The central nervous system (CNS) includes the brain and spinal cord. One of the major factors distinguishing animals into different classes is the degree of brain development in response to evolution. This process, called cephalization, has led to increases in the size and complexity of the rostral, or front, portion of the brain. The central nervous system derives from the ectoderm, the outermost layer of the embryo. During gastrulation, the notochord develops from the chordamesodermal tissue. The notochord elicits secretion of a protein called noggin, which induces thickening of the overlying ectoderm to create the neural plate (Fig. 9.1). Shortly after the formation of the neural plate, its lateral edges elevate, creating the neural folds, which flank the neural groove. As the neural plate invaginates, the neural folds surround it. The lateral edges of the neural folds eventually migrate toward the longitudinal midline of the embryo to produce the neural tube. The neural tube then separates from the overlying cutaneous ectoderm. The cavity inside the neural tube is called the neurocoele. Closure of the tube first occurs in the upper spinal cord and progresses to both cephalad (toward the head) and caudad (toward the tail). In the chick, neurulation occurs in the cephalic region while gastrulation is still occurring in the caudal region. The opened ends of the tube are called the anterior and posterior neuropore, respectively. In mammals, neural tube closure is initiated at multiple sites along the tube. Failure of the tube to close at different sites results in various birth defects. Spina bifida occurs if the posterior neural tube does not close, whereas anencephaly is a lethal condition that results when the anterior neural tube fails to close. Craniorachischisis is a failure of the entire tube to close. There are different forms of spina bifida. In spina bifida occulta, often called hidden spina bifida, the spinal cord and the nerves are usually normal, and there is no visible opening on the back (Fig. 9.2). Usually harmless, there is a small defect or gap in a few of the vertebrae. When the meninges protrude from the spine, it is called a myelocele (or meningomyelocele). The sac is filled with cerebrospinal fluid (CSF), but there is generally no nerve damage. In myelomeningocele, the meninges and spinal nerves push through an opening in the vertebrae (Box 9.1). Fig. 9.1 Neurulation. (A) During early development, shortly after the formation of the primitive streak, the notochord sends a signal to the overlying ectoderm to begin to flatten and the cells elongate, thus forming the neural plate. (B) After induction from the notochord, the overlying ectoderm begins to involute, and a neural groove is formed in the midline while the sides of the area that is involuting form the neural folds. (C) Later in embryonic development, the neural tube forms and is covered by the overlying ectoderm. Fig. 9.2 Spina bifida. Spina bifida aperta produces a noticeable sac in the back. A meningocele, in which a portion of the meninges protrudes, produces little or no muscle paralysis or incontinence once it is repaired. However, in 90% of all spina bifida cases, a portion of the undeveloped spinal cord itself protrudes through the spine and forms a sac called a myelocele. Any portion of the spinal cord outside the vertebrae is undeveloped or damaged, causing paralysis and incontinence. As the neural tube is closing, a group of ectodermal cells separates from the neural tube and locates on the dorsal–lateral edge of the tube. These cells become the neural crest cells, which eventually migrate throughout the body, producing all neurons that have cell bodies in the peripheral nervous system, including (1) neurons and glial cells of the sensory, sympathetic, and parasympathetic nervous system; (2) norepinephrine‐ and epinephrine‐producing cells of the adrenal gland; (3) pigment‐containing cells of the epidermis; and (4) skeletal and connective tissues of the head. The neural crest cells develop in conjunction with the underlying mesoderm. The mesoderm on either side of the neural tube forms the somites. The somites produce the vertebrae and the associated skeletal muscle. The nerves that innervate the skeletal muscle are called somatic motor neurons because they are derived from somites. As the brain develops from the neural tube, three swellings form at its rostral end. These three vesicles include the prosencephalon, the mesencephalon or midbrain, and the rhombencephalon or hindbrain. The rhombencephalon connects the brain with the spinal cord (Fig. 9.3). Fig. 9.3 Brain development. During embryonic development, the brain begins as three vesicles. The most rostral is the prosencephalon, which gives rise to the telencephalon and diencephalon vesicles. The most caudal, the rhombencephalon, gives rise to the metencephalon and myelencephalon. During the next stage of development, two secondary vesicles, the optic vesicles and telencephalic vesicles, form from the prosencephalon. The remaining unpaired vesicle in the middle is called the diencephalon, or “between brain.” The telencephalon vesicles grow to become the two cerebral hemispheres, collectively called the cerebrum. Finally, paired vesicles form on the ventral surface of the telencephalic vesicles and eventually become the olfactory bulbs. The olfactory bulbs participate in the sense of smell. The mesencephalon does not divide but instead remains the midbrain, while the rhombencephalon divides into the metencephalon and myelencephalon. The metencephalon includes the pons (pons = bridge) and cerebellum, and the myelencephalon includes the medulla oblongata. Collectively, the midbrain, pons, and medulla oblongata constitute the brain stem. The telencephalic vesicles form the telencephalon, becoming the two cerebral hemispheres. As the brain develops, the telencephalic vesicles grow posteriorly and laterally until they encase the diencephalon. There is a proliferation of neurons resulting in the formation of three major white matter systems, including the cortical white matter, the corpus callosum, and the internal capsule. Cortical white matter includes neurons that run to and from the cerebral cortex. The corpus callosum includes neurons that connect the two cerebral hemispheres, and the internal capsule connects the cortex with the brain stem. The surface of the brain is marked by many convolutions. The grooves are called sulci (singular = sulcus); the ridges are called gyri (singular = gyrus). The outer six layers of neurons constitute the cerebral cortex. The convolutions greatly increase the surface area, thus increasing the amount of cortex. Whereas the brain of a chicken has a relatively flat surface, the brain of most domestic animals is highly convoluted (Fig. 9.4). The larger grooves that separate brain regions are called fissures. The longitudinal fissure separates the two cerebral hemispheres, whereas the cerebral hemispheres are separated from the cerebellum by the perpendicular transverse fissure. When viewing an intact brain, four lobes—frontal, parietal, temporal, and occipital—are visible in the cerebral hemispheres. In most animals, the lobes are not delineated by sulci but are named for adjacent cranial bones (Fig. 9.5). In the human brain, various sulci separate the lobes, as will be described below. The frontal lobe lies just under the frontal bone, making up the most rostral lobe of the cerebral hemispheres. Immediately caudal to the frontal lobe is the parietal lobe, which is separated from the frontal lobe by the central sulcus. Fig. 9.4 Cytoarchitecture of the goat brain surface. (A) Lateral view. (B) Dorsal view. Cephalization has resulted in the formation of ridges, called gyri, and valleys, called sulci. Fissures are larger grooves separating major brain areas, such as the longitudinal fissure that separates the two cerebral hemispheres. (Reprinted from Constantinescu, 2001. Used by permission of the publisher.). Fig. 9.5 Lobes of the brain. Four lobes are visible on the surface of the brain. The frontal lobe is most rostral. Caudad to the frontal lobe is the parietal lobe. Below the lateral fissure is the temporal lobe. The most caudal lobe is the occipital lobe. (Modified from Constantinescu, 2001). Two important gyri are also bordered by the central sulcus: the precentral gyrus anterior and the postcentral gyrus posterior to the central sulcus. Below the parietal lobe is the temporal lobe, which is found under the temporal bone. The temporal lobe is separated from the frontal and parietal lobes by the lateral sulcus (lateral fissure, Sylvian fissure). Finally, at the back surface of the cerebral hemispheres is the occipital lobe, which is separated from the parietal lobe by the parieto‐occipital sulcus. Although not visible on the surface, there is a fifth lobe called the insula that is found by spreading the brain apart at the lateral fissure. It is covered by parts of the temporal, parietal, and frontal lobes. As development progresses, the neurocoel expands to produce four chambers, called cerebroventricles. Ependymal cells line the cerebroventricles. Each cerebral hemisphere contains a lateral ventricle, also called the first and second ventricle. A thin layer of tissue called the septum pellucidum separates the two lateral ventricles from each other. The third ventricle is found in the diencephalon, and the fourth ventricle extends from the posterior surface of the pons and the anterior surface of the cerebellum to the superior portion of the medulla oblongata (Fig. 9.6). The fourth ventricle is continuous with the central canal of the spinal cord. The lateral ventricles are connected to the third ventricle via the interventricular foramen of Monroe (Fig. 9.7). The third ventricle connects to the fourth ventricle via the mesencephalic aqueduct, also called the aqueduct of Sylvius or cerebral aqueduct. CSF flows by bulk flow from the lateral ventricles to the third ventricle to the fourth ventricle. CSF can leave the fourth ventricle through the medial aperture called the foramen of Magendie and the two lateral apertures called the foramina of Luschka and enter the subarachnoid space (Fig. 9.8). This space is found between the arachnoid and pia mater, which, along with the dura mater, form the three meningeal layers covering the brain. The fluid in the subarachnoid space bathes the surface of the brain and spinal cord. Should the CSF not be able to flow through the ventricular system, it will back up in the ventricles, causing hydrocephalus or swelling of the ventricles. Because the skull cannot expand, increased pressure in the ventricles causes the soft tissue of the brain to be compressed, which leads to impaired brain function and death if untreated. The adult human neural system contains about 150 ml of CSF, and it is estimated that 430–450 ml are produced daily. Therefore, CSF is turned over every 6 to 7 hours. After entering the subarachnoid space, CSF moves through one‐way valves called arachnoid villi that project into the superior sagittal sinus formed by the dura mater located in the longitudinal fissure. The superior sagittal sinus is filled with venous blood. This means once the CSF enters the superior sagittal sinus, it returns to the circulatory system. Fig. 9.6 Cerebroventricles. The neurocoele, or central cavity, in the neural tube, eventually enlarges into four cerebroventricles filled with cerebrospinal fluid. Two lateral ventricles, one in each cerebral hemisphere, empty into the medially located third ventricle located in the diencephalon. This empties into the fourth ventricle, located between the cerebellum and the brain stem. Fig. 9.7 Cerebroventricles of the sheep. Cerebrospinal fluid (CSF) is formed in the choroid plexus located in each lateral ventricle. Via bulk flow, the CSF flows from the lateral ventricles through the interventricular foramen of Monroe to the third ventricle. It then flows through the cerebral aqueduct to the fourth ventricle. Fig. 9.8 Cerebrospinal fluid (CSF) movement out of ventricles. CSF travels from the fourth ventricle to the subarachnoid space via the medially located foramen of Magendie and the two lateral foramina of Luschka. The cerebral cortex is arranged as layers of cells that lie parallel to the surface of the brain. The layer closest to the surface of the brain is separated from the pia mater by a zone, called the molecular layer or layer I, that lacks neurons. Furthermore, at least one layer contains pyramidal cells that have large dendrites, called apical dendrites, that project to layer I, where they form multiple synapses. The neocortex is found only in mammals and is associated with higher brain functions such as conscious behavior. It is found over most of the surface of the cerebral hemisphere and consists of six layers of cells (Fig. 9.9). Medial to the lateral ventricles is an area of cortex named for its unique shape called the hippocampus (Greek for “seahorse”). It is only a single‐cell layer. Ventral and lateral to the hippocampus is the third area of the cortex called the olfactory cortex (piriform, or pyriform cortex), which consists of two cell layers. The olfactory cortex connects with the olfactory bulbs. The olfactory cortex is separated from the neocortex by the rhinal fissure. In humans, the neocortex was extensively mapped by Korbinian Brodmann in 1906, in which he numbered 52 different cortical areas, each having a common cytoarchitecture. Such an extensive mapping has not been done on other animals. The cortex can be designated into three areas: (1) motor areas responsible for the control of voluntary motor functions, (2) sensory areas responsible for the perception of various sensations, and (3) association areas that integrate the motor and sensory signals. Fig. 9.9 Three types of mammalian cortex. The neocortex is found on the outer surface of the cerebral hemispheres. Medial to the lateral ventricles is a second type of cortex called the hippocampus, part of the limbic system. The third type of cortical tissue is the olfactory cortex, located ventrally and laterally in the hippocampus. Fig. 9.10 Evolutionary changes in the cerebrocortex. Through evolutionary development, the ability to display higher‐order thinking is associated with enhanced development of the association areas. (Modified from King, 1987) The motor and sensory areas can be grouped together into projection areas, thus allowing the cortex to be subdivided into three components, including the projection areas, rhinencephalon (olfactory and limbic) areas, and association areas. The projection and association areas comprise the neocortex. Association areas receive sensory information, process that information, develop a response, and predict its consequences. As animals became more evolutionarily advanced, the association areas also became more developed (Fig. 9.10). The white matter in the cerebral cortex forms three types of fibers: These are cortical areas responsible for motor functions. The primary motor cortex is the final site for the cortical processing of motor commands before messages are then sent to the somatic muscles. Although in humans, the primary motor area lies in the posterior part of the frontal lobes just anterior to the central sulcus, in other mammals this area lies in the rostral region of the frontal lobes (Fig. 9.11). Unlike birds, reptiles, amphibians, and fish, mammals possess a pyramidal system. The pyramidal system consists of corticospinal fibers that travel from the primary motor area, through the medullary pyramids located at the base of the medulla oblongata, to the somatic motor neurons found in the spinal cord. It also includes the corticonuclear fibers that project to the nuclei of cranial nerves that innervate striated muscles in the head. The pyramidal fibers decussate, or cross over, in the pyramids of the medulla. The extrapyramidal system includes all the descending somatic motor pathways excluding those described above that constitute the pyramidal system. The extrapyramidal system is phylogenetically old and is found in all but the lowest vertebrates. It consists of nine main motor areas located in the forebrain, midbrain, and hindbrain (Fig. 9.12). These will be discussed in depth later in the chapter. Fig. 9.11 Primary projection areas of the cat. The primary motor and somatic sensory areas spread over the medial surface of the frontal lobes. The visual area is in the occipital lobe; the auditory area is in the temporal lobe. Fig. 9.12 Extrapyramidal system. The extrapyramidal system is a multisynaptic motor pathway that includes all those motor neurons not part of the pyramidal system. It consists of nine motor centers scattered throughout the brain. Unlike the motor areas that are in the frontal lobe, sensory areas are located throughout the cortex. The primary somatosensory cortex receives information from sensory receptors located in the skin and proprioceptors in skeletal muscles. In addition to the cerebral cortex discussed above, its commissures, and association and projection fibers, there are some deep subcortical nuclei called the basal ganglia or nuclei (Fig. 9.13). Although the definition of the structures included in the basal ganglion varies, it generally includes the caudate nucleus, putamen, globus pallidus, substantia nigra (consisting of the pars reticulata and pars compacta), and subthalamic nucleus. The putamen and globus pallidus (or pallidum) together form the lentiform nucleus that laterally borders the internal capsule. The lentiform nucleus and caudate are collectively called the corpus striatum because the fibers of the internal capsule, a collection of fibers that runs between the neocortex and thalamus, pass through them, giving them a striated appearance. The corpus striatum receives most of the inputs to the basal ganglion from the cerebral cortex, thalamus, and brain stem. The corpus striatum sends projections to the globus pallidus and substantia nigra that provide the major output projections from the basal ganglion. These projection fibers travel through the thalamus to the premotor and prefrontal cortex and therefore affect motor movement. Fig. 9.13 Basal ganglion. The basal ganglion, or nucleus, is a collection of brain nuclei that function in motor movement as part of the extrapyramidal system. The basal ganglion includes the globus pallidus and putamen, which together constitute the lentiform nucleus. Also included is the caudate, which, combined with the lentiform nucleus, forms the corpus striatum. Finally, also included in the basal ganglion are the amygdala, subthalamic nucleus, and substantia nigra. The limbic system consists of a group of structures located in the medial region of each cerebral hemisphere (Fig. 9.14). These structures encircle (limbus = ring or border) the brain stem. The limbic lobe of the cerebral hemisphere includes gyri surrounding the diencephalon and other underlying structures. Specifically, it consists of three gyri. The cingulate gyrus is dorsal to the corpus callosum. The dentate gyrus and parahippocampal gyrus form the inferior and posterior portions of the limbic lobe. These later two gyri conceal the hippocampus, which is a nucleus lying inferior to the lateral ventricle. The fornix is a fiber tract running inferior to the corpus callosum and connecting the hippocampus with the hypothalamus, where it ends in the mammillary bodies. Also included in the limbic system is the anterior nucleus of the thalamus, which relays information from the mammillary bodies to the cingulate gyrus. The amygdaloid body (nucleus) serves as the interface between the limbic system, cerebrum, and various sensory systems. Fig. 9.14 Limbic system. The limbic lobe includes the cingulate gyrus, parahippocampal gyrus, and the hippocampal formation that lies deep in the parahippocampal gyrus. The hippocampal formation includes the hippocampus, dentate gyrus (deep to the parahippocampal gyrus), and subiculum. The limbic system additionally includes parts of the rhinencephalon, amygdala, hypothalamus, and anterior nucleus of the thalamus. The fornix helps connect parts of the limbic system. The limbic system is involved in emotional and behavioral patterns. The functions of the limbic system include (1) establishment of emotional states, (2) linking of conscious functions with unconscious, autonomic functions, and (3) long‐term memory storage and retrieval. The rabies virus generally attacks the hippocampus and results in emotional changes, including bouts of terror and rage. The amygdala is believed to be the major component of the limbic system involved in emotion because electrical stimulation of this region produces feelings of fear and apprehension, whereas damage to this region causes tameness. Removal of the amygdala will allow a cat to wander through a colony of monkeys, ignoring the monkey’s hoots and threats. It is believed that the hippocampal formation processes information from the cingulated gyrus. This information is sent to the mammillary bodies of the hypothalamus via the fornix (Fig. 9.14). The hypothalamus gives feedback to the cingulate gyrus by a pathway from the mammillary bodies to the anterior thalamus via the mammillothalamic tract, and then to the cingulate gyrus. The telencephalon and diencephalon make up the forebrain and are derived from the rostral‐most vesicle called the prosencephalon. Surrounded by the cerebral hemispheres, the diencephalons consist of three paired structures—the thalamus, hypothalamus, and epithalamus. The thalamus lies dorsally to the hypothalamus and is bordered by the caudate nucleus dorsally and the internal capsule laterally. Its two halves are separated by the third ventricle. There is a collection of nuclei found in each half of the thalamus (Fig. 9.15). The intermediate mass of the thalamus extends through the third ventricle, connecting the two halves. The thalamus is the major relay station for sensory information generated in the periphery and transferred for processing to the cerebral hemispheres. It also integrates motor information from the cerebellum and basal ganglion and transfers such information to the motor regions of the cortex. The nuclei of the thalamus are generally classified into four groups—anterior, ventrolateral, medial, and posterior. The ventrolateral and posterior groups are located lateral to the medullary lamina, a fiber tract that runs the rostrocaudal length of the thalamus. The medial group of nuclei is located medial to the medullary lamina. The medullary lamina splits at the rostral end of the thalamus and encases the anterior group. Fig. 9.15 The thalamus. The thalamus consists of four groups of nuclei. The anterior nucleus and pulvinar constitute the anterior and posterior groups, respectively. The medial group consists mainly of the mediodorsal nuclei (not shown). The posterior nuclear group consists of the pulvinar and medial and lateral geniculate bodies, whereas the anterior group consists only of the anterior nucleus. The anterior nucleus receives input from the mammillary nuclei of the hypothalamus and is thought to participate in memory and emotion. It is also interconnected with the cingulated and frontal cortices and is therefore part of the limbic system.
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Central Nervous System
Embryonic Development
Three Brain Vesicles
Organization of the Brain
The Cerebral Hemispheres
Ventricles of the Brain
Cerebral Cortex
Motor Areas
Sensory Areas
Cerebral White Matter
Basal Ganglion
Limbic System
Diencephalon
Thalamus
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