eye. The most anterior region of invade the eyelids. The muscles that widen the palpebral fissure develop the stroma (Bowman’s layer) develops as an acellular zone packed with within the orbit from mesenchymal cells; the superior and inferior collagen fibrils that confer strength on the layer. From the third month, tarsal muscles are smooth muscle, whereas levator palpebrae superioris the endothelium is organized as a monolayer of cells and develops a is striated muscle that is attached to each upper eyelid by a tendon strong, laminated basal lamina (Descemet’s membrane) adjacent to the derived from the neural crest (Plock et al 2005). (For a detailed time- stroma. The endothelium maintains corneal transparency by regulating table for upper eyelid development in staged human embryos and 664 the water content of the stroma (p. 689). fetuses, see Byun et al (2011).) Key references

Lacrimal apparatus Hartnett et al 2014). At full term, the eye is 65% of its adult size. It grows rapidly for the first year, slightly slower until 3 years and then The ectodermal epithelium of the superior conjunctival fornix prolifer- more slowly until puberty. There is proportionately less growth of ante- ates and gives rise to a series of tubular buds that form the alveoli and rior, compared to posterior, structures and the globe of the eye becomes ducts of the lacrimal gland. The buds are arranged in two groups: one more spherical. The neonatal lens is more spherical than that of the forms the gland proper and the other forms its palpebral process (de adult, which helps to compensate for the relative shortness of the eye. la Cuadra-Blanco et al 2003). The lacrimal sac and nasolacrimal duct The visual acuity of neonates is estimated to be 20/400 and can reach are derived from ectoderm in the nasomaxillary groove (between the 20/30–20/20 by 2–3 years of age; there is a general trend for infants to lateral nasal process and the maxillary process of the developing face) be far-sighted (Olitsky et al 2011). The visual pathways, lateral genicu- (see Fig. 36.11). The ectoderm thickens to form a solid cord of cells, the late body and occipital visual cortex are patterned postnatally by expo- nasolacrimal ridge, which subsequently sinks into the mesenchyme and sure to visual stimuli. Visual loss with no structural anomaly of the eye, becomes canalized during the third month to form the nasolacrimal amblyopia, is caused by abnormal visual stimulation during infancy duct. The lacrimal canaliculi arise from the cranial extremity of the cord and early childhood (up to 6–7 years of age) (Ruiz de Zárate and as buds that establish openings (puncta lacrimalia) on the margins of Tejedor 2007). Neonatal extraocular muscle coordination is usually the lids. The inferior canaliculus isolates a small part of the lower eyelid achieved by 3–6 months of age and persistent deviation of an eye to form the lacrimal caruncle and plica semilunaris. requires evaluation. Preterm infants have reduced reflex and tear secre- tion, and tears may not be present with crying until more than 3 months of age (Olitsky et al 2011). Neonatal and infant eye

Low-birth-weight and preterm infants are at risk of developing retin- opathy of prematurity, a proliferative retinopathy (Kashani et al 2014,

KEY REFERENCES 40

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665 Development of the eye

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Bharti K, Nguyen MT, Skuntz S et al 2006 The other pigment cell: specifica- McMenamin PG 1989 Human fetal iridocorneal angle: a light and scanning tion and development of the pigmented epithelium of the vertebrate electron microscopic study. Br J Ophthalmol 73:871–9. eye. Pigment Cell Res 19:390–4. Nakano T, Ando S, Takata N et al 2012 Self-formation of optic cups and Bohnsack BL, Gallina D, Thompson H et al 2011 Development of extraocu- storable stratified neural retina from human ESCs. Cell Stem Cell lar muscles requires early signals from periocular neural crest and the 10:771–85. developing eye. Arch Ophthalmol 129:1030–41. Olitsky SE, Hug D, Plummer LS et al 2011 Disorders of the eye. Growth and Byun TH, Kim JT, Park HW et al 2011 Timetable for upper eyelid develop- development. In: Kliegman RM, Stanton BF, St Geme JW et al (eds) ment in staged human embryos and fetuses. Anat Rec (Hoboken) 294: Nelson Textbook of Pediatrics, 19th ed. Philadelphia: Elsevier, Saunders; 789–96. Ch. 610. Cepko CL, Austin CP, Yang X et al 1996 Cell fate determination in the ver- This chapter considers the postnatal growth of the eye. tebrate retina. Proc Natl Acad Sci U S A 93:589–95. O’Rahilly R 1966 The early development of the eye in staged human Chow RL, Lang RA 2001 Early eye development in vertebrates. Annu Rev embryos. Contrib Embryol Carnegie Inst 38:1. Cell Dev Biol 17:255–96. This paper presents the fundamental information of human eye development de la Cuadra-Blanco C, Peces-Peña MD, Mérida-Velasco JR 2003 Morpho- in staged embryos. genesis of the human lacrimal gland. J Anat 203:531–6. O’Rahilly R 1983 The timing and sequence of events in the development of Dubourg C, Lazaro L, Pasquier L 2004 Molecular screening of SHH, ZIC2, the human eye and ear during the embryonic period proper. Anat SIX3, and TGIF genes in patients with features of holoprosencephaly Embryol (Berl) 168:87–99. spectrum: mutation review and genotype-phenotype correlations. Hum This paper presents the stages of human eye development. Mutat 24:43–51. Pedrosa-Domellöf F, Holmgren Y, Lucas CA et al 2000 Human extraocular Eiraku M, Sasai Y 2012 Self-formation of layered neural structures in three- muscles: unique pattern of myosin heavy chain expression during dimensional culture of ES. Curr Opin Neurobiol 22:768–77. myotube formation. Invest Ophthalmol Vis Sci 41:1608–16. 40 Fuhrmann S 2010 Eye morphogenesis and patterning of the optic vesicle. This paper presents a study of extraocular muscle development in human Curr Top Dev Biol 93:61–84. embryos and fetuses. R This paper considers the factors involved in the patterning of the optic vesicle into neural retina and pigmented retinal epithelium. Plock J, Contaldo C, von Lüdinghausen M 2005 Levator palpebrae superioris PTE muscle in human fetuses: anatomical findings and their clinical rele- A H

Gould DB, Smith RS, John SW 2004 Anterior segment development relevant vance. Clin Anat 18:473–80. C to glaucoma. Int J Dev Biol 48:1015–29. Porter JD, Baker RS 1996 Muscles of a different ‘color’: the unusual proper- Graw J 2003 The genetic and molecular basis of congenital eye defects. Nat ties of the extraocular muscles may predispose or protect them in neu- Rev Genet 4:876–88. rogenic and myogenic disease. Neurology 46:30–37. Graw J 2010 Eye development. Curr Top Dev Biol 90:343–86. Ruiz de Zárate B, Tejedor J 2007 Current concepts in the management of This paper presents the transcription factors in eye development and amblyopia. Clin Ophthalmol 1:403–14. discusses their relevance to human eye disorders. Saha MS, Servetnick M, Grainger RM 1992 Vertebrate eye development. Curr Hartnett ME, Morrison MA, Smith S et al 2014 Genetic variants associated Opin Genet Dev 2:582–8. with severe retinopathy of prematurity in extremely low birth weight A review of the interactions involved in eye development and discussion of infants. Invest Ophthalmol Vis Sci 55:6194–203. the genes responsible for development of the eye. Hartnett ME 2015 Pathophysiology and mechanisms of severe retinopathy Sinn R, Wittbrodt J 2013 An eye of eye development. Mech of Dev 130: of prematurity. Ophthalmology 122:200–10. 347-58. Hendrickson AE, Yuodelis C 1984 The morphological development of the This paper reviews the transcription factors in development of the eye. human fovea. Ophthalmology 91:603–12. Wachtler F, Jacob M 1986 Origin and development of the cranial skeletal Hughes S, Yang H, Chan-Ling T 2000 Vascularization of the human fetal muscles. Bibl Anat 29:24–46. retina: roles of vasculogenesis and angiogenesis. Invest Ophthalmol Vis This paper considers the origin of the extraocular muscles. Sci 41:1217–28. Zhu Y, Carido M, Meinhardt A et al 2013 Three-dimensional neuroepithelial Kashani AH, Drenser KA, Capone Jr A 2014 Retinopathy of prematurity. culture from human embryonic stem cells and its use for quantitative In: Yanoff M, Duker J (eds) Ophthalmology; 4th ed. Oxford: Elsevier, conversion to retinal pigment epithelium. PLoS One 8:e54552. Saunders; Ch. 6.20, pp. 535–40. This chapter reviews the genetic factors and molecular causes of retinopathy in preterm, low birthweight infants.

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Astrocyte Internal limiting membrane Nerve fibre layer

Ganglion cell layer Retinal ganglion cell

Inner plexiform layer Displaced amacrine cell

Bipolar cell

Inner nuclear layer Müller cell Amacrine cell Horizontal cell

Outer plexiform layer

External limiting Outer nuclear layer membrane Cell bodies of rods and cones 4

Cone

Photoreceptor cell inner Rod and outer segments

SECTION Pigment epithelium Choroid

Fig. 42.21 Neural cells whose cell bodies and interconnections account for the layered appearance of the retina in histological section (compare with Fig. 42.22). Also shown are the two principal types of neuroglial cell in the retina (although microglia are also present, they are not shown).

10 Foveola Papillomacular bundle Optic nerve head 9 8 7 GCL NFL IPL INL 6 ELM OPL IS/OS ONL 5 RPE/CH 4 3 2 1 CC 100 m 500 µm µ Fig. 42.23 A high-definition optical coherence tomography (OCT) in vivo SC image of the human retina. The image has approximately 2 μm axial resolution, is 8 mm long and consists of 10,000 axial scans. The image is Fig. 42.22 A transverse section of the retina and choroid. The 10 layers expanded in the vertical direction to permit better visualization of retinal of the retina are shown. Key: 1, pigment epithelial layer; 2, rod and cone layers. Abbreviations: ELM, external limiting membrane; GCL, ganglion layer; 3, external limiting membrane; 4, outer nuclear layer; 5, outer cell layer; INL, inner nuclear layer; IPL, inner plexiform layer; IS/OS, plexiform layer; 6, inner nuclear layer; 7, inner plexiform layer; 8, ganglion boundary between the photoreceptor inner and outer segments; NFL, cell layer; 9, nerve fibre layer; 10, internal limiting membrane. nerve fibre layer; ONL, outer nuclear layer; OPL, outer plexiform layer; Abbreviations: CC, choriocapillaris; SC, suprachoroid. RPE/CH, retinal pigment epithelium and choriocapillaris. (Courtesy of Professor James Fujimoto, Department of Electrical Engineering and Computer Science, MIT, Boston, USA.)

photoreceptors towards the brain, some information flow occurs in the opposite direction via centripetal fibres in the optic nerve and inter- plexiform cells in the retina that connect the inner and outer plexiform layers. Cells of the retina The classic 10-layered appearance of the retina is absent in the optic nerve head, the fovea and foveola, and the ora serrata. At the optic Retinal pigment epithelium nerve head, the axons of the retinal ganglion cells leave the retina to The retinal pigment epithelium is composed of approximately cuboidal form the optic nerve and all the other neural cell types are missing. At cells that form a single continuous layer extending from the periphery the fovea and foveola, the inner five layers of the retina are ‘pushed of the optic disc to the ora serrata, where it continues as the outer ciliary aside’. At the ora serrata, where the retina borders the ciliary body (see epithelium. The cells are flat in radial section and hexagonal or pen- Fig. 42.10), the retinal pigment epithelium merges with the outer pig- tagonal in surface view, and number 4–6 million in the human retina. mented epithelium of the ciliary body, while the neural retina borders Their cytoplasm contains numerous melanosomes. Apically (towards the inner unpigmented ciliary epithelium; the retina is thinnest at this the rods and cones), the cells bear long (5–7 μm) microvilli that point. The normal layered arrangement of the neural retina approach- contact, or project between, the outer segments of rods and cones. The ing the ora serrata is frequently disrupted by cysts in older individuals tips of rod outer segments are deeply inserted into invaginations in 700 (Fig. 42.24). the apical membrane of the retinal pigment epithelium. The different Retina embryological origins of the retinal pigment epithelium and neural Retinal pigment epithelium cells play a major role in the turnover retina mean that the attachments between these two layers are unsup- of rod and cone photoreceptive components. Their cytoplasm contains ported by junctional complexes; the neural retina and retinal pigment the phagocytosed tips of rods and cones undergoing lysosomal destruc- epithelium are therefore easily parted (retinal detachment) due to tion. The final products of this process are lipofuscin granules, which trauma or disease. accumulate in these cells with age. Disturbances in this phagocytic activity of the retinal pigment epithelium can lead to retinal disease Ora serrata Retinal cyst (Kevany and Palczewski 2010). Light reaching the outer retina but missing the photoreceptors is absorbed by the retinal pigment epithelium, which, like melanin else- where in the eye, prevents such stray light degrading image quality. The zone of tight junctions between adjacent cells also allows the epithe- lium to function as an important blood–retinal barrier between the retina and the vascular system of the choroid. The retinal pigment epi- thelium is required for the regeneration of bleached visual pigment and may have antioxidant properties. It also secretes a variety of growth factors necessary for the integrity of the choriocapillaris endothelium and the photoreceptors, and produces a number of immunosuppressive factors. A failure of any of the diverse functions of the retinal pigment epithelium could result in compromised retinal function and eventual Unpigmented epithelium of Pigmented epithelium of Retinal pigment epithelium blindness (Strauss 2005). ciliary body ciliary body Fig. 42.24 The junction between the retina and ciliary body (ora serrata). Rods and cones The retinal pigment epithelium is continuous with the outer, pigmented Rods and cones are the ‘image-forming’ photoreceptors of the outer epithelium of the ciliary body, while the neural retina abuts the inner, retina and function at low (scotopic) and higher (photopic) light levels, unpigmented epithelium of the ciliary body. The layered appearance that respectively. Both are long, radially orientated structures with a similar 42 is apparent elsewhere in the neural retina is disrupted adjacent to the ora organization, although details differ (Fig. 42.25). From the choroidal serrata by cystic degeneration. end inwards, the cells consist of outer and inner segments connected

A B Fig. 42.25 The major features of a retinal rod cell (A) and a retinal cone CHAPTER cell (B). The relative size of the pigment epithelial cells has been Lateral process exaggerated for illustrative purposes. Rod spherule with gap junction Cone pedicle

Müller cell cytoplasm Cell body and nucleus

Cell body and nucleus

Myoid

Inner segment

Müller cell microvilli Ellipsoid

Centriole

Cilium Calycal process Cone lamellae

Rod lamellae Pigment epithelial microvilli Outer segment

Phagosomal vacuole Pigment granule

Pigment epithelial Pigment cell epithelium

Bruch’s membrane 701 EYE

by a thin connecting cilium (together making up layer 2 of the retina), populations of cones absorbing maximally at the short- (λmax 420 nm), a cell body containing the nucleus, and a synaptic terminal (either a middle- (λmax 534 nm) and longer-wave (λmax 563 nm) end of the more complex pedicle for cones or a simpler rod spherule) where they visible spectrum. The three cone classes are sometimes referred to as make synaptic connections with adjacent bipolar and horizontal cells the blue, green and red cones but are better classed as S, M and L cones. and with other cone or rod cells within the outer plexiform layer. The action of light is to isomerize the retinal, separating it from the The nuclei of the rods and cones form the outer nuclear layer (layer opsin, a process which, via a G-protein coupled enzyme cascade and a 4). The cone nuclei are relatively large and oval, and generally form a second messenger system, results in the closure of cation channels in single layer that often penetrates the external limiting membrane (layer the receptor outer segment membrane, a hyperpolarization of the 3). They also contain less heterochromatin and thus usually stain more photoreceptor, and a consequent decrease in the release of the neuro- lightly. Rod nuclei are round and smaller, stain more darkly, and form transmitter glutamate from its synapses. several layers vitreal to the cone nuclei. The human retina contains, on average, 4.6 million cones and The external limiting is not, in fact, a membrane at all, although it 92 million rods, although there is significant inter-individual variation appears as such in the light microscope; rather, it is a series of zonulae (Curcio et al 1990). Although cones populate the whole retina, their adherentes between photoreceptors and the glial (Müller) cells that density is highest in the foveola, where approximately 7000 cones reach separate them. These junctions most likely serve to anchor the photo- an average density of 199,000 cones/mm2; this area is entirely rod-free. receptors and prevent leakage of the interphotoreceptor matrix that Going outwards from the foveola, rod numbers rise, reaching a peak surrounds the photoreceptor outer and inner segments. density in a horizontal elliptical ring at the eccentricity of the optic disc, Rod outer segments are cylindrical and consist of around 1000 flat- before declining once more towards the periphery. Cone density is tened, lobulated, membranous discs. These form as deep infoldings of 40–45% higher in the nasal compared to the temporal retina, and the plasma membrane at the base of the outer segment; they ‘bud off’ slightly higher inferiorly than superiorly. after formation so that the discs are not attached to the plasma mem- The number of S cones in all human retinae is similar, making up brane and are free-floating within the outer segment. Cone outer seg- less than 10% of all cones (Curcio et al 1991, Hofer et al 2005). The ments are generally shorter and, as their name implies, often conical distribution of S cones is relatively even throughout the retina, although (especially in the peripheral retina). Cone discs do not bud off after they are absent from the central fovea. The relative proportions of L and

4 formation and remain as infoldings of the plasma membrane. M cones shows a much greater degree of variation between individuals, The inner segment of both rods and cones is divided into an outer, the L : M cone ratio varying from close to unity to over 10. The distribu- mitochondria-rich, ellipsoid and an inner myoid that contains endo- tion of L and M cones is more irregular than that of S cones, and appears plasmic reticulum. In most of the retina, these inner segments are random with some indication of clumping (Bowmaker et al 2003, much wider in cones (5–6 μm at their widest point) than rods (1.5 μm) Hofer et al 2005).

SECTION (Fig. 42.26). In both rods and cones, proteins are manufactured within The high packing density of cones at the foveola, achieved by de- the myoid and incorporated into the newly formed discs at the base of creasing inner segment size, ensures maximal resolution, while the the outer segment. In rods, as new discs are added, and old discs are presence of more than one spectral cone type allows colour vision. S pushed up the outer segment and eventually phagocytosed by the cones probably contribute little to spatial resolution because they are retinal pigment epithelium. Cone discs are also phagocytosed but the absent from the foveola. Rod-based vision provides high sensitivity, but incorporation of new proteins within the discs is more diffuse (Nguyen- with relatively low spatial discrimination and no ability to distinguish Legros and Hicks 2000). While all rods within the retina have a similar wavelengths. Although many of the functional differences between rods structure, the cones at the foveola are highly modified compared to and cones rely on the different properties of the photoreceptors them- those situated more peripherally and, in many ways, resemble rods selves, their connectivity to other retinal neurones is equally important. with a longer outer segment and a thinner inner segment. Light is absorbed by rhodopsins, visual pigments consisting of a Horizontal cells protein, opsin, that spans the membrane of the outer segment discs, Horizontal cells are inhibitory interneurones. Their dendrites and axons bound to a light-absorbing chromophore, retinal, which is an aldehyde extend laterally within the outer plexiform layer, making synaptic con- of vitamin A1. Such rhodopsins have a smooth, bell-shaped absorption tacts with cone pedicles and rod spherules, and, via gap junctions at profile with a point of maximum absorbance (λmax), indicating the the tips of their dendrites, with each other. Their cell bodies lie in the wavelength at which they are most sensitive. Humans possess four dif- outer part of the inner nuclear layer (layer 6). Three morphological ferent opsins, resulting in four spectrally distinct visual pigments: one types of horizontal cell can be distinguished in the human retina (Kolb located within the rods (λmax 498 nm) and three within different et al 1992). The dendrites of HI and HIII cells contact cones, and their axons terminate on rods. Both the axons and dendrites of HII cells synapse only with cones. Bipolar cells Bipolar cells are radially orientated neurones. Their dendrites synapse on photoreceptors, horizontal cells and interplexiform cells in the outer plexiform layer. Their somata are located in the inner nuclear layer, and axonal branches in the inner plexiform layer synapse with dendrites of ganglion cells or amacrine cells. Golgi staining has identified nine dis- tinct types of bipolar cell in the human retina (Kolb et al 1992), eight of which contact cones exclusively, and the remaining type synapses only on rods. Cone bipolars are of three major morphological types: midget, S (blue) cone and diffuse, according to their connectivity and size. Midget cone bipolar cells either invaginate the cone pedicle or synapse on its base (flat subtype). In the central retina, each midget bipolar cell con- tacts only a single cone (2–3 in the periphery), forming part of a one- to-one channel from cone to ganglion cell that mediates high spatial resolution. S cones form part of a short-wavelength mediating channel, while the larger diffuse cone bipolars are connected to up to 10 cones and are thought to signal luminosity rather than colour. Cone bipolar cells can also be of two physiological types, according Fig. 42.26 A tangential section of a human retina in the parafovea cut at to their response to the light-induced decrease in glutamate release from the level of the inner/outer segments, showing the ‘mosaic’ of both the larger cones and the more numerous, but smaller, rods. Photoreceptors the photoreceptors to which they are synaptically connected. If illumi- at the top of the image are sectioned at a level closer to the retinal nation of the photoreceptors with a point of light causes a depolariza- pigment epithelium than receptors lower in the figure. The reduction in tion of the connected bipolar cell, it is said to be an ‘ON’ bipolar, size of the cones at the top of the figure is explained by the conical contacting the cone by ‘sign-inverting’ synapses with metabotropic shape of their outer segment. If the figure were continued upwards, receptors. However, if cones are connected to an ‘OFF’ bipolar cell via representing sections closer to the retinal pigment epithelium, the size of ‘sign-conserving’ synapses with ionotropic receptors, illumination of the cones would continue to decrease and the amount of surrounding the photoreceptor will result in hyperpolarization of the bipolar cell. white space would increase. Rod outer segment diameter, however, Illumination of a concentric area of surrounding photoreceptors causes 702 would change little. the opposite response in bipolar cells to illumination within their Retina dendritic field. This inhibition is mediated via horizontal cells and gives layer. Ganglion cell bodies, together with displaced amacrine cells, form rise to the antagonistic centre-surround type receptive field that is char- the ganglion cell layer of the retina (layer 8). Throughout most of the acteristic of all levels of the visual system up to and including the retina, they form a single layer; they become progressively more numer- occipital cortex. ous near the macula, where they are ranked in up to 10 rows, reaching The single morphological type of rod bipolar cell contacts 30–35 a peak density of up to 38,000/mm2 in a horizontally orientated ellipti- rods in the central retina, increasing to 40–45 rods in the periphery. cal ring 0.4–2.0 mm from the foveal centre. Their number diminishes Such convergence serves to increase the absolute sensitivity of the rod again towards the fovea, from which they are almost totally excluded. system. All rod bipolar cells are ‘ON-centre’ and do not contact ganglion Up to 15 ganglion cell types have been identified in the mammalian cells directly, but synapse with a class of amacrine cell (AII), which then retina based on morphology, physiology, and target area in the brain, contacts cone bipolar cells. each of them presumably functionally distinct. For example, some The inner plexiform layer can be divided into two main layers: an project to different regions of the lateral geniculate nucleus and form outer layer containing the synaptic endings of ‘OFF’ cone bipolar cells, three parallel visual pathways involved in conscious visual perception, and an inner layer of ‘ON’ cone and rod bipolar cell synapses. namely: the magnocellular and parvocellular systems and a pathway carrying the S cone signal (Wässle 2004). Midget ganglion cells (P cells) Amacrine cells contact only single midget bipolar cells in the central retina, which, in Most amacrine cells lack typical axons and, consequently, their den- turn, connect to single cones, giving each cone a ‘private line’ out of the drites make both incoming and outgoing synapses. Each neurone has retina and ensuring optimal acuity. The large dendritic field of parasol a cell body either in the inner nuclear layer near its boundary with the cells (M cells) is consistent with a role in motion detection. Parasol and inner plexiform layer, or on the outer aspect of the ganglion cell layer, midget ganglion cells together make up around 80% of human retinal when it is known as a displaced amacrine cell. The processes of ama- ganglion cells. The remaining cells (approximately 200,000) project to crine cells make a variety of synaptic contacts in the inner plexiform the superior colliculus of the midbrain, the thalamic pulvinar, the pre- layer with bipolar and ganglion cells, as well as with other amacrine tectum and the accessory optic system, and contribute to various sub- cells. conscious visual reflexes such as the pupillary and accommodation The various classes of amacrine cell serve a number of important responses (see Fig. 42.12). In addition, a population of around 3000 functions. AII cells play an essential role in the rod pathway (see above). large, intrinsically light-sensitive ganglion cells form a network com- 42 Other cells appear to be important modulators of photoreceptive posed of extensive overlapping dendrites (Dacey et al 2005). Such signals, and serve to adjust or maintain relative colour and luminosity ‘inner retinal photoreceptors’ contain a retinal-based visual pigment inputs under changing light conditions. They are probably also respon- (melanopsin; λmax 479 nm), which resembles an invertebrate-type sible for some of the complex forms of image analysis known to occur visual pigment in many of its characteristics. These light-sensitive gan- within the retina, such as directional movement detection. Up to 24 glion cells are part of a pathway parallel to the rod- and cone-mediated different morphological types are recognized in humans (Kolb et al ‘image-forming’ system that monitors overall levels of illumination. CHAPTER 1992); coupled to their neurochemical complexity, this makes them This ‘non-imaging’ pathway is the major route by which the eye influ- perhaps the most diverse neural cell type in the body. ences circadian rhythms via the suprachiasmatic nucleus; it also con- tributes to light-evoked pupillary constriction via projections to the Interplexiform cells olivary pretectal nucleus. Although the axons of some of these photo- Interplexiform cells, often regarded as a subclass of amacrine cells, sensitive ganglion cells also project to the lateral geniculate nucleus, generally have cell bodies in the inner nuclear layer. They are postsyn- their wider contribution to conscious visual perception remains incom- aptic to cells in the inner retina, and send signals against the general pletely understood. direction of information flow in the retina, synapsing with bipolar, Ganglion cell axons, which form the nerve fibre layer on the inner horizontal and photoreceptor cells in the outer plexiform layer. surface of the retina, run parallel to the surface of the retina, and con- Although their function is uncertain, it is likely that, through the release verge on the optic nerve head where they leave the eye as the optic nerve. of γ-aminobutyric acid (GABA) and dopamine, they adjust some aspect Fibres from the medial (nasal) retina approach the disc in a simple of retinal function such as sensitivity. radial pattern (Fig. 42.27), whereas axons from the lateral (temporal) retina take an arcuate route as they avoid the fovea. Axons from the Ganglion cells macula form a papillomacular fasciculus that passes almost straight to The human retina contains 0.7–1.5 million ganglion cells, the output the disc. The thickness of the nerve fibre layer increases dramatically neurones of the retina (Curcio and Allen 1990). Their dendrites synapse near the optic disc as fibres from the peripheral retina traverse more with processes of bipolar and amacrine cells in the inner plexiform central areas. Towards the edge of the disc, the other retinal layers thin,

Temporal side Nasal side of Fig. 42.27 Directions of axons (dashed lines) and of optic disc optic disc blood vessels of the nerve fibre layer of the retina of the right eye. Axons pass radially on the nasal side of the optic disc, whereas fibres on the temporal side avoid crossing the fovea by arching around it. Some of the fibres from the fovea and central region pass straight to the optic disc and others arch above and below the horizontal; together, these form the papillomacular bundle. A Raphe raphe is formed by the central fibres temporal to Fovea the fovea. Venules are shown crossing in front of arteries; the reverse relationship is probably the Papillomacular more common pattern. All vessels issue from the bundle disc on the right of the figure; the larger temporal branches tend to arch around the central region of the retina and do not approach the fovea. The peripheral retina and most of the nasal retina are not shown.

703 EYE

and, at the disc, all neural elements of the retina other than ganglion cell axons are excluded. Axons of ganglion cells are surrounded by the processes of radial glial cells and retinal astrocytes, and are almost always unmyelinated within the retina, which is an optical advantage because myelin is refractile. Although a few small myelinated axons may occur, myelin- ation does not generally start until axons enter the optic disc to become the optic nerve. Retinal glial cells There are three types of retinal glial cells: radial Müller cells, astrocytes and microglia. Müller cells form the predominant glial element of the retina; retinal astrocytes are largely confined to the ganglion cell and nerve fibre layers; and microglial cells are scattered throughout the neural part of the retina in small numbers. Müller cells span almost the entire thickness of the neural retina, ensheathing and separating the various neural cells except at synaptic sites. They constitute much of the total retinal volume, and almost totally fill the extracellular space between neural elements. Their nuclei lie within the inner nuclear layer, and from this region each cell body extends a single thick fibre that runs radially outwards, giving off Fig. 42.28 A section through the fovea centralis. (With permission from complex lateral lamellae that branch among the processes of the outer Young B, Heath JW 2000 Wheater’s Functional Histology. Edinburgh: plexiform layer. Apically, each central process terminates at the external Churchill Livingstone.) limiting membrane, from which microvilli project for a short distance

4 into the space between the rod and cone inner segments (fibre baskets)

(see Fig. 42.25). On the inner surface of the retina, the main Müller cell process expands into a terminal foot plate that contacts those of neigh- fovea. The Henle fibres contain two xanthophyll carotenoid pigments bouring glial cells and forms part of the internal limiting membrane (lutein and zeaxanthin), which create an elliptical yellowish area (see below). (approximately 2 mm horizontally and 1 mm vertically): the macula

SECTION The functions of Müller cells are numerous (Reichenbach and Bring- lutea. Macular pigment density varies by more than an order of magni- mann 2013). Like astrocytes, Müller cells contact blood vessels, espe- tude between individuals, is influenced by several environmental cially capillaries of the inner nuclear layer, and their basal laminae fuse factors, including diet, and is negligible in the central foveola. Low with those of perivascular cells or vascular endothelia, contributing to levels of macular pigment are likely to be associated with retinal path- the formation of the blood–retinal barrier. They also maintain the ologies such as age-related macular degeneration (Beatty et al 2008). stability of the retinal extracellular environment by, for example, regula- The absence of the inner retinal layers, including blood vessels (see tion of K+ levels, uptake of neurotransmitter, removal of debris, storage Fig. 42.20A), reduces light scatter, which, along with the increased of glycogen, providing neuroprotective support to the photoreceptors packing density of cones in the foveola and their lack of convergence and mechanical support to the whole neural retina. Recently, it has been with ganglion cells, ensures that visual resolution is highest in this part shown that they are also involved in the regeneration of cone visual of the retina. Acuity may be further enhanced by the macular pigment, pigments, that some are a source of stem cells and that they may even which, apart from having antioxidant properties and removing poten- act as light guides, conveying light from the inner retina to the photo- tially harmful short-wave radiation, will absorb those wavelengths most receptors and overcoming some of the optical disadvantages of an prone to chromatic aberration and Rayleigh scatter. ‘inverted’ retina. The cell bodies of retinal astrocytes lie within the nerve fibre layer and their processes branch to form sheaths around ganglion cell axons. VASCULAR SUPPLY The close association between astrocytes and blood vessels in the inner retina suggests that they contribute to the blood–retinal barrier. Retinal The retina has a dual arterial supply and both parts are necessary to microglia are scattered mostly within the inner plexiform layer. Their maintain retinal function. The outer five layers of the retina are avascu- radiating branched processes spread mainly parallel to the retinal plane, lar and rely on an indirect supply from the choroidal capillaries. The giving them a star-like appearance when viewed microscopically from inner retina receives a direct blood supply through capillaries connected the surface of the retina. They can act as phagocytes, and their number to branches of the central retinal artery and vein. Only the inner retinal increases in the injured retina. circulation is described here. The inner border of the retina is formed by the internal limiting The central retinal artery enters the optic nerve as a branch of the membrane (layer 10), which consists of collagen fibres and proteo- ophthalmic artery 6.4–15.2 mm behind the eyeball (Kocabiyik et al glycans from the vitreous, a basement membrane (which is continuous 2005), and travels within the optic nerve to its head, where it passes with the basal lamina of the ciliary epithelium), and the plasma mem- through the lamina cribrosa. At this level, the central artery divides into brane of expanded Müller cell terminal foot plates. It is 0.5–2 μm thick equal superior and inferior branches, which, after a few millimetres, in the posterior retina and thickens with age. The internal limiting divide into superior and inferior nasal, and superior and inferior tem- membrane is involved in fluid exchange between the vitreous and the poral, branches, each supplying a ‘quadrant’ of the retina (see Fig. retina, and, perhaps through the latter, with the choroid. It also has 42.27). Although similar retinal veins unite to form the central retinal various other functions, including anchorage of retinal glial cells, and vein, the courses of the arteries and veins do not correspond exactly. inhibition of cell migration into the vitreous body. These vessels mainly run within the nerve fibre and ganglion cell layers of the retina, accounting for their clarity when seen through an oph- thalmoscope (see Fig. 42.20A). Arteries often cross veins, usually lying Modifications of the central retina superficial to them; in severe hypertension, the arteries may press on the veins and cause visible dilations distal to these crossings. The vitreal The central retina, clinically referred to as the macula, is composed of location of arteries, their lighter, bright red colouration and smaller four concentric areas, which, starting with the innermost, are: the diameter in comparison to veins allow the two vessel types to be dis- foveola (0.35 mm diameter, equivalent of an angular subtense at the tinguished ophthalmoscopically. nodal point of around 1.25°), the fovea (1.5 mm, 5.2°), the parafovea From the four major arteries within the inner retina, dichotomous (2.5 mm, 8.6°) and the vaguely defined perifovea (5–6 mm, 20°). The branches run from the posterior pole to the periphery, supplying the foveola, which contains no rods or S cones, is centred about 3 mm whole retina (Zhang 1994). Arteries and veins ramify in the nerve fibre temporal and 1 mm inferior to the optic disc (see Fig. 42.20A). In the layer, near the internal limiting membrane, and arterioles pass deeper foveola and surrounding fovea, all the inner layers of the neural retina into the retina to supply capillary beds. Venules return from these beds beyond the outer nuclear layer have been displaced peripherally, result- to larger superficial veins that converge towards the disc to form the ing in a retinal thickness around half of that elsewhere in the retina (see central retinal vein. Fig. 42.23; Fig. 42.28). This foveal pit is created by the cone ‘axons’, Retinal capillary networks can occur in three different layers, the known here as Henle fibres, running almost parallel to the retinal number of layers depending on location (Zhang 1994). Radial peri- 704 surface before connecting to postreceptoral retinal neurones outside the papillary capillaries are the most superficial of the capillary networks Retina and lie within the inner nerve fibre layer. A layer of inner capillaries The adrenergic postganglionic sympathetic supply originates in the runs within the nerve fibre and ganglion cell layers, while an outer superior cervical ganglion and travels via a plexus around the internal capillary layer is located in the inner plexiform and inner nuclear layers carotid and ophthalmic arteries; it is unclear whether it elicits vasocon- (Fig. 42.29). Approaching the fovea, capillaries are restricted to two striction or dilation (Bergua et al 2013). layers, and terminal capillaries eventually join to form a single-layered macular capillary ring, producing a capillary-free zone 450–500 μm in diameter at the fovea. This avascular region is clearly visible in a fluo- RETINOPETAL INNERVATION OF THE RETINA rescein angiogram (Fig. 42.20B). Capillaries become less numerous in the peripheral retina and are absent from a zone approximately 1.5 mm The human retina receives input from the brain via retinopetal axons wide adjoining the ora serrata. within the optic nerve. Although there are only a small number of these The territories of the arteries that supply a particular quadrant do (usually less than 10), they branch extensively within the retina. They not overlap, nor do the branches within a quadrant anastomose with emerge at the optic disc, course through the nerve fibre layer, and give each other; consequently, a blockage in a retinal artery causes loss of off orthogonal branches that ramify in the inner plexiform layer vision in the corresponding part of the visual field. The only exception (Repérant et al 2006). One set of such axons arises from perikarya in to this end-arterial pattern is in the vicinity of the optic disc. Here, the the posterior hypothalamus and uses histamine as a neurotransmitter, posterior ciliary arteries enter the eye near the disc (Fig. 42.30), and while other, serotoninergic, fibres arise from cell bodies in the dorsal their rami not only supply the adjacent choroid, but also form an raphe. These neurones are not specialized for vision; they project to anastomotic circle in the sclera around the head of the optic nerve. many other targets in the central nervous system besides the retina, Branches from this ring join the pial arteries of the nerve, and small forming components of an ascending arousal system (Gastinger et al cilioretinal arteries from any arteries in this region may enter the eye 2006). Histamine is released during the day in the inner plexiform layer, and contribute to the retinal vasculature, possibly resulting in the pres- but it activates receptors located on cone pedicles, horizontal cell peri- ervation of visual function following central retinal artery occlusion. karya and ON bipolar cell dendrites via volume transmission. Hista- Similarly, small retinociliary veins may sometimes also be present. mine decreases the absolute sensitivity of the retina to light and, The structure of retinal blood vessels resembles that of vessels else- possibly, modulates retinal blood flow (Vila et al 2012). where, except that the internal elastic lamina is absent from the arteries, 42 and muscle cells may appear in their adventitia. Capillaries are non- fenestrated and endothelial cells are joined by complex tight junctions, OPTIC NERVE HEAD fulfilling the requirements of a functional blood–retinal barrier. Within the optic nerve, the central artery is innervated by both branches of the The axons of more than a million retinal ganglion cells converge at the autonomic nervous system; this innervation does not extend to the optic nerve head and leave the eye by penetrating the sclera to form the vessels in the retina. The cholinergic parasympathetic supply is derived optic nerve. The optic nerve head represents that part of the optic nerve CHAPTER mainly from the pterygopalatine ganglion and is vasodilatory (Ch. 32). lying within the bulb of the eye. Since all retinal neural elements, apart from ganglion cell axons, are absent from this region, it is insensitive to light and forms the ‘blind spot’. Histologically, the optic nerve head can be divided into three zones (Fig. 42.31): the prelaminar (the anterior part terminating at the vitre- ous), laminar (formed by the lamina cribrosa) and postlaminar (con- tinuous with the retrobulbar optic nerve). The surface view of the optic nerve head, usually seen with an ophthalmoscope, is referred to as the optic disc (Fig. 42.20). Prelaminar zone

The inner surface of the optic nerve head is covered by an astroglial membrane (of Elschnig) that is continuous with the internal limiting membrane of the retina. At the centre of the disc, the layer of astrocytes thickens into a central meniscus (of Kuhnt). Retinal ganglion cells turn into the optic nerve head accompanied by astrocytes, which gradually increase in number posteriorly, eventually forming a sieve-like structure, the glial lamina cribrosa, through which the nerve fibres pass as separate fasciculi. At the perimeter of the optic nerve head, a collar of astrocytes several cells thick (the intermediary tissue of Kuhnt) separates the optic nerve from the terminating outer layers of the retina. This layer contin- Fig. 42.29 A tangential section of the retina at the level of the inner ues posteriorly and forms a barrier between the optic nerve head and nuclear layer, highlighting the dense network of capillaries. the choroid (the border tissue of Jacoby).

Fig. 42.30 Vessels of the optic nerve head. Retina The dotted areas represent the principal glial membranes. Arteries are represented on the right Choroid and veins on the left.

Sclera

Circle of Zinn/Haller

Short posterior ciliary artery

Lamina Central canal of Arachnoid mater cribrosa the optic nerve Central retinal Central retinal Pia mater Dura mater vein artery 705 EYE

1b 1a

2 Retina 5

Choroid 3 4 6 Sclera

Circle of Zinn 7

Sep 4

Sep SECTION Du

Ar GI.M

Pia GI.C

Fig. 42.31 The optic nerve head, showing the distribution of collagenous tissue (grey) and neuroglial nuclei (solid blue circles). Key: 1a, retinal internal limiting membrane; 1b, inner limiting membrane of Elschnig; 2, central meniscus of Kuhnt; 3, spur of collagenous tissue separating the anterior lamina cribrosa (6) from the choroid; 4, border tissue of Jacoby; 5, intermediary tissue of Kuhnt; 7, posterior lamina cribrosa. Abbreviations: Ar, arachnoid mater; Du, dura mater; Gl.C, astrocytes and oligodendrocytes among the fibres in their fascicles; Gl.M, astroglial membrane; Pia, pia mater; Sep, connective tissue septa from pia mater. The dotted lines represent the borders of the lamina cribrosa. (With permission from Anderson DR, Hoyt W 1969 Ultrastructure of intraorbital portion of human and monkey optic nerve. Arch Ophthalmol 82:506–30.)

Laminar zone general retinal hue is a bright terracotta-red, with which the pale pink of the disc contrasts sharply; its central part is usually even paler and The lamina cribrosa is composed of discrete trabeculae of collagenous may be light grey. These differences are due in part to the degree of and elastic connective tissue, which extend from the sclera to form a vascularization of the two regions, which is much less at the optic disc, meshwork through which the optic nerve fascicles and central retinal and also to the total absence of choroidal or retinal pigment cells. In vessels pass. Each trabecula has a lining of astrocytes that are continu- subjects with strongly melanized skins, both retina and disc are darker. ous with those of the glial lamina cribrosa. The optic disc rarely projects sufficiently to justify the term papilla, although it is usually a little elevated on its lateral side, where the papillomacular nerve fibres turn into the optic nerve (see Fig. 42.27). Postlaminar zone There is usually a slight depression where the retinal vessels traverse its centre. The optic nerve thickens in the postlaminar zone as its axons become myelinated. The reflected sclera, and the dura mater with which it is Vascular supply continuous, invest the nerve together with the other two meningeal sheaths, the arachnoid and pia mater. Fine fibrous septa penetrate the The blood supply to the three regions of the optic nerve head differs optic nerve from the pia mater, dividing it into 300–400 fascicles, giving (see Fig. 42.30). The prelaminar region is supplied mainly by branches pial blood vessels access to the nerve. of the central retinal artery. Branches from the short posterior ciliary arteries form an often incomplete circle within the sclera around the Optic disc optic nerve head (circle of Zinn/Haller); centripetal branches from this structure supply the laminar region of the optic nerve head. The short As it is visible by ophthalmoscopy (see Fig. 42.20A), the disc is a region posterior ciliary arteries may also give off centripetal branches directly of great clinical importance. Oedema of the disc (papilloedema) may to supply the lamina, and branches that pass anteriorly to augment the be the first sign of raised intracranial pressure, which is transmitted into prelaminar blood supply. In the postlaminar region, arteries from the the subarachnoid space around the optic nerve. The disc is also sensitive prepapillary choroid and circle of Zinn pass retrogradely as pial vessels, to the raised intraocular pressure that occurs in glaucoma and shows providing centripetal branches that supply the optic nerve. More poste- characteristic structural changes due to retinal ganglion cell loss. riorly, the optic nerve receives pial arterioles directly from the posterior The optic disc is superomedial to the posterior pole of the eye, and ciliary arteries. The central retinal artery may also contribute some so lies away from the visual axis. It is round or oval, and usually centrifugal branches in this region. approximately 1.6 mm in transverse diameter and 1.8 mm in vertical The central retinal vein drains the optic nerve head at all levels; other 706 diameter; its appearance is very variable. In light-skinned subjects, the drainage pathways are minor. Visual pathway

Left visual field Binocular field Right visual field Fig. 42.32 A simplification of the visual pathway, showing the spatial arrangement of neurones and their fibres in relation to the quadrants of the retinae and visual fields. The proportions at various levels are not exactly to scale. In particular, the macula is exaggerated in size in the visual fields and retinae. In each quadrant of the visual field, and in the parts of the visual pathway subserving it, two shades of each respective Left Binocular Right monocular monocular Note optical colour are used; the paler shade denotes the inversion peripheral field and the darker shade denotes the macular part of the quadrant. From the lateral geniculate nucleus onwards, these two shades are Right retina both made more saturated to denote intermixture of neurones from both retinae, the palest shade being reserved for parts of the visual pathway concerned with monocular vision.

Macula

Optic nerve 42 Chiasma CHAPTER

Lateral geniculate nucleus

Visual cortex Optic radiation

developing earlier during axogenesis than the more superficial ones VISUAL PATHWAY (Reese 1993). The lateral geniculate nucleus contains cells arranged in six laminae The visual pathway includes the interneurones of the retina, retinal (see Fig. 23.6). Each layer receives input from either crossed or uncrossed ganglion cells whose axons project via the optic nerve, chiasma, and projections from the retina. The contralateral nasal retina projects to optic tract to the lateral geniculate nucleus and neurones within the laminae 1, 4 and 6, whereas the ipsilateral temporal retina projects to lateral geniculate nucleus that project via the optic radiation to the layers 2, 3 and 5. Layers 1 and 2 contain magnocellular cells; the primary visual cortex (Fig. 42.32). It is important to remember that remaining layers are parvocellular. Unlike in the optic tract, there is a visual space is optically inverted by the crystalline lens when relating point-to-point retinotopic arrangement between corresponding points the spatial location of neurones within the visual pathway to corres- in each hemi-retina so that the contralateral visual field is mapped ponding visual field locations. within each lateral geniculate nucleus. Retinal ganglion cell axons, on entering the optic nerve, initially Axons from the lateral geniculate nucleus run in the retrolenticular maintain their relative retinal positions, with axons from the fovea part of the internal capsule and form the optic radiation. This curves forming a lateral wedge. Such retinotopic mapping is largely main- dorsomedially to the primary visual cortex, located around and within tained within the optic nerve, although nearer the chiasma the foveal the depths of the calcarine sulcus in the occipital lobe (also known as axons take a position in the centre of the optic nerve while temporal the striate cortex, Brodmann area 17, or V1 (see Fig. 25.19). The visual fibres occupy their previous lateral location. cortex also has a strict retinotopic organization. Fibres representing the At the chiasma, a substantial rearrangement of axons occurs. Most lower half of the visual field sweep superiorly to reach the visual cortex axons arising from the nasal half of a line bisecting the fovea within above the calcarine sulcus, while those representing the upper half of each retina cross in the chiasma to enter the contralateral optic tract. the visual field curve inferiorly into the temporal lobe (Meyer’s loop) Fibres from the temporal hemi-retinas do not generally cross in the before reaching the visual cortex below the calcarine sulcus. The chiasma. Classically, the axons within the optic tract were thought to periphery of the retina is represented anteriorly within the visual maintain their topographic order and each tract was assumed to be a cortex, and the macula is represented towards the posterior pole, occu- single representation of the contralateral hemifield. However, it is now pying a disproportionately large area that reflects the high number of clear that axons are mainly organized in functional groupings, larger foveal retinal ganglion cells that subserve the enhanced acuity of this superficial axons representing the magnocellular pathway and deeper region. axons originating from midget ganglion cells and forming the parvo- The primary visual cortex is connected to prestriate and other cortical cellular pathway. This arrangement is chronotopic, the deeper axons regions where further processing of visual stimuli occurs. 707 EYE

Almost all of the retinal ganglion cell axons (90%) terminate on Since the optic tract contains contralateral nasal and ipsilateral tem- neurones in the lateral geniculate nucleus. Extrageniculate axons (10%) poral retinal projections, damage to it will cause a homonymous con- leave the optic tract before the lateral geniculate nucleus; they may leave tralateral visual field loss. Although complete disruption of the tract the optic chiasma dorsally and project to the suprachiasmatic nucleus results in contralateral hemianopia, the incomplete spatial segregation of the hypothalamus, while others branch off the optic tract at the of axons within the tract, described above, makes the field losses fol- superior brachium and project to the superior colliculus, pretectal areas lowing smaller lesions harder to interpret. They do, however, show and inferior pulvinar. substantial incongruity (dissimilar defects in the fields of the two eyes) and often specific functional deficits, consistent with the partial segrega- tion of functionally distinct axons from the two half-retinas (Reese VISUAL FIELD DEFECTS 1993). Incongruity is most marked in defects of the optic tract, less obvious in optic radiation defects, and usually absent in cortically The basis for clinical assessment of damage to the visual pathway is an induced field defects, thus providing an additional clue in assessing understanding of the retinotopic projections within the pathway. More- location of the cause. over, plotting visual field loss frequently reveals the approximate loca- Lesions of the optic radiations are usually unilateral, and commonly tion of the causative lesion and sometimes its nature. Since retinal vascular in origin. Field defects therefore develop abruptly, in contrast lesions can be visualized with an ophthalmoscope, field testing might to the slow progression of defects associated with tumours, and the appear to be redundant for such defects, but visual field measurement resulting hemifield loss follows the general rule that visual field defects is still helpful in assessing the extent of the damage and may be the key central to the chiasma are on the opposite side to the lesion. Little or factor in confirming a diagnosis. Field defects in glaucoma, for example, no incongruity is seen in visual cortical lesions, but they commonly that occur as a consequence of damage to the nerve fibre bundles at the display the phenomenon of macular sparing, the central 5–10° field optic nerve head, may be detectable ophthalmoscopically, but confir- being retained in an otherwise hemianopic defect. mation of the diagnosis frequently depends on field assessment. Early defects consist of one or more areas of paracentral focal field loss, pro- gressing to arcuate scotomas. The shape of the defect corresponds to

4 the anatomical arrangement of ganglion cell axons. Bonus e-book image As far as the location of lesions central to the retina is concerned, deficits in the vision of one eye are usually attributable to optic nerve lesions. Lesions of the optic chiasma, involving crossing nerve fibres, Fig. 42.20B Fluorescein angiogram showing the macular region of produce a bilateral field loss, as exemplified by a pituitary adenoma. a right eye. The main macular vessels are approaching from the right. The subject was an elderly person with considerable macular SECTION The tumour expands upwards from the pituitary fossa, compressing the inferior midline of the chiasma, and eventually produces bitemporal pigmentation, which masks fluorescence from the choroidal hemianopia, starting with an early loss in the upper temporal quadrants circulation. (bitemporal quadrantanopia).

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