Chapter 5

Comparative anatomy shows that the mass of the cerebral lobes, and more especially their superficial ridging by fissures and gyri, increase with increasing intelligence of the animal (Wundt, 1874, p. 285). Translated from the German publication of the very first psychology text: Principles of

In this chapter the focus moves away from the molecular or cellular view of the to the global anatomical view. The goal is to provide a basic framework of important structures within the brain onto which human behaviors can be mapped. Neuroanatomy is generally considered to be universal (i.e., invariant across cultures). The central is made up of: (a) groups of , known as nuclei; (b) groups of axons that project away from the nuclei, known as tracts; and (c) glia cells. Neuroanatomy is mostly concerned with nuclei and tracks, as these represent the communication system within the brain that is responsible for producing behavior. The distinction between nuclei and tracts is also the distinction between gray and , respectively, in reference to visual inspection of the brain. The delineation of nuclei into distinct regions by neuroanatomists was done on the assumption that groupings of adjacent neurons with similar cytoarchitecture (structure and organization) formed a functionally homogeneous region that modulated (changed) the flow of information (action potentials) through that region. Once these neuroanatomical units had been delineated it was expected that the function of each unit in terms of effects on behavior could then be determined. Brain/behavior relationships are arranged within a hierarchy whereby the most basic human behaviors (e.g., respiration; control of the cardiac system) are controlled from nuclei located at the bottom of the brain stem where the brain intersects with the and the most complex of human behaviors (e.g., language; conscious volition) are controlled by nuclei within the at the top of the hierarchy. In between, behaviors modulated by nuclei become increasingly more complex (from an evolutionary standpoint) as one moves from the bottom of the to the anterior cerebral cortex. Commonly used terminology for indicating direction within the three dimensions of the brain includes: (a) anterior (also called rostral) vs. posterior (caudal), referring to the front and the back, respectively; (b) superior (dorsal) vs. inferior (ventral), referring to the top and bottom, respectively; and (c) lateral vs. medial, referring to the sides and the middle, respectively. Common cross-sections of the brain that are used in brain imaging are: (a) horizontal (parallel to the ground); (b) coronal (side to side: see Figure 5.1); and (c) sagittal (front to back: see Figures 5.7 and 5.11). The coronal section shows that the distribution of grey matter (nuclei: darker matter in figure) within the cerebral cortex is on the surface compared with the white matter (tracks: lighter matter) that are interior. The mid-sagittal section pictured in Figures 5.7 and 5.11 divides the corpus collosum. The corpus collosum is the main pathway that allows information to flow between the two hemispheres of the cerebral cortex. Aside from midline structures like the corpus collosum, all brain structures are doubled,

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having a left and a right version. When the brain becomes the brain stem and spinal cord the orien- tation changes by 90º and the back of the body becomes dorsal and the front ventral. Another often used distinction is between ispilateral meaning structures on the same side (e.g., the left innervates the left eye) and contralateral meaning structures on opposite sides of the body (e.g., the right motor cortex controls the left arm and leg).

Figure 5.1 coronal section through the brain

Source: Wikimedia

Meninges

The nervous system is covered by protective tissue. The protective tissue around the brain and spinal cord is called the (see Figure 5.2). The meninges are composed of three layers: (a) dura mater; (b) arachnoid membrane; and (c) pia mater (contains small surface blood vessels). Between the pia mater and the arachnoid membrane is the subarachnoid space that is filled with cerebrospinal fluid.

Ventricular System

Cerebrospinal fluid is produced in the ventricles of the brain that consist of a series of four interconnected spaces. As shown in Figure 5.3, the largest ventricles are the two lateral ventricles that occupy space within the two cerebral hemispheres. These lateral ventricles connect with the third ventricle and, via the cerebral aqueduct, to the fourth ventricle. The third and fourth ventricles and the cerebral aqueduct are in the midline. Cerebrospinal fluid is a clear, colorless liquid that is manufactured from materials in the blood by specialized tissue within the ventricles known as the choroid plexus. It is produced continuously and has a half-life of about three hours. It circulates from the ventricles into the subarachnoid space around the Neuroanatomy 73 brain and spinal cord before being reabsorbed back into the blood supply. Cerebrospinal fluid provides buoyancy to the mass of the brain by suspending it in fluid. It also circulates nutrients from the blood supply and cleans the brain of metabolic waste.

Figure 5.2 meninges

Source: Shutterstock

Figure 5.3 ventricular system of the brain

Source: Wikimedia 74 Chapter FIVE

Cerebral Cortex

When the skull and meninges are removed the cerebral cortex of the brain becomes visible (see Figure 5.4). The exterior of the cerebral cortex is characterized by ridges (gyri) and valleys (fissures or sulci). All of these have a specific name. As illustration, one of the most prominent fissures is the central fissure that divides the frontal and parietal lobes. The presence of gyri and fissures triples the area of the cerebral cortex that can be contained within the skull. The introductory quote to this chapter is from the very first physiological psychology textbook (Wundt, 1874) that describes a steady increase in the size of the cerebral cortex among animals as one progresses up the evolutionary scale, corresponding to an increase in the complexity of behavior. The more cerebral cortex a species has the more sophisticated its behavior. The cerebral cortex is responsible for all intricate human behaviors, including all conscious behaviors. It is divided into two hemispheres that are differentially specialized for neurocognitive behavior. In most people, the left hemisphere (the dominant hemisphere) is more specialized for language processing. The right hemisphere is called the non-dominant hemisphere and is more specialized for visual-perceptual processing. Some 30% of left handed individuals (who themselves number about 10% of the population) have right hemispheric or mixed language dominance.

Figure 5.4 major lobes of the cerebral cortex

Source: Shutterstock

Primary, Secondary, and Association Cortex Each hemisphere of the cerebral cortex is divided into four lobes that are named for the bones of the skull directly above them, namely the occipital, temporal, parietal, and frontal lobes (see Figure 5.4). There is also a “fifth ” known as theinsula (or island of Reil, , or intrasylvian cortex) The insula (see Figure 5.5) is shaped like an inverted triangle and forms the base of the lateral (sylvian fissure) that separates the from the parietal and frontal lobes. It is completely obscured in the lateral view of the exposed cerebral cortex. It was named by Johann-Christian Reil (1809), the father of German psychiatry. Insula means island in Latin. Neuroanatomy 75

Figure 5.5 insular cortex (the “fifth lobe”) shown in a lateral view of the left with the overlying opercula of the temporal, parietal, and frontal lobes removed

Source: Gray’s Anatomy

As a general rule, all regions of the cerebral cortex posterior to the central fissure (i.e., the occipital, temporal, parietal and insular lobes) are involved in processing sensory information while all regions of the cerebral cortex anterior to the central fissure (i.e., the frontal lobes) are involved in motor or response behaviors. Each lobe contains a region of primary cortex. The contains primary ; the temporal lobe has primary ; the has primary somatosensory cortex; the insular lobe has primary interoceptive cortex (Gasquoine, 2014a: interoceptive being a generic term that includes sensory information from the internal milieu including temperature, pain, itch, tickle, visceral sensations, and others) and primary gustatory cortex (Gorschkow, 1901); and the has . The primary sensory cortices are the first regions of the cerebral cortex to receive modality-specific sensory information. Primary motor cortex is the last region of the cerebral cortex to send information to the limbs for execution of movements. Organization in primary regions of cerebral cortex is typically contralateral. An exception is taste in the insula that has ipsilateral representation. There are also regions of cerebral cortex that are known as secondary cortex. Regions of secondary sensory cortex receive modality-specific input directly from primary sensory cortical regions while secondary motor cortex transmits information directly to primary motor cortex. Regions of cerebral cortex outside primary and secondary cortex are known as association cortex. Association cortex is the least specialized cortex in terms of sensory or motor specificity and consequently it is harder to pinpoint its behavioral function than those regions of primary and secondary cortex that are modality specific.

Identifying Regions of Cerebral Cortex There are three different ways in which a distinct region of cerebral cortex can be identified: (a) by its given name (e.g., primary visual cortex); (b) by its location within the lobes of the brain (e.g., occipital lobe); or (c) by its number (e.g., area 24). The last refers to a cytoarchitectonic map of the outer surface of the human cerebral cortex compiled by German neuroanatomist Korbinian 76 Chapter FIVE

Brodmann (1868-1918) in 1909 that is still widely used in modern times (Zilles & Amunts, 2010). This map delineated different regions of the cerebral cortex containing neurons of similar organization, structure, and distribution that were each assigned a number between 1 and 52. These numbers have ordinal value only and were assigned in no specific sequence. It was thought by Brodmann that each distinct numbered region would come to have a different behavioral function, although this did not quite work out and the numbers remain structurally, but not necessarily functionally, defined. Figure 5.6 and 5.7 shows Brodmann’s cytoarchitectonic map of the lateral and medial surfaces of the cerebral cortex, respectively. Numbers 13-16 are missing as they refer to areas of insular cortex that are hidden from view. Numbers 48-51 are also missing because these brain regions were too small to clearly label (Simic & Hof, 2015).

Columnar Organization of Cerebral Cortex Grey Matter Within the brain, grey matter (the nuclei of the neurons) is located on the outside, whereas white matter (the axons forming tracts) is on the inside. The grey matter nuclei on the outer surface of the cerebral cortex are organized within vertical columns with six distinct layers that contain different types of neurons in differing densities (see Figure 5.8). These layers have different general functions: (a) the top three layers (I, II, and III) involve communication with other cortical columns: (b) layer IV receives input into a particular column from subcortical structures (the cerebral cortex has bidirectional connec- tions with virtually every other part of the brain: collectively the term subcortical structures represents all of the brain under the cerebral cortex); and (c) layers V and VI are mostly concerned with sending output to subcortical structures. Sensory regions of the cerebral cortex have a much larger layer IV than motor regions that have much larger layers V and VI. The significance of this vertical organization has been the subject of much debate that remains unresolved. Cerebral cortex with these six layers is known as and this constitutes the majority of this structure. In contrast, there is another form of cerebral cortex located just above the corpus collosum on the medial surface of each hemisphere with just three layers. This includes the cingulate gyrus (Brodmann’s areas 24 and 32: see Figure 5.7) and is known as limbic cortex as it is part of the limbic system. Neuroanatomy 77

Figure 5.6 brodmann’s cytoarchitectonic map of the lateral surface of the cerebral cortex

Source: Wikimedia

Figure 5.7 brodmann’s cytoarchitectonic map of the medial surface of the cerebral cortex

Source: Wikimedia 78 Chapter FIVE

Figure 5.8 layers of the cerebral cortex (ramón y cajal, 1899): (a) nissl- stained visual cortex of a human adult showing nine layers (p. 314); (b) nissl-stained motor cortex of a human adult showing eight layers (p. 361); and (c) golgi-stained motor cortex of a 1 1/2 month old infant showing seven layers (p. 363).

Source: Wikimedia Neuroanatomy 79

Limbic System

The limbic system is a set of brain regions that are grouped together due to their close functional connectivity. Figure 5.9 shows the location within the brain of four main structures of the limbic system: (a) cingulate (limbic) cortex; (b) ; (c) ; and (d) mammillary bodies. The hippo- campus and amygdala lie beneath the cerebral cortex of the temporal lobe within each hemisphere. Like the cingulate gyrus, the hippocampus is limbic cortex (it forms part of the cerebral cortex infolded upon itself) with three layers. The amygdala and mammillary bodies are subcortical structures.

Figure 5.9 major structures of the limbic system

Source: www.uni.edu

The term limbic (from limbus meaning border in Latin), was originally used by Broca (1878) to describe a “lobe” on the medial and inferior surfaces of the cerebral hemispheres that formed the border around the brainstem. Historically, these structures were linked with olfaction (the lies directly anterior to the limbic system) until Papez (1937) used data from neuroanatomical dissection and animal lesion studies to conclude: “the , the anterior thalamic nuclei, the gyrus cinguli, the hippocampus and their interconnections constitute a harmonious mechanism which may elaborate the functions of central ” (p. 743). This circuit forms part (but not all) of the modern day conception of the limbic system. MacLean (1952) is credited with illuminating the central function of the limbic system as: “a visceral brain that interprets and gives expression to its incoming information in terms of feeling” (p. 415: italics as in original). Later the hippocampus was linked with the encoding of . It is now clear that the limbic system does not operate as a single unit (Devinsky, Morrell, & Vogt, 1995; LeDoux, 1995) as was historically hypothesized, but has both emotional and neurocognitive functions. All behavior mediated by the limbic system takes place below the level of . 80 Chapter FIVE

Basal Ganglia

The consist of a group of three subcortical nuclei that lie just below the white matter of the cerebral cortex, the: (a) caudate nucleus; (b) putamen; and (c) globus pallidus (see Figure 5.10). The caudate nucleus and the putamen are together known as the striatum. All three nuclei are involved in the control of movement, specifically the control and coordination of involuntary movement patterns, but not the activation of specific muscles (that role goes to primary motor cortex in the cerebral cortex). The function of the basal ganglia (actually a misnomer as the term “ganglia” should only be used within the peripheral nervous system) is known chiefly from the effects of two movement disorders, Parkinson’s disease and Huntington’s disease. Parkinson’s disease produces bilateral tremor, stiff muscles (rigidity), facial masking (blank facial expression), and difficulty initiating voluntary movements. It is associated with diminished levels of within the brain from dysfunction of the substantia nigra, a brainstem nucleus that innervates the basal ganglia. Huntington’s disease is a fatal genetic disorder that involves neuronal loss in the basal ganglia. The primary manifestation of Huntington’s disease is the occurrence of involuntary bodily movements.

Figure 5.10 major nuclei of the basal ganglia

Source: www.alinenewton.com

Thalamus and Hypothalamus

The (see Figure 5.10) has two lobes, a left and a right, connected by the massa intermedia. The massa intermedia is a structure of unclear functional significance surrounded by cerebrospinal fluid Neuroanatomy 81 within the third ventricle. As evidence that not all human are the same, the massa intermedia is missing in about 20-30% of individuals with no apparent adverse effect. The thalamus functions as a sensory relay station. Most sensory input (olfaction excepted) passes through the thalamus on its way from the organs to primary modality-specific (e.g., visual, auditory) regions of the cerebral cortex. The thalamus is comprised of groups of nuclei that are modality-specific (e.g., the lateral geniculate is a relay center for visual information). The hypothalamus (see Figure 5.11) is located beneath the anterior thalamus. The role of the hypothalamus in controlling the and (e.g., blood sugar levels; sleep duration) was reviewed in chapter 3.

Figure 5.11 nuclei of the hypothalamus

Source: Shutterstock

Cerebellum

The is located in the posterior fossa of the skull below the occipital lobes and dorsal to the brain stem (see Figure 5.4). The cerebellum is associated with fine motor control and motor coordi- nation, including acquiring and maintaining skilled movements required to play a musical instrument. It has ipsilateral representation. The major manifestation of cerebellar injury is ataxia. Ataxia refers to motor incoordination that can affect the fingers, hands, arms, legs, body, speech, or eyes. There are different types of ataxias. For example, ataxia that affects speech is a form of dysarthria. There is a form of ataxia known as optic ataxia, with characteristic misreaching that is not associated with cerebellar injury, but injury to the posterior parietal lobe in the cerebral cortex. 82 Chapter FIVE

Cerebellar injury can also result in neurocognitive impairment in , language, visual- , and (O’Halloran, Kinsella, & Storey, 2012). These impairments rarely persist and are hypothesized to occur from disconnection syndromes, whereby the flow of information between the cerebellum and regions of the cerebral cortex that are primarily involved in the control of these aspects of neurocognition are disrupted. The occurrence of these impairments, even temporarily, after cerebellar injury does suggest that the cerebellum plays some role in , but exactly what remains speculative. Based on a study of eight patients with focal cerebellar lesions it was proposed that “the detection of a sequence and the acquisition of declarative knowledge about it” (Molinari et al., 1997, p. 1759) was the common denominator.

Brainstem: and medulla

Beneath the thalamus and hypothalamus are the two main parts of the brain stem, the pons and the medulla. The lower extremity of the medulla extends into the spinal cord (see Figure 5.12). Nuclei within the pons include the: (a) substantia nigra: this nucleus connects to the basal ganglia and is involved in movement and the production of the neurotransmitter dopamine; (b) superior and inferior colliculi: these two groups of nuclei are involved in orienting movements towards visual and auditory stimuli, respectively; and (c) the ascending reticular activating system (see Figure 3.4), a large system of gray and white matter that is involved in (waking). The medulla controls bodily functions like breathing, , and blood pressure, and the reflexes of vomiting, coughing, sneezing, and .

Cranial nerves

There are 12 that represent the extension of the spinal nerves for the face and head region. With the exception of the Optic nerve (II) that carries information from the into the brain, the cranial nerves are part of the peripheral nervous system. They originate from the brainstem in descending numerical order. Except for the Trochlear nerve (IV) that controls rotational eye-movements, they all have ipsilateral innervation. The 12 cranial nerves are listed in Table 5.1. A useful mnemonic for remembering the names of the cranial nerves in order is: “On Old Olympus Towering Top A Finn And German Viewed A Hop”. Prominent behavioral changes associated with injury to each cranial nerve are: I – loss of smell; II – loss of vision (blindness in the ipsilateral eye); III – outward eye deviation (patient reports double vision) and (possibly) difficulty raising the eyelid; IV – one eye is higher by an amount that varies with the rotation of the head (double vision); V – facial numbness; VI – inward eye deviation (double vision); VII – facial paralysis and reduction of taste (anterior 2/3rds of tongue); VIII – loss of hearing and dizzy spells; IX – reduction of taste (posterior 1/3rd of tongue) and loss of gag reflex; X – difficulties with swallowing, hoarseness, and nausea; XI – difficulty shrugging the shoulders; and XII – difficulty moving the tongue from side to side. As the cranial nerves (excepting II) are part of the peripheral nervous system, all of Neuroanatomy 83 these behavioral changes have the capacity to resolve as the nerve tries to regenerate. The regeneration process may take up to six months and may not be successful if the severed nerve ends cannot align.

Figure 5.12 mid -sagittal section through the brain showing the location of the pons and medulla of the brain stem

Source: Shutterstock

Table 5.1 cranial nerves

Numeral Name Function Sensory/Motor I Olfactory Smell Sensory II Optic Vision Sensory III Oculomotor Eye-movement Motor IV Trochlear Eye-movement Motor V Trigeminal Facial touch Sensory Jaw muscles Motor VI Abducens Eye-movement Motor VII Facial Taste (anterior 2/3rds tongue) Sensory Facial muscles Motor VIII Auditory Audition Sensory Balance (vestibular) Sensory IX Glossopharyngeal Taste (posterior 1/3rd tongue) Sensory Throat muscles Motor X Vagus Internal organs Sensory/motor Larynx muscles Motor XI Accessory Shoulder and neck muscles Motor XII Hypoglossal Tongue muscles Motor 84 Chapter FIVE

Spinal cord

The spinal cord is protected by the bones of the vertebral column that comprise 24 individual vertebra divided into: 7x cervical (neck) segments (C1; C2;…C7); 12x thoracic (chest) segments (T1; T2;…T12); and 5x lumbar (lower back) segments (L1; L2:…L5). The fused vertebra at the bottom of the spinal cord is called the sacrum. In contrast to the cerebral cortex, within the spinal cord the white matter tracks are on the outside, while the gray matter nuclei are on the inside. The spinal nerves that are part of the peripheral nervous system begin at the junction of the dorsal (sensory fibers) and ventral (motor fibers) roots of the spinal cord. Spinal nerves connect to muscles, internal organs, and sensory receptors. Figure 5.13 diagrams the contrasting effects of the sympathetic and parasympathetic branches of the on various bodily organs. The sympathetic branch (innervated from the thoracic and lumbar regions of the spinal cord) prepares the body to counter perceived stressors and the parasym- pathetic branch (innervated from some of the nuclei of the cranial nerves, especially the Vagus [X] nerve, and the sacral region of the spinal cord) relaxes the body and conserves energy.

Figure 5.13 sympathetic and parasympathetic branches of the autonomic nervous system

Source: Wikimedia Neuroanatomy 85

Summary

Neuroanatomy identifies the main groupings of nuclei ( cell bodies) and tracts (axons) within the brain. The organization of brain/behavior relationships is hierarchical whereby the most complex of human behaviors (e.g., conscious behaviors; language) are controlled by structures in anterior cerebral cortex and the most basic of human behaviors (e.g., respiration) are controlled from the bottom of the brain stem. The has a protective covering known as the meninges composed of: (a) dura mater; (b) arachnoid membrane; and (c) pia mater. The mass of the brain is supported by cerebrospinal fluid produced within the ventricular system that consists of the four main cavities within the brain comprising two lateral, a third, and fourth ventricle. Cerebrospinal fluid is manufactured by thechoroid plexus and it flows through the ventricles and within the subarachnoid space between the arachnoid membrane and pia mater of the meninges before being reabsorbed back into the blood supply. The visible part of the brain when the skull and meninges are removed is the cerebral cortex. It is covered by ridges (gyri) and valleys (fissures or sulci) that triples its area. It is responsible for intricate neurocognitive behaviors including all conscious behavior. It is divided into two hemispheres, the left or dominant hemisphere being more specialized for language processing and the right or non-dominant hemisphere being more specialized for visual-perceptual processing. These hemispheres are connected by the corpus collosum. Each hemisphere is divided into occipital, temporal, parietal, and frontal lobes that are named for bones of the skull. There is also a fifth lobe known as insular cortex that is hidden from view in the exposed brain. The occipital lobe contains primary visual cortex, the temporal lobe has primary auditory cortex, the parietal lobe has primary somatosensory cortex, the insula contains primary interoceptive, and gustatory cortices, and the frontal lobe has primary motor cortex. In addition to primary cortical regions there are also modality-specific regions of cerebral cortex that are known as secondary cortex and non-modality-specific regions that are known as association cortex. Distinct regions of cerebral cortex can be identified by: (a) name; (b) location within the lobes of the brain; or (c) Brodmann area number. The grey matter nuclei of the cerebral cortex are organized within vertical columns with six distinct layers that contain different types of neurons in differing densities. The limbic system mediates the unconscious processes of encoding. Its main structures are the: (a) cingulate (limbic) cortex; (b) hippocampus; (c) amygdala; and (d) mammillary bodies. Limbic cortex has only three layers. Medial to the limbic system lie the basal ganglia that consist of a group of three subcortical nuclei involved in the coordination of involuntary movement patterns: (a) caudate nucleus; (b) putamen; and (c) globus pallidus. Medial to the basal ganglia is the thalamus that functions as a sensory relay station and the hypothalamus that is implicated in homeostasis (e.g., temperature regulation; blood sugar control), control of the autonomic nervous system, and control of the endocrine (hormone) system. Beneath the thalamus and hypothalamus are the two main parts of the brain stem, the pons and the medulla. The pons contains the reticular activating system that is involved in sleep and arousal. The medulla controls bodily functions like breathing, heart rate, and blood pressure. Posterior to the brainstem is the cerebellum that is associated with fine motor control and motor coordination, including acquiring and maintaining skilled movements. 86 Chapter FIVE

There are 12 cranial nerves that represent the extension of the spinal nerves for the face and head region: I = Olfactory (smell); II = Optic (vision); III = Oculomotor (eye-movement); IV = Trochlear (eye-movement); V = Trigeminal (facial touch); VI = Abducens (eye-movement); VII = Facial (face muscles and taste); VIII = Auditory (hearing and balance); IX = Glossopharyngeal (taste); X = Vagus (internal organs); XI = Accessory (shoulder muscles); and XII = Hypoglossal (tongue movement). A useful mnemonic for remembering the names of the cranial nerves in order is: “On Old Olympus Towering Top A Finn And German Viewed A Hop”. The spinal cord is protected by the bones of the vertebral column composed of 24 individual segments: 7x cervical (neck); 12x thoracic (chest); and 5x lumbar (lower back). The spinal nerves are part of the peripheral nervous system and connect to muscles, internal organs, and sensory receptors. The sympathetic branch of the autonomic nervous system prepares the body to meet stressors, while the parasympathetic branch allows the body to relax and conserve energy.