Chapter 5 Neuroanatomy
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CHAPTER 5 Neuroanatomy 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 Physiological Psychology In this chapter the focus moves away from the molecular or cellular view of the brain 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 nervous system is made up of: (a) groups of neurons, 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 white matter, 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 spinal cord and the most complex of human behaviors (e.g., language; conscious volition) are controlled by nuclei within the cerebral cortex 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 brainstem 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, 71 72 CHAPTER FIVE 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 optic nerve 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 meninges (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 lobe” known as theinsula (or island of Reil, insular cortex, or intrasylvian cortex) The insula (see Figure 5.5) is shaped like an inverted triangle and forms the base of the lateral sulcus (sylvian fissure) that separates the temporal lobe 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 CEREBRAL HEMISPHERE 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 occipital lobe contains primary visual cortex; the temporal lobe has primary auditory cortex; the parietal lobe 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 frontal lobe has primary motor cortex. 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.