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Home , Autonomic ganglion, Axon, Histology, Internodal segment, Soma (biology), Theodor Schwann

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LABORATORY ACTIVITY FOR STUDENT

Module author : Dr. Wida Purbaningsih, dr., M.Kes Person : Dr. Wida Purbaningsih, dr., M.Kes Subject : Department : Histology

A. Sequence I. Introduction : 40 min II. Pre Test : 5 min III. Activity Lab : 70 min - Discussion 20 min - Identify 50 min IV. Post Test : 5 min B. Topic 1. General microstructure of the , neuroglia, and nerve tissue 2. General microstructure of the central nerve tissue (CNS)   Medulla spinalis 3. Genral microstructure of the peripheral nerve tissue (PNS)

C. Venue Biomedical Laboratory Faculty of , Bandung Islamic Universtity

D. Equipment 1. Light 2. Stained tissue section : 1. Neuron and neuroglia 2. Cerebrum/Cerebellum/Medulla spinalis 3. Cerebrum Nerve tissue Meningens 4. Cerebellum 5. Medulla spinalis 6. otonom Peripheral Nerve 7. Ganglion Snesorik Tissue 8. Nervus ischiadicus 3. Colouring pencils

E Pre-requisite - Before following the laboratory activity, the students must prepare : 1. Draw the schematic picture of neuron and give explanation 2. Mention four type of neuroglia and their functional (, , oligodendrosit, sel schwan, and ependymal ), then draw the schematic neuroglia and give explanation 3. Draw the schematic picture of nerve tissue microsructure and give explanation 2

4. Draw the schematic picture of cerebrum microsructure and give explanation 5. Draw the schematic picture of cerebellum microstructure and give explanation 6. Draw the schematic picture medulla spinalis microstrusture and give explanation 7. Draw the schematic picture of meningens microstructure and give explanation about tissue type 8. Draw the schematic ganglion sensorik microstrusture and give explanation 9. Draw the schematic ganglion otonom microstrusture and give explanation. 10. Draw the schematic nervus (nerve fiber) microstrusture and give explanation

- Content lab in manual book ( pre and post test will be taken from the manual, if scorring pre test less than 50, can not allowed thelab activity ) - Bring your text book, reference book e.q atlas of Histology, e-book etc. (minimal 1 group 1 atlas ). - Bring colouring pencils for drawing

NERVE TISSUE The human , by far the most in the , is formed by a network of many billion nerve cells (), all assisted by many more supporting cells called glial cells. Each neuron has hundreds of interconnections with other neurons, forming a very complex system for processing information and generating responses. Nerve tissue is distributed throughout the body as an integrated communications network.(Gartner 2017) , comprising perhaps as many as a trillion neurons with multitudes of interconnections, forms the complex system of neuronal communication within the body. Certain neurons have receptors, elaborated on their terminals, that are specialized for receiving different types of stimuli (e.g., mechanical, chemical, thermal) and transducing them into nerve impulses that may eventually be conducted to nerve centers. These impulses are then transferred to other neurons for processing.(Mescher 2018) The nervous system develops from the ectoderm of the embryo in response to signaling from the (Gartner 2017). Cells of the nervous system are classified into two categories: neurons and neuroglia. Neurons are responsible for the receptive, integrative, and motor functions of the nervous system. Neuroglial cells are responsible for supporting, protecting, and assisting neurons in neural transmission.

I. NEURONS The cells responsible for the reception and transmission of nerve impulses to and from the CNS are the neurons. Ranging in diameter from 5 to 150 μm, neurons are among both the smallest and the largest cells in the body. Most neurons are composed of three distinct parts: o A cell body (perikaryon or ): the central portion of the cell where the and perinuclear are contained. o multiple , which are the numerous elongated processes extending from the perikaryon and specialized to receive stimuli from other neurons at unique sites called 3

. Dendrites processes specialized for receiving stimuli from sensory cells, , and other neurons o a single . which is a single long process ending at synapses specialized to generate and conduct nerve impulses to other cells (eg, nerve, muscle, and gland cells). Axons may also receive information from other neurons, information that mainly modifies the transmission of action potentials to those neurons. Axon have varying diameter and up to 100 cm in length (Gartner 2017). o Nerve Impulses o Synaptic Communication Generally, neurons in the CNS are polygonal (Fig. 1.a), with concave surfaces between the many cell processes, whereas neurons in the (a sensory ganglion of the PNS) have a round cell body from which only one process exits (Fig. 1.c). Cell bodies exhibit different sizes and shapes that are characteristic for their type and location.

Figure 1. Shown are the four main types of neurons, with short descriptions. (a) Most neurons, including all motor neurons and CNS , are multipolar. (b) Bipolar neurons include sensory neurons of the , , and inner ear. (c) All other sensory neurons are unipolar or pseudounipolar. (d) Anaxonic neurons of the CNS lack true axons and do not produce action potentials, but regulate local electrical changes of adjacent neurons.

I.1 Cell Body (Perikaryon or Soma) The neuronal cell body contains the nucleus and surrounding cytoplasm, exclusive of the cell processes (Figure 4–a and b). It acts as a trophic center, producing most cytoplasm for the processes. Most cell bodies are in contact with a great number of nerve endings conveying excitatory or inhibitory stimuli generated in other neurons. A typical neuron has an unusually large, euchromatic nucleus with a prominent nucleolus, indicating intense synthetic activity. Cytoplasm of perikarya often contains numerous free polyribosomes and highly developed RER, indicating active production of both cytoskeletal and proteins for transport and . Histologically these regions with concentrated RER and other 4 polysomes are and are distinguished as chromatophilic substance (or Nissl substance, Nissl bodies) (Figure 4-a and b). The amount of this material varies with the type and functional state of the neuron and is particularly abundant in large nerve cells such as motor neurons (Figure 4–a and b). The Golgi apparatus is located only in the cell body, but mitochondria can be found throughout the cell and are usually abundant in the axon terminals. In both perikarya and processes , filaments and intermediate filaments are abundant, with the latter formed by unique subunits and called in this cell type. Cross-linked with certain fixatives and impregnated with silver stains, neurofilaments are also referred to as neurofibrils by light microscopists. Some nerve cell bodies also contain inclusions of pigmented material, such as lipofuscin, consisting of residual bodies left from lysosomal . I.2 Dendrites Dendrites (Gr. dendron, tree) are typically short, small processes emerging and branching off the soma (Figure 2). Usually covered with many synapses, dendrites are the principal signal reception and processing sites on neurons. The large number and extensive arborization of dendrites allow a single neuron to receive and integrate signals from many other nerve cells. For example, up to 200,000 axonal endings can make functional contact with the dendrites of a single large of the cerebellum. Dendrites become much thinner as they branch, with cytoskeletal elements predominating in these distal regions. In the CNS most synapses on dendrites occur on dendritic spines, which are dynamic membrane protrusions along the small dendritic branches, visualized with silver (Figure 2) and studied by confocal or electron microscopy. Dendritic spines serve as the initial processing sites for synaptic signals and occur in vast numbers, estimated to be on the order of 1014 for cells of the human . depends on actin filaments and changes continuously as synaptic connections on neurons are modified. Changes in dendritic spines are of key importance in the constant changes of the neural plasticity that occurs during embryonic development and underlies , learning, and postnatally.

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DS

D

D

D

CB

Figure 2. The large Purkinje neuron in this silver-impregnated section of cerebellum has many dendrites (D) emerging from its cell body (CB) and forming branches. The small dendritic branches each have many tiny projecting dendritic spines (DS) spaced closely along their length, each of which is a site of a with another neuron. Dendritic spines are highly dynamic, the number of synapses changing constantly. (X650; Silver stain)

I.3 Axon Most neurons have only one axon, typically longer than its dendrites. Axonal processes vary in length and diameter according to the type of neuron. Axons of the motor neurons that innervate the foot muscles have lengths of nearly a meter; large cell bodies are required to maintain these axons, which contain most of such neurons’ cytoplasm. The plasma membrane of the axon is often called the and its contents are known as (contains mitochondria, microtubules, neurofilaments, and transport vesicles, but very few polyribosomes or cisternae of RER, features that emphasize the dependence of axoplasm on the perikaryon). Axons originate from a pyramid-shaped region of the perikaryon called the (Figure 4), just beyond which the axolemma has concentrated ion channels that generate the . At this initial segment of the axon, the various excitatory and inhibitory stimuli impinging on the neuron are algebraically summed, resulting in the decision to propagate—or not to propagate—a nerve impulse. Axons generally branch less profusely than dendrites, but do undergo terminal arborization (Figure 4). Axons of interneurons and some motor neurons also have major branches called collaterals that end at smaller branches with synapses influencing the activity of many other neurons. Each small axonal branch ends with a dilation called a terminal bouton (Fr. bouton, button) that contacts another neuron or non-nerve cell at a synapse to initiate an impulse in that cell. If an axon is severed from its cell body, its distal part quickly degenerates and undergoes phagocytosis.

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a b

Figure 4. (a) A “typical” neuron has three major parts: (1) The cell body (also called the perikaryon or soma) is often large, with a large, euchromatic nucleus and well-developed nucleolus. The cyto- plasmic contains basophilic Nissl substance or Nissl bodies, which are large masses of free polysomes and RER indicating the cell’s high rate of protein synthesis. (2) Numerous short dendrites extend from the perikaryon, receiving input from other neurons. (3) A long axon carries impulses from the cell body and is covered by a sheath composed of other cells. The ends of axons usually have many small branches (telodendria), each of which ends in a knob-like structure that forms part of a functional connection (synapse) with another neuron or other cell. (b) Micrograph of a large motor neuron showing the large cell body and nucleus (N), a long axon (A) emerging from an axon hillock (AH), and several dendrites (D). Nissl substance (NS) can be seen throughout the cell body and cytoskeletal elements can be detected in the processes. Nuclei of scattered glial II. GLIAL CELLS Glial cells support neuronal survival and activities, and are 10 times more abundant than neurons in the mammalian brain. Like neurons most glial cells develop from progenitor cells of the embryonic neural plate. In the CNS glial cells surround both the neuronal cell bodies, which are often larger than the glial cells, and the processes of axons and dendrites occupying the spaces between neurons. Except around the larger vessels, the CNS has only a very small amount of and . Glial cells substitute for cells of connective tissue in some respects, supporting neurons and creating immediately around those cells microenvironments that are optimal for neuronal activity. The fibrous 7 intercellular network of CNS tissue superficially resembles collagen by light microscopy, but is actually the network of fine cellular processes emerging from neurons and glial cells. Such processes are collectively called the (Figure 5).

Figure 5. (a) Most neuronal cell bodies (N) in the CNS are larger than the much more numerous glial cells (G) that surround them. The various types of glial cells and their relationships with neurons are difficult to distinguish by most routine light microscopic methods. However, have condensed, rounded nuclei and unstained cytoplasm due to very abundant Golgi complexes, which stain poorly and are very likely represented by the cells with those properties seen here. The other glial cells seen here similar in overall size, but with very little cytoplasm and more elongated or oval nuclei, are mostly . Routine H&E staining does not allow neuropil to stand out well. (X200; H&E) (b) With the use of gold staining for neurofibrils, neuropil (Np) is more apparent. (X200; Gold chloride and hematoxylin) There are six major kinds of glial cells, as shown schematically in Figure 9–9, four in the CNS and two in the PNS. Their main functions, locations, and origins are summarized in Table 9–2.

2.1 Oligodendrocytes Oligodendrocytes (Gr. oligos, small, few + dendron, tree + kytos, cell) extend many processes, each of which becomes sheetlike and wraps repeatedly around a portion of a nearby CNS axon (Figure 9–9a). During this wrapping most cytoplasm gradually moves out of the growing extension, leaving multiple compacted layers of collectively termed myelin. An axon’s full length is covered by the action of many oligodendrocytes. The resulting myelin sheath electrically insulates the axon and facilitates rapid transmission of nerve impulses. Found only in the CNS oligodendrocytes are the predominant glial cells in , which is white because of the concentrated in the wrapped membrane sheaths. The processes and sheaths are not visible by routine light staining, in which oligodendrocytes usually appear as small cells with rounded, condensed nuclei and unstained cytoplasm (Figure 5.a).(Mescher 2018)

2.2 Astrocytes Also unique to the CNS astrocytes (Gr. astro-, star + kytos) have a large number of long radiating, branching processes (Figures 9–6 and 9–9a). Proximal regions of the astrocytic processes are reinforced with bundles of intermediate filaments made of glial fibrillary protein (GFAP), which serves as a unique marker for this glial cell. Distally the processes lack GFAP, are not readily seen by microscopy, and form a vast network of delicate terminals contacting synapses and other structures. Terminal processes of a single astrocyte typically occupy a large volume and associate with over a million synaptic sites. Astrocytes originate from progenitor cells in the embryonic neural tube and are by far the most numerous glial cells of the brain, as well as the most diverse structurally and func- 8 tionally. Fibrous astrocytes, with long delicate processes, are abundant in white matter; those with many shorter processes are called protoplasmic astrocytes and predominate in the gray matter. The highly variable and dynamic processes mediate most of these cells many functions.

a b c

Figure 6. (a) Astrocytes are the most abundant glial cells of the CNS and are characterized by numerous cytoplasmic processes (P) radiating from the glial cell body or soma (S). Astrocytic processes are not seen with routine light microscope staining but are easily seen after gold staining. Morphology of the processes allows astrocytes to be classified as fibrous (relatively few and straight processes) or protoplasmic (numerous branching processes), but functional dif- ferences between these types are not clear. (X500; Gold chloride) (b) All astrocytic processes contain intermediate filaments of GFAP, and against this protein provide a simple method to stain these cells, as seen here in a fibrous astrocyte (A) and its processes. The small pieces of other GFAP- positive processes in the neuropil around this cell give an idea of the density of this glial cell and its processes in the CNS. Astrocytes form part of the blood-brain barrier (BBB) and help regulate entry of molecules and ions from blood into CNS tissue. at the extreme upper right and lower left corners are enclosed by GFAP-positive perivascular feet (PF) at the ends of numerous astrocytic processes. (X500; Anti-GFAP immunoperoxidase and hematoxylin counterstain) (c) A length of (C) is shown here completely covered by silver-stained terminal processes extending from astrocytes (A). (X400; Rio Hortega silver)

MEDICAL APPLICATION Most brain tumors are astrocytomas derived from fibrous astrocytes. These are distinguished pathologically by their expression of GFAP. (Mescher 2018)

Functions attributed to astrocytes of various CNS regions include the following: 1. Extending processes that associate with or cover synapses, affecting the formation, , and plasticity of these structures. 2. Regulating the extracellular ionic concentrations around neurons, with particular importance in buffering extracellular K+ levels. 3. Guiding and physically supporting movements and locations of differentiating neurons during CNS development. 4. Extending fibrous processes with expanded perivascular feet that cover capillary endothelial cells and modulate blood flow and help move nutrients, wastes, and other metabolites between neurons and capillaries (Figure 9–9a). 5. Forming a barrier layer of expanded protoplasmic processes, called the glial limiting membrane, which lines the at the external CNS surface. 6. Filling tissue defects after CNS injury by proliferation to form an astrocytic .

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Finally, astrocytes communicate directly with one another via gap junctions, forming a very large cellular network for the coordinated regulation of their various activities in different brain regions.

MEDICAL APLICATION Alzheimer disease, a common type of dementia in the elderly, affects both neuronal perikarya and synapses within the cerebrum. Functional defects are due to neurofibrillary tangles, which are accumulations of associated with microtubules of the neuronal perikaryon and axon hillock regions, and neuritic plaques, which are dense aggregates of β- protein that form around the outside of these neuronal regions.

(Mescher 2018)

2.3 Ependymal Cells Ependymal cells are columnar or cuboidal cells that line the fluid-filled ventricles of the brain and the central canal of the (Figures 9–9a and 9–7). In some CNS locations, the apical ends of ependymal cells have cilia, which facilitate the movement of cerebrospinal fluid (CSF), and long microvilli, which are likely involved in absorption. Ependymal cells are joined apically by apical junctional complexes similar to those of epithelial cells. However, unlike a true there is no basal lamina. Instead, the basal ends of ependymal cells are elongated and extend branching processes into the adjacent neuropil.

Figure 7 Ependymal cells are epithelial-like cells that form a single layer lining the fluid-filled ventricles and central canal of the CNS. (a) Lining the ventricles of the cerebrum, columnar ependymal cells (E) extend cilia and microvilli from the apical surfaces into the ventricle (V). These modifications help circulate the CSF and monitor its contents. Ependymal cells have junctional complexes at their apical ends like those of epithelial cells but lack a basal lamina. The cells’ basal ends are tapered, extending processes that branch and penetrate some distance into the adjacent neuropil (N). Other areas of are responsible for production of CSF. (X100; H&E) (b) Ependymal cells (E) lining the central canal (C) of the spinal cord help move CSF in that CNS region. (X200; H&E)

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2.4 Microglia Less numerous than oligodendrocytes or astrocytes but nearly as common as neurons in some CNS regions, microglia are small cells with actively mobile processes evenly distributed throughout gray and white matter (Figures 9–9a and 9–12). Unlike other glial cells microglia migrate, with their processes scanning the neuropil and removing damaged or effete synapses or other fibrous components. Microglial cells also constitute the major mechanism of immune defense in the CNS, removing any microbial invaders and secreting a number of immunoregulatory cytokines. Microglia do not originate from neural progenitor cells like other , but from circulating blood monocytes, belonging to the same family as and other antigen-presenting cells. Nuclei of microglial cells can often be recognized in routine hematoxylin and (H&E) preparations by their small, dense, slightly elongated structure, which contrasts with the larger, spherical, more lightly stained nuclei of other glial cells. using antibodies against cell surface antigens of immune cells demonstrates microglial processes. When activated by damage or microglia retract their processes, proliferate, and assume the morphologic characteristics and functions of antigen-presenting cells. Figure 8. Microglia are monocyte-derived, antigen- presenting cells of the CNS, less numerous than astrocytes but nearly as common as neurons and evenly distributed in both gray and white matter. By immunohistochemistry, here using a monoclonal against human leukocyte antigens (HLA) of immune-related cells, the short branching processes of microglia can be seen. Routine staining demonstrates only the small dark nuclei of the cells. Unlike other glia of the CNS, microglia are not interconnected; they are motile cells, constantly used in immune surveillance of CNS tissues. When activated by products of cell damage or by invading microorganisms, the cells retract their processes, begin phagocytosing the damage- or danger- related material, and behave as antigen-presenting cells. (X500; Antibody against HLA-DR and peroxidase) (Used with permission from Wolfgang Streit, Department of , University of Florida College of Medicine, Gainesville.) 2.5 Schwann Cells Schwann cells (named for 19th century German histologist ), sometimes called neurolemmocytes, are found only in the PNS and differentiate from precursors in the neural crest. Schwann cells are the counterparts to oligodendrocytes of the CNS, having trophic interactions with axons and most importantly forming their myelin sheathes. However unlike an , a forms myelin around a portion of only one axon. Figure 9–9b shows a series of Schwann cells sheathing the full length of an axon, a process described more fully with peripheral .

2.6 Satellite Cells of Ganglia Also derived from the embryonic neural crest, small satellite cells form a thin, intimate glial layer around each large neuronal cell body in the ganglia of the PNS (Figures 9–9b and 9–13). Satellite cells exert a trophic or supportive effect on these neurons, insulating, nourishing, and regulating their microenvironments. 11

Dikutip dari: (Mescher 2018)

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B. (CNS)

The CNS is completely covered by connective tissue layers, the meninges, but CNS tissue contains very little collagen or similar material, making it relatively soft and easily damaged by injuries affecting the protective skull or vertebral . I. White matter The main components of white matter are myelinated axons (Figure 9–14), often grouped together as tracts, and the myelin-producing oligodendrocytes. Astrocytes and microglia are also present, but very few neuronal cell bodies. Most white matter is found in deeper region of both the cerebrum and the cerebellum

II. Gray matter Gray matter contains abundant neuronal cell bodies, dendrites, astrocytes, and microglial cells, and is where most synapses occur. Gray matter makes up the thick cortex or surface layer of both the cerebrum and the cerebellum.

Many regions show organized areas of white matter and gray matter, differences caused by the differential distribution of lipid-rich myelin. Deep within the brain are localized, variously shaped darker areas called the cerebral nuclei, each containing large numbers of aggregated neuronal cell bodies. In the folded cerebral cortex, neuroscientists recognize six layers of neurons with different sizes and shapes. The most conspicuous of these cells are the efferent pyramidal neurons (Figure 9–15). Neurons of the cerebral cortex function in the integration of sensory information and the initiation of voluntary motor responses. The sharply folded cerebellar cortex coordinates muscular activity throughout the body and is organized with three layers (Figure 9–16): 1. A thick outer molecular layer has much neuropil and scattered neuronal cell bodies. 2. A thin middle layer consists only of very large neurons called Purkinje cells (named for the 19th century Czech histologist Jan Purkinje). These are conspicuous even in H&E- stained sections, and their dendrites extend throughout the molecular layer as a branching basket of nerve fibers (Figures 9–16c and d). 3. A thick inner granular layer contains various very small, densely packed neurons (including granule cells, with diameters of only 4-5 μm) and little neuropil. 13

In cross sections of the spinal cord, the white matter is peripheral and the gray matter forms a deeper, H-shaped mass (Figure 9–17). The two anterior projections of this gray matter, the anterior horns, contain cell bodies of very large motor neurons whose axons make up the ventral roots of spinal nerves. The two posterior horns contain interneurons which receive sensory fibers from neurons in the spinal (dorsal root) ganglia. Near the middle of the cord the gray matter surrounds a small central canal, which develops from the lumen of the neural tube, is continuous with the ventricles of the brain, is lined by ependymal cells, and contains CSF.

III. Meninges The skull and the vertebral column protect the CNS, but between the and nervous tissue are membranes of connective tissue called the meninges. Three meningeal layers are distinguished: the dura, arachnoid, and pia maters (Figures 9–18 and 9–19).

3.1 Dura Mater The thick external dura mater (L. dura mater, tough mother) consists of dense irregular connective tissue organized as an outer periosteal layer continuous with the periosteum of the skull and an inner meningeal layer. These two layers are usually fused, but along the superior sagittal surface and other specific areas around the brain they separate to form the blood-filled dural venous sinuses (Figure 9–19). Around the spinal cord the dura mater is separated from the periosteum of the vertebrae by the epidural space, which contains a plexus of thin-walled and (Figure 9–18). The dura mater may be separated from the arachnoid by formation of a thin subdural space.

3.2 Arachnoid The arachnoid (Gr. arachnoeides, spider web-like) has two components: (1) a sheet of connective tissue in contact with the dura mater and (2) a system of loosely arranged trabeculae composed of collagen and , continuous with the underlying pia mater layer. Surrounding these trabeculae is a large, sponge-like cavity, the subarachnoid space, filled with CSF. This fluid-filled space helps cushion and protect the CNS from minor trauma. The subarachnoid space communicates with the ventricles of the brain where the CSF is produced. The connective tissue of the arachnoid is said to be avascular because it lacks nutritive capillaries, but larger blood vessels run through it (Figures 9–18 and 9–19). Because the arachnoid has fewer trabeculae in the spinal cord, it can be more clearly distinguished from the pia mater in that area. The arachnoid and the pia mater are intimately associated and are often considered a single membrane called the pia-arachnoid. In some areas, the arachnoid penetrates the dura mater and protrudes into blood-filled dural venous sinuses located there (Figure 9–19). These CSF-filled protrusions, which are covered by the vascular endothelial cells lining the sinuses, are called arachnoid villi and function as sites for absorption of CSF into the blood of the venous sinuses.

3.3 Pia Mater The innermost pia mater (L. pia mater, tender mother) consists of flattened, mesenchymally derived cells closely applied to the entire surface of the CNS tissue. The pia does not directly contact nerve cells or fibers, being separated from the neural elements by 14 the very thin superficial layer of astrocytic processes (the glial limiting membrane, or glia limitans), which adheres firmly to the pia mater. Together, the pia mater and the layer of astrocytic end feet form a physical barrier separating CNS tissue from CSF in the subarachnoid space (Figure 9–19). Blood vessels penetrate CNS tissue through long perivascular spaces covered by pia mater, although the pia disappears when the blood vessels branch to form the small capillaries. However, these capillaries remain completely covered by the perivascular layer of astrocytic processes (Figures 9–9a and 9–10c).

IV. Blood-Brain Barrier The blood-brain barrier (BBB) is a functional barrier that allows much tighter control than that in most tissues over the passage of substances moving from blood into the CNS tissue. The main structural component of the BBB is the capillary , in which the cells are tightly sealed together with well-developed occluding junctions, with little or no transcytosis activity, and surrounded by the basement membrane. The limiting layer of perivascular astrocytic feet that envelops the basement membrane of capillaries in most CNS regions (Figure 9–10c) contributes to the BBB and further regulates passage of molecules and ions from blood to brain. The BBB protects neurons and glia from bacterial toxins, infectious agents, and other exogenous substances, and helps maintain the stable composition and constant balance of ions in the interstitial fluid required for normal neuronal function. The BBB is not present in regions of the where plasma components are monitored, in the which releases hormones, or in the choroid plexus where CSF is produced.

V. Choroid Plexus The choroid plexus consists of highly , elaborately folded and projecting into the large ventricles of the brain (Figure 9–20a). It is found in the roofs of the third and fourth ventricles and in parts of the two lateral ventricular walls, all regions in which the ependymal lining directly contacts the pia mater. Each villus of the choroid plexus contains a thin layer of well-vascularized pia mater covered by cuboidal ependymal cells (Figure 9–20b). The function of the choroid plexus is to remove water from blood and release it as the CSF. CSF is clear, contains Na+, K+, and Cl– ions but very little protein, and its only cells are normally very sparse lymphocytes. It is produced continuously and it completely fills the ventricles, the central canal of the spinal cord, the subarachnoid and perivascular spaces. It provides the ions required for CNS neuronal activity and in the arachnoid serves to help absorb mechanical shocks. Arachnoid villi (Figure 9–19) provide the main pathway for absorption of CSF back into the venous circulation. There are very few lymphatic vessels in CNS tissue.

PERIPHERAL NERVOUS SYSTEM The main components of the peripheral nervous system (PNS) are the nerves, ganglia, and nerve endings. Peripheral nerves are bundles of nerve fibers (axons) individually sur- rounded by Schwann cells and connective tissue. 1. Nerve Fibers Nerves are analogous to tracts in the CNS, containing axons enclosed within sheaths of glial cells specialized to facilitate axonal function. In peripheral nerves, axons are sheathed by 15

Schwann cells or neurolemmocytes (Figure 9–9b). The sheath may or may not form myelin around the axons, depending on their diameter. 1.a Myelinated Fibers As axons of large diameter grow in the PNS, they are engulfed along their length by a series of differentiating neurolemmocytes and become myelinated nerve fibers. The plasma membrane of each covering Schwann cell fuses with itself at an area termed the mesaxon and a wide, flattened process of the cell continues to extend itself, moving circumferentially around the axon many times (Figure 9–21). The multiple layers of Schwann cell membrane unite as a thick myelin sheath. Composed mainly of lipid bilayers and membrane proteins, myelin is a large lipoprotein complex that, like cell membranes, is partly removed by standard histologic procedures (Figures 9–14 and 9–17d). Unlike oligodendrocytes of the CNS, a Schwann cell forms myelin around only a portion of one axon. With high-magnification TEM, the myelin sheath appears as a thick electron-dense axonal covering in which the concentric membrane layers may be visible (Figure 9–22). The prominent electron-dense layers visible ultrastructurally in the sheath, the major dense lines, represent the fused, protein-rich cytoplasmic surfaces of the Schwann cell membrane. Along the myelin sheath, these surfaces periodically separate slightly to allow transient movement of cytoplasm for membrane maintenance; at these myelin clefts (or Schmidt- Lanterman clefts) the major dense lines temporarily disappear (Figure 9–23). Faintly seen ultrastructurally in the light staining layers are the intraperiod lines that represent the apposed outer bilayers of the Schwann cell membrane. Membranes of Schwann cells have a higher proportion of than do other cell membranes, and the myelin sheath serves to insulate axons and maintain a constant ionic microenvironment most suitable for action potentials. Between adjacent Schwann cells on an axon the myelin sheath shows small nodes of Ranvier (or nodal gaps, Figures 9–9b, 9– 23, and 9–24), where the axon is only partially covered by interdigitating Schwann cell processes. At these nodes the axolemma is exposed to ions in the interstitial fluid and has a much higher concentration of voltage-gated Na+ channels, which renew the action potential and produce (L. saltare, to jump) of nerve impulses, their rapid movement from node to node. The length of axon ensheathed by one Schwann cell, the , varies directly with axonal diameter and ranges from 300 to 1500 μm.

1.b. Unmyelinated Fibers Unlike the CNS where many short axons are not myelinated at all but course among other neuronal and astrocytic processes, the smallest diameter axons of peripheral nerves are still enveloped within simple folds of Schwann cells (Figure 9–25). These very small unmyelinated fibers do not however undergo multiple wrapping to form a myelin sheath (Figure 9–21). In unmyelinated nerves each Schwann cell can enclose portions of many axons with small diameters. Without the thick myelin sheath, nodes of Ranvier are not seen along unmyelinated nerve fibers. Moreover, these small-diameter axons have evenly distributed voltage-gated ion channels; their impulse conduction is not saltatory and is much slower than that of myelinated axons. Nerve Organization In the PNS, nerve fibers are grouped into bundles to form nerves. Except for very thin nerves containing only unmyelinated fibers, nerves have a whitish, glistening appearance because of their myelin and collagen content. 16

Axons and Schwann cells are enclosed within layers of connective tissue (Figures 9–24, 9–26, and 9–27). Immediately around the external lamina of the Schwann cells is a thin layer called the , consisting of reticular fibers, scattered fibroblasts, and capillaries. Groups of axons with Schwann cells and endoneurium are bundled together as fascicles by a sleeve of , containing flat with their edges sealed together by tight junctions. From two to six layers of these unique connective tissue cells regulate diffusion into the fascicle and make up the blood-nerve barrier which helps maintain the fibers’ microenvironment. Externally, peripheral nerves have a dense, irregular fibrous coat called the , which extends deeply to fill the space between fascicles. Very small nerves consist of one fascicle (Figure 9–28). Small nerves can be found in sections of many organs and often show a winding disposition in connective tissue. Peripheral nerves establish communication between centers in the CNS and the sense organs and effectors (muscles, glands, etc). They generally contain both afferent and efferent fibers. Afferent fibers carry information from internal body regions and the environment to the CNS. Efferent fibers carry impulses from the CNS to effector organs commanded by these centers. Nerves possessing only sensory fibers are called sensory nerves; those composed only of fibers carrying impulses to the effectors are called motor nerves. Most nerves have both sensory and motor fibers and are called mixed nerves, usually also with both myelinated and unmyelinated axons. 2. Ganglia Ganglia are typically ovoid structures containing neuronal cell bodies and their surrounding glial satellite cells supported by delicate connective tissue and surrounded by a denser capsule. Because they serve as relay stations to transmit nerve impulses, at least one nerve enters and another exits from each ganglion. The direction of the nerve impulse determines whether the ganglion will be a sensory or an . 2.a Sensory Ganglia Sensory ganglia receive afferent impulses that go to the CNS. Sensory ganglia are associated with both (cranial ganglia) and the dorsal roots of the spinal nerves (spinal ganglia). The large neuronal cell bodies of ganglia (Figure 9–29) are associated with thin, sheetlike extensions of small glial satellite cells (Figures 9–9b and 9–13). Sensory ganglia are supported by a distinct connective tissue capsule and an internal framework continuous with the connective tissue layers of the nerves. The neurons of these ganglia are pseudounipolar and relay information from the ganglion’s nerve endings to the gray matter of the spinal cord via synapses with local neurons. 2.b Autonomic Ganglia Autonomic (Gr. autos, self + nomos, law) nerves effect the activity of , the secretion of some glands, heart rate, and many other involuntary activities by which the body maintains a constant internal environment (). Autonomic ganglia are small bulbous dilations in autonomic nerves, usually with multipolar neurons. Some are located within certain organs, especially in the walls of the digestive tract, where they constitute the intramural ganglia. The capsules of these ganglia may be poorly defined among the local connective tissue. A layer of satellite cells also envelops the neurons of autonomic ganglia (Figure 9–29), although these may also be inconspicuous in intramural ganglia. Autonomic nerves use two neuron circuits. The first neuron of the chain, with the preganglionic fiber, is located in the CNS. Its axon forms a synapse with postganglionic 17 fibers of the second in the chain located in a peripheral ganglion system. The chemical mediator present in the synaptic vesicles of all preganglionic axons is acetylcholine. As indicated earlier autonomic nerves make up the . This has two parts: the sympathetic and the parasympathetic divisions. Neuronal cell bodies of preganglionic sympathetic nerves are located in the thoracic and lumbar segments of the spinal cord and those of the parasympathetic division are in the medulla and midbrain and in the sacral portion of the spinal cord. Sympathetic second neurons are located in small ganglia along the vertebral column, while second neurons of the parasympathetic series are found in very small ganglia always located near or within the effector organs, for example in the walls of the stomach and intestines. may lack distinct capsules altogether, perikarya and associated satellite cells simply forming a loosely organized plexus within the surrounding connective tissue.

NEURAL PLASTICITY & REGENERATION Despite its general stability, the nervous system exhibits neuronal differentiation and formation of new synapses even in adults. Embryonic development of the nervous system produces an excess of differentiating neurons, and the cells which do not establish correct synapses with other neurons are eliminated by . In adult mammals after an injury, the neuronal circuits may be reorganized by the growth of neuronal processes, forming new synapses to replace ones lost by injury. Thus, new communications are established with some degree of functional recovery. This neural plasticity and reformation of processes are controlled by several growth factors produced by both neurons and glial cells in a family of proteins called . Neuronal stem cells are present in the adult CNS, located in part among the cells of the ependyma, which can supply new neurons, astrocytes, and oligodendrocytes. Fully differentiated, interconnected CNS neurons cannot temporarily disengage these connections and divide to replace cells lost by injury or disease; the potential of neural stem cells to allow tissue regeneration and functional recovery within the CNS components is a subject of intense investigation. Astrocytes do proliferate at injured sites and these growing cells can interfere with successful axonal regeneration in structures such as spinal cord tracts. In the histologically much simpler peripheral nerves, injured axons have a much greater potential for regeneration and return of function. If the cell bodies are intact, damaged, or severed PNS axons can regenerate as shown in the sequence of diagrams in Figure 9–30. Distal portions of axons, isolated from their source of new proteins and , degenerate; the surrounding Schwann cells dedifferentiate, shed the myelin sheaths, and proliferate within the surrounding layers of connective tissue. Cellular debris including shed myelin is removed by blood-derived macrophages, which also secrete neurotrophins to promote anabolic events of axon regeneration. The onset of regeneration is signaled by changes in the perikaryon that characterize the process of : the cell body swells slightly, Nissl substance is initially dimin- ished, and the nucleus migrates to a peripheral position within the perikaryon. The proximal segment of the axon close to the wound degenerates for a short distance, but begins to grow again distally as new Nissl substance appears and debris is removed. The new Schwann cells align to serve as guides for the regrowing axons and produce polypeptide factors which promote axonal outgrowth. Motor axons reestablish synaptic connections with muscles and function is restored. 18

F Activity Lab 1. Students will be divided into 17 groups 2. Discussion in 20 minute: 3. Identify tissue section using light microscopy and draw it , in 50 minute 4. LIST MICROSTRUCTURE IDENTIFY REVIEW ( give the checklist √ if you have already known) I. Neuron and neuroglia Check list 1. Neuron (Preparat: Motor neuron) - Identify part of the neuron cell (cell body, axon and dendrit). Draw ! 100 x

1000X

19

Identify the structure below, then fill in the box below with the appropriate structure !

20

2. Neuroglia ( Preparat : Medulla spinalis/cerebrum cortex/) Identify type cell of neuroglia : astrocyte, microglia, oligodendrosit.)

Draw !

21

Ada halo Bening di sekitar sel

22

II. Nerve tissue ( of cerebrum)

1. Cerebrum - Indentify part of cerebrum (grey matter/cortex) and medulla (white matter)  40X - Identify of neuron, Neuroglia , meningens (piamater, arachnoid space and arachnoid) 400x/1000x

Mention layer of cerebrum

23

Grey matter 1000X White matter 1000X

Meningens: Piamater and Arachnoid 1000x Indentify meningens of cerebrum and fill in the box below with the appropriate structure name.

2. Cerebellum 24

- Identify three layer of cerebellum -- 100X - Indentify of purkinje cell in middle layer  400x malphigian layer

Draw ( 100x)

Draw (400x/1000X)

Identify the structure below 25

Cerebellum

…………. layer

………….. layer

Layers of the skin

]

Neuron cell

2. Medulla spinalis 26

- Indentify part of medulla spinalis (grey matter/central) and medulla (white matter/perifer) -> 40X - Identify of neuron, meningens, canalis centralis, duramater  400x/1000x Draw !

Medulla spinalis 40 X Canalis ……….

III. Peripheral Nerve Tissue 27

3.1 Ganglion 1. Sensorik Ganglion (somatik) - Identify distribution of ganglion cell and ganglion microstructure - Indentify morfology of the ganglion cells bellow Draw

2. Autonom genglion - Identify distribution of ganglion cell and ganglion organ microstructure - Indentify morfology of the ganglion cells bellow

Draw

Identify the structure bellow and fill in the box below with the appropriate structure 28

Sensorik Ganglion 100X

Ganglion Cells 1000X Give an arrow to the appropriate structure !

Satelit cells

Ganglion cells

29

Ganglion Autonom 40X

Ganglion Autonom 1000X

3.2 Nerve fiber 30

1. Cross section - Identify epineurium, perineurium and endoneurium Draw !

2. Longitudinal section Draw !

Identify teh structure bellow, fill 31

IV. Motor end plate (sceletal muscle, Ag Nitrat) 32

1. Motor end plate Identify motor end plate at scleletal muscle preparat Draw 400X

Identify the structure below, and fill in the box below with the appropriate structure

G Reference Gartner, L.P., 2017. Text Book of Histology Fourth., Mescher, A.L., 2018. The text book of Histology Edisi 15.,