Chapter 1: Organization of the Central Nervous System

The human nervous system carries out an enormous number of functions by means of many subdivisions. Indeed, the brain's complexity has traditionally made the study of neuroanatomy a demanding task. This task can be greatly simplified by approaching the study of the nervous system from the dual perspectives of its regional and functional anatomy. Regional neuroanatomy examines the spatial relations between brain structures within a portion of the nervous system. Regional neuroanatomy defines the major brain divisions as well as local, neighborhood relationships within the divisions. In contrast, functional neuroanatomy examines those parts of the nervous system that work together to accomplish a particular task, for example, visual perception. Functional systems are formed by specific neural connections within and between regions of the nervous system; connections that form complex neural circuits. A goal of functional neuroanatomy is to develop an understanding of the neural circuitry underlying behavior. By knowing regional anatomy together with the functions of particular brain structures, the clinician can determine the location of nervous system damage in a patient who has a particular neurological or psychiatric impairment. Combined knowledge of what structures do and where they are located is essential for a complete understanding of nervous system organization. The term neuroanatomy is therefore misleading because it implies that knowledge of structure is sufficient to master this discipline. Indeed, in the study of neuroanatomy, structure and function are tightly interwoven, so much so that they should not be separated. The interrelationships between structure and function underlie functional localization, a key principle of nervous system organization.

This chapter examines the organization of the nervous system and the means to study it by developing the vocabulary to describe its regional anatomy. First, the cellular constituents of the nervous system are described briefly. Then the chapter focuses on the major regions of the nervous system and the functions of these regions. This background gives the reader insight into functional localization.

The nerve cell, or neuron, is the functional cellular unit of the nervous system. Neuroscientists strive to understand the myriad functions of the nervous system partly in terms of the interconnections between neurons. The other major cellular constituent of the nervous system is the neuroglial cell, or glia. Glia provide structural and metabolic support for neurons during development and in maturity.

All Neurons Have a Common Morphological Plan

It is estimated that there are about 100 billion neurons in the adult human brain. Although neurons come in different shapes and sizes, each has four morphologically specialized regions with particular functions: dendrites, cell body, axon, and axon terminals (Figure 1–2A). Dendrites receive information from other neurons. The cell body contains the nucleus and cellular organelles critical for the neuron's survival and function. The cell body also receives information from other neurons and serves important integrative functions. The axon conducts information, which is encoded in the form of action potentials, to the axon terminal. Connections between two neurons in a neural circuit are made by the axon terminals of one and the dendrites and cell body of the other, at the synapse (discussed below).

Despite a wide range of morphology, we can distinguish three classes of neuron based on the configuration of their dendrites and axons: unipolar, bipolar, and multipolar (Figure 1–2B). These neurons were drawn by the distinguished Spanish neuroanatomist Santiago Ramón y Cajal at the beginning of the twentieth century. Unipolar neurons are the simplest in shape (Figure 1–2B1). They have no dendrites; the cell body of unipolar neurons receives and integrates incoming information. A single axon, which originates from the cell body, gives rise to multiple processes at the terminal. In the human nervous system, unipolar neurons are the least common. They control exocrine gland secretions and smooth muscle contractility.

Bipolar neurons have two processes that arise from opposite poles of the cell body (Figure 1–2B2). The flow of information in bipolar neurons is from one of the processes, which function like a dendrite, across the cell body to the other process, which functions like an axon. A bipolar neuron subtype is a pseudounipolar neuron (see Figure 6–2 top). During development the two processes of the embryonic bipolar neuron fuse into a single process in the pseudounipolar neuron, which bifurcates a short distance from the cell body. Many sensory neurons, such as those that transmit information about odors or touch to the brain, are bipolar and pseudounipolar neurons.

Multipolar neurons feature a complex array of dendrites on the cell body and a single axon that branches extensively (Figure 1–2B3). Most of the neurons in the brain and spinal cord are multipolar. Multipolar neurons that have long axons, with axon terminals located in distant sites, are termed projection neurons. Projection neurons mediate communication between regions of the nervous system and between the nervous system and peripheral targets, such as striated muscle cells. The neuron in Figure 1–2B3 is a particularly complex projection neuron. The terminals of this neuron are not shown because they are located far from the cell body. For this type of neuron in the human, the axon may be up to 1-m long, about 50,000 times the width of the cell body! Other multipolar neurons, commonly called interneurons, have short axons that remain in the same region of the nervous system in which the cell body is located. Interneurons help to process neuronal information within a local brain region.

Neurons Communicate With Each Other at Synapses

Information flow along a neuron is polarized. The dendrites and cell body receive and integrate incoming information, which is transmitted along the axon to the terminals. Communication of information from one neuron to another also is polarized and occurs at specialized sites of contact called synapses. The neuron that sends information is the presynaptic neuron and the one that receives the information is the postsynaptic neuron. The information carried by the presynaptic neuron is most typically transduced at the synapse into a chemical signal that is received by specialized membrane receptors on the dendrites and cell body of the postsynaptic neuron.

The synapse consists of three distinct elements: (1) the presynaptic terminal, the axon terminal of the presynaptic neuron, (2) the synaptic cleft, the narrow intercellular space between the neurons, and (3) the receptive membrane of the postsynaptic neuron. Synapses are present on dendrites, the cell body, the initial segment of the axon, or the portion of the axon closest to the cell body, and the presynaptic axon terminal. Synapses located on different sites can serve different functions.

To send a message to its postsynaptic neurons, a presynaptic neuron releases neurotransmitter, packaged into vesicles, into the synaptic cleft. Neurotransmitters are small molecular weight compounds; among these are amino acids (eg, glutamate; glycine; and γ-aminobutyric acid, or GABA), acetylcholine, and monoaminergic compounds such as norepinephrine and serotonin. Larger molecules, such as peptides (eg, enkephalin and substance P), also can function as neurotransmitters. After release into the synaptic cleft, the neurotransmitter molecules diffuse across the cleft and bind to receptors on the postsynaptic membrane. Neurotransmitters change the permeability of particular ions across the neuronal membrane. A neurotransmitter can either excite the postsynaptic neuron by depolarizing it, or inhibit the neuron by hyperpolarizing it. For example, excitation can be produced by a neurotransmitter that increases the flow of sodium ions into a neuron (ie, depolarization), and inhibition can be produced by a neurotransmitter that increases the flow of chloride ions into a neuron (ie, hyperpolarization). Glutamate and acetylcholine typically excite neurons, whereas GABA and glycine typically inhibit neurons. Many neurotransmitters, like dopamine and serotonin, have more varied actions, exciting some neurons and inhibiting others. Their action depends on a myriad of factors, such as the particular receptor subtype that the neurotransmitter engages and whether the binding of the neurotransmitter leads directly to the change in ion permeability or if the change is mediated by the actions on second messengers and other intracellular signaling pathways (eg, G protein coupled receptors). For example, the dopamine receptor subtype 1 is depolarizing, whereas the type 2 receptor is hyperpolarizing; both act through G protein coupled mechanisms. A neurotransmitter can even have opposing actions on the same neuron depending on the composition of receptor subtypes on the neuron's membrane. Action through second messengers and other intracellular signaling pathways can have short-term effects, such as changing membrane ion permeability, or long-term effects, such as gene expression. Many small molecules that produce strong effects on neurons are not packaged into vesicles; they are thought to act through diffusion. These compounds, for example, nitric oxide, are produced in the postsynaptic neuron and are thought to act as retrograde messengers that serve important regulatory functions on pre- and postsynaptic neurons, including maintaining and modulating the strength of synaptic connections. These actions are important for learning and memory.

Although chemical synaptic transmission is the most common way of sending messages from one neuron to another, purely electrical communication can occur between neurons. At such electrical synapses, there is direct cytoplasmic continuity between the presynaptic and postsynaptic neurons.

Glial Cells Provide Structural and Metabolic Support for Neurons

Glial cells comprise the other major cellular constituent of the nervous system; they outnumber neurons by about 10 to 1. Given this high ratio, the structural and metabolic support that glial cells provide for neurons must be a formidable task! There are two major classes of glia: microglia and macroglia. Microglia subserve a phagocytic or scavenger role, responding to nervous system infection or damage. They are rapidly mobilized—they become activated—in response to different pathophysiological conditions and trauma. Activated microglia can destroy invading microorganisms, remove debris, and promote tissue repair. Interestingly, they also mediate changes in neuronal properties after nervous system damage; sometimes maladaptive changes, so they may also hinder recovery after injury. For example, neurons often become hyperexcitable after nervous system damage, and microglia can be involved in this process. Macroglia, of which there are four separate types—oligodendrocytes, Schwann cells, astrocytes, and ependymal cells—have a variety of support and nutritive functions. Schwann cells and oligodendrocytes form the myelin sheath around peripheral and central axons, respectively (Figures 1–2A and 1–3). The myelin sheath increases the velocity of action potential conduction. It is whitish in appearance because it is rich in a fatty substance called myelin, which is composed of many different kinds of myelin proteins. Schwann cells also play important roles in organizing the formation of the connective tissue sheaths surrounding peripheral nerves during development and in axon regeneration following damage in maturity. Astrocytes have important structural and metabolic functions. For example, in the developing nervous system, astrocytes act as scaffolds for growing axons and guides for migrating immature neurons. Many synapses are associated with astrocyte processes that may monitor synaptic actions and provide chemical feedback. Astrocytes also contribute to the blood-brain barrier, which protects the vulnerable environment of the brain from invasion of chemical from the periphery, which can influence neuronal firing. The last class of macroglia, ependymal cells, line fluid-filled cavities in the central nervous system (see below). They play an important role in regulating the flow of chemicals from these cavities into the brain.

Neurons and glial cells of the nervous system are organized into two anatomically separate but functionally interdependent parts: the peripheral and the central nervous systems (Figure 1–4A). The peripheral nervous system is subdivided into somatic and autonomic divisions. The somatic division contains the sensory neurons that innervate the skin, muscles, and joints. These neurons detect and, in turn, inform the central nervous system of stimuli. This division also contains the axons of motor neurons that innervate skeletal muscle, although the cell bodies of motor neurons lie within the central nervous system. These axons transmit control signals to muscle to regulate the force of muscle contraction. The autonomic division contains the neurons that innervate glands and the smooth muscle of the viscera and blood vessels (see Chapter 15). This division, with its separate sympathetic, parasympathetic, and enteric subdivisions, regulates body functions based, in part, on information about the body's internal state.

The central nervous system consists of the spinal cord and brain (Figure 1–4B), and the brain is further subdivided into the medulla, pons, cerebellum, midbrain, diencephalon, and cerebral hemispheres (Figure 1–4C). Within each of the seven central nervous system divisions resides a component of the ventricular system, a labyrinth of fluid-filled cavities that serve various supportive functions (see Figure 1–13). Box 1–1 shows how all of the divisions of the central nervous system and the components of the ventricular system are present from very early in development, from about the first month after conception.

Neuronal cell bodies and axons are not distributed uniformly within the nervous system. In the peripheral nervous system, cell bodies collect in peripheral ganglia and axons are contained in peripheral nerves. In the central nervous system, neuronal cell bodies and dendrites are located in cortical areas, which are flattened sheets of cells (or laminae) located primarily on the surface of the cerebral hemispheres, and in nuclei, which are clusters of neurons located beneath the surface of all of the central nervous system divisions. Nuclei come in various sizes and shapes; they are commonly oval and columnar but sometimes occur in complex three-dimensional configurations (see Figure 1–10). Regions of the central nervous system that contain axons have an unwieldy number of names, the most common of which is tract. In fresh tissue, nuclei and cortical areas appear grayish and tracts appear whitish, hence the familiar terms gray matter and white matter. The whitish appearance of tracts is caused by the presence of the myelin sheath surrounding the axons (Figure 1–3). The gray and white matter can be distinguished in fixed tissue using anatomical methods and in the living brain using radiological methods (see Chapter 2, Boxes 2–1, 2–2).

The spinal cord participates in processing sensory information from the limbs, trunk, and many internal organs; in controlling body movements directly; and in regulating many visceral functions (Figure 1–6). It also provides a conduit for the transmission of both sensory information in the tracts that ascend to the brain and motor information in the descending tracts. The spinal cord is the only part of the human central nervous system that has an external segmental organization (Figure 1–6B, C).

The spinal cord has a modular organization, in which every segment has a similar basic structure (Figure 1–6C). Each spinal cord segment contains a pair of nerve roots (and associated rootlets) called the dorsal and ventral roots. (The terms dorsal and ventral describe the spatial relations of structures; these and other anatomical terms are explained later in this chapter.) Dorsal roots contain only sensory axons, which transmit sensory information into the spinal cord. By contrast, ventral roots contain motor axons, which transmit motor commands to muscle and other body organs. Dorsal and ventral roots exemplify the separation of function in the nervous system, a principle that is examined further in subsequent chapters. These sensory and motor axons, which are part of the peripheral nervous system, become intermingled in the spinal nerves en route to their peripheral targets (Figure 1–6C).

The next three divisions—medulla, pons, and midbrain—comprise the brain stem (Figure 1–7). The brain stem has three general functions. First, it receives sensory information from cranial structures and controls the muscles of the head. These functions are similar to those of the spinal cord. Cranial nerves, the sensory and motor nerve roots that enter and exit the brain stem, are parts of the peripheral nervous system and are analogous to the spinal nerves (Figure 1–7). Second, similar to the spinal cord, the brain stem is a conduit for information flow because ascending sensory and descending motor tracts travel through it. Finally, nuclei in the brain stem integrate information from a variety of sources for arousal and other higher brain functions.

In addition to these three general functions, the various divisions of the brain stem each subserve specific sensory and motor functions. For example, portions of the medulla participate in essential blood pressure and respiratory regulatory mechanisms. Indeed, damage to these parts of the brain is almost always life threatening. Parts of the pons and midbrain play a key role in the control of eye movement.

The principal functions of the cerebellum are to regulate eye and limb movements and to maintain posture and balance (Figure 1–8). Limb movements become poorly coordinated when the cerebellum is damaged. In addition, parts of the cerebellum play a role in higher brain functions, including language, cognition, and emotion (Chapter 13).

The two principal parts of the diencephalon participate in diverse sensory, motor, and integrative functions. One component, the thalamus (Figure 1–9), is a key structure for transmitting information to the cerebral hemispheres. The thalamus is composed of numerous nuclei. Neurons in separate thalamic nuclei transmit information to different cortical areas. In the brains of most people, a small portion of the thalamus in each half adheres at the midline, the thalamic adhesion. The other component of the diencephalon, the hypothalamus (Figure 1–9A; see also Figure 1–12A), controls endocrine hormone release from the pituitary gland and the overall functions of the autonomic nervous system.

The cerebral hemispheres are the most highly developed portions of the human central nervous system. Each hemisphere is a distinct half, and each has four major components: cerebral cortex, hippocampal formation, amygdala, and basal ganglia. Together, these structures mediate the most sophisticated of human behaviors, and they do so through complex anatomical connections.

The Subcortical Components of the Cerebral Hemispheres Mediate Diverse Motor, Cognitive, and Emotional Functions

The hippocampal formation is important in learning and memory, whereas the amygdala not only participates in emotions but also helps to coordinate the body's response to stressful and threatening situations, such as preparing to fight (Figure 1–10A). These two structures are part of the limbic system (see Chapter 16), which includes other parts of the cerebral hemispheres, diencephalon, and midbrain. Because parts of the limbic system play a key role in mood, it is not surprising that psychiatric disorders are often associated with limbic system dysfunction.

The basal ganglia are another deeply located collection of neurons. The portion of the basal ganglia that has the most complex shape is called the striatum (Figure 1–10B). The importance of the basal ganglia in the control of movement is clearly revealed when they become damaged, as in Parkinson disease. Tremor and a slowing of movement are some of the overt signs of this disease. The basal ganglia also participate in cognition and emotions in concert with the cerebral cortex and are key brain structures involved in addiction.

The Four Lobes of the Cerebral Cortex Each Have Distinct Functions

The cerebral cortex, which is located on the surface of the brain, is highly convoluted (Figures 1–11 and 1–12). The human cerebral cortex is approximately 2,500 cm². Convolutions are an evolutionary adaptation to fit a greater surface area within the confined space of the cranial cavity. In fact, only one quarter to one third of the cerebral cortex is exposed on the surface. The elevated convolutions on the cortical surface, called gyri, are separated by grooves called sulci or fissures (which are particularly deep sulci). The cerebral hemispheres are separated from each other by the sagittal (or interhemispheric) fissure (Figure 1–12B).

The four lobes of the cerebral cortex are named after the cranial bones that overlie them: frontal, parietal, occipital, and temporal (Figure 1–11, inset). The functions of the different lobes are remarkably distinct, as are the functions of individual gyri within each lobe.

The frontal lobe serves diverse behavioral functions, from thoughts to action, cognition, and emotions. The precentral gyrus contains the primary motor cortex, which participates in controlling the mechanical actions of movement, such as the direction and speed of reaching. Many projection neurons in the primary motor cortex have an axon that terminates in the spinal cord. The superior, middle, and inferior frontal gyri form most of the remaining portion of the frontal lobe. The premotor areas, which are important in motor decision making and planning movements, are adjacent to the primary motor cortex in these gyri. The inferior frontal gyrus in the left hemisphere in most people contains Broca's area, which is essential for the articulation of speech. Much of the frontal lobe is association cortex. Association cortical areas are involved in the complex processing of sensory and other information for higher brain functions, including emotions, organizing behavior, thoughts, and memories. Areas closer to the frontal pole comprise the frontal association cortex. The prefrontal association cortex is important in thought, cognition, and emotions. The cingulate gyrus (Figure 1–11B), medial frontal lobe, and most of the orbital gyri (Figure 1–12A) are important in emotions. Psychiatric disorders of thought, as in schizophrenia, and mood disorders, such as depression, are linked with abnormal functions of frontal association cortex. The basal forebrain, which is on the ventral surface of the frontal lobe (Figure 1–12A), contains a special population of neurons that uses acetylcholine to regulate cortical excitability. These neurons are examined further in Chapter 2. Although the olfactory sensory organ, the olfactory bulb, is located on the ventral surface of the frontal lobe, its connections are predominantly with the temporal lobe (Figure 1–12A).

The parietal lobe, which is separated from the frontal lobe by the central sulcus, mediates our perceptions of touch, pain, and limb position. These functions are carried out by the primary somatic sensory cortex, which is located in the postcentral gyrus. Primary sensory areas are the initial cortical processing stages for sensory information. The remaining portion of the parietal lobe on the lateral brain surface consists of the superior and inferior parietal lobules, which are separated by the intraparietal sulcus. The superior parietal lobule contains higher-order somatic sensory areas, for further processing of somatic sensory information, and other sensory areas. Together these areas are essential for a complete self-image of the body, and they mediate behavioral interactions with the world around us. A lesion in this portion of the parietal lobe in the right hemisphere, the side of the human brain that is specialized for spatial awareness, can produce bizarre neurological signs that include neglecting a portion of the body on the side opposite the lesion. For example, a patient may not dress one side of her body or comb half of her hair. The inferior parietal lobule is involved in integrating diverse sensory information for perception and language, mathematical reasoning, and visuospatial cognition.

The occipital lobe is separated from the parietal lobe on the medial brain surface by the parietooccipital sulcus (Figure 1–11B). On the lateral and inferior surfaces, there are no distinct boundaries, only an imaginary line connecting the preoccipital notch (Figure 1–11A) with the parietooccipital sulcus. The occipital lobe is the most singular in function, subserving vision. The primary visual cortex is located in the walls and depths of the calcarine fissure on the medial brain surface (Figure 1–11B). Whereas the primary visual cortex is important in the initial stages of visual processing, the surrounding higher-order visual areas play a role in elaborating the sensory message that enables us to see the form and color of objects. For example, on the ventral brain surface is a portion of the occipitotemporal gyrus in the occipital lobe (also termed the fusiform gyrus) that is important for recognizing faces (Figure 1–12A). Patients with a lesion of this area can confuse faces with inanimate objects, a condition termed prosopagnosia.

The temporal lobe, separated from the frontal and parietal lobes by the lateral sulcus (or Sylvian fissure) (Figure 1–11A), mediates a variety of sensory functions and participates in memory and emotions. The primary auditory cortex, located on the superior temporal gyrus, works with surrounding areas on the superior temporal gyrus and within the lateral sulcus and the superior temporal sulcus for perception and localization of sounds (Figure 1–11A). The superior temporal gyrus on the left side is specialized for speech. Lesion of the posterior portion of this gyrus, which is the location of Wernicke's area, impairs the understanding of speech. The middle temporal gyrus, especially the portion close to the occipital lobe, is essential for perception of visual motion. The inferior temporal gyrus mediates the perception of visual form and color (Figures 1–11A and 1–12A). The cortex located at the temporal pole (Figure 1–12A), together with adjacent portions of the medial temporal lobe and inferior and medial frontal lobes, is important for emotions.

Deep within the lateral sulcus are portions of the frontal, parietal, and temporal lobes. This territory is termed the insular cortex (Figure 1–11, inset). It becomes buried late during prenatal development (see Figure 1-14). Portions of the insular cortex are important in taste, internal body senses, pain, and balance.

The corpus callosum contains axons that interconnect the cortex on the two sides of the brain (Figure 1–11B). Tracts containing axons that interconnect the two sides of the brain are called commissures, and the corpus callosum is the largest of the brain's commissures. To integrate the functions of the two halves of the cerebral cortex, axons of the corpus callosum course through each of its four principal parts: rostrum, genu, body, and splenium (Figure 1–11B). Information between the occipital lobes travels through the splenium of the corpus callosum, whereas information from the other lobes travels through the rostrum, genu, and body.

The central nervous system has a tubular organization. Within it are cavities, collectively termed the ventricular system, that contain cerebrospinal fluid (Figure 1–13). Cerebrospinal fluid is a watery fluid that cushions the central nervous system from physical shocks and is a medium for chemical communication. An intraventricular structure, the choroid plexus, secretes most of the cerebrospinal fluid. Cerebrospinal fluid production is considered in Chapter 3.

The ventricular system consists of ventricles, where cerebrospinal fluid accumulates, and narrow communication channels. There are two lateral ventricles, each within one cerebral hemisphere, the third ventricle, between the two halves of the diencephalon, and the fourth ventricle, which is located between the brain stem and cerebellum. Development of the lateral ventricles, along with the sulci and gyri of the cerebral cortex, is discussed in Box 1-2. The ventricles are interconnected by narrow channels: The interventricular foramina (of Monro) connect each of the lateral ventricles with the third ventricle, and the cerebral aqueduct (of Sylvius), in the midbrain, connects the third and fourth ventricles. The ventricular system extends into the spinal cord as the central canal. Cerebrospinal fluid exits the ventricular system through several apertures in the fourth ventricle and bathes the surface of the entire central nervous system.

The meninges consist of the dura mater, the arachnoid mater, and the pia mater (Figure 1–15). (The meninges are more commonly called the dura, arachnoid, and pia, without using the term mater.) The dura mater is the thickest and outermost of these membranes and serves a protective function. (Dura mater means "hard mother" in Latin.) Ancient surgeons knew that patients could survive even severe skull fractures if bone fragments had not penetrated the dura. Two important partitions arise from the dura and separate different components of the cerebral hemispheres and brain stem (Figure 1–15B): (1) The falx cerebri separates the two cerebral hemispheres, and (2) the tentorium cerebelli separates the cerebellum from the cerebral hemispheres.

The arachnoid mater adjoins but is not tightly bound to the dura mater, thereby allowing a potential space, the subdural space, to exist between them. This space is important clinically. Because the dura mater contains blood vessels, breakage of one of its vessels due to head trauma can lead to subdural bleeding and to the formation of a blood clot (a subdural hematoma). In this condition the blood clot pushes the arachnoid mater away from the dura mater, fills the subdural space, and compresses underlying neural tissue.

The innermost meningeal layer, the pia mater, is very delicate and adheres to the surface of the brain and spinal cord. (Pia mater means "tender mother" in Latin.) The space between the arachnoid mater and pia mater is the subarachnoid space. Filaments of arachnoid mater pass through the subarachnoid space and connect to the pia mater, giving this space the appearance of a spider's web. (Hence the name arachnoid, which derives from the Greek word arachne, meaning "spider.")

The terminology of neuroanatomy is specialized for describing the brain's complex three-dimensional organization. The central nervous system is organized along the rostrocaudal and dorsoventral axes of the body (Figure 1–16). These axes are most easily understood in animals with a central nervous system that is simpler than that of humans. In the rat, for example, the rostrocaudal axis runs approximately in a straight line from the nose to the tail (Figure 1–16A). This axis is the longitudinal axis of the nervous system and is often termed the neuraxis because the central nervous system has a predominant longitudinal organization. The dorsoventral axis, which is perpendicular to the rostrocaudal axis, runs from the back to the abdomen. The terms posterior and anterior are synonymous with dorsal and ventral, respectively.

The longitudinal axis of the human nervous system is not straight as it is in the rat (Figure 1–16B). During development the brain—and therefore its longitudinal axis—undergoes a prominent bend, or flexure, at the midbrain. Instead of describing structures located rostral to this flexure as dorsal or ventral, we typically use the terms superior and inferior. As described in Box 1–1, this axis bend reflects the persistence of the cephalic flexure (see Figure 1–5).

We define three principal planes relative to the longitudinal axis of the nervous system in which anatomical sections are made (Figure 1–17). Horizontal sections are cut parallel to the longitudinal axis, from one side to the other. Transverse sections are cut perpendicular to the longitudinal axis, between the dorsal and ventral surfaces. Transverse sections through the cerebral hemisphere are roughly parallel to the coronal suture of the skull and, as a consequence, are also termed coronal sections. Sagittal sections are cut parallel both to the longitudinal axis of the central nervous system and to the midline, between the dorsal and ventral surfaces. A midsagittal section divides the central nervous system into two symmetrical halves, whereas a parasagittal section is cut off the midline. Radiological images are also obtained in these planes. This will be described in Chapter 2.

Cellular Organization of the Nervous System

The cellular constituents of the nervous system are neurons (Figure 1–2) and glia (Figure 1–3). Neurons have four specialized regions: (1) the dendrites, which receive information, (2) the cell body, which receives and integrates information, and (3) the axon, which transmits information from the cell body to (4) the axon terminals. There are three neuron classes: unipolar, bipolar, and multipolar (Figure 1–2B). Intercellular communication occurs at synapses, where a neurotransmitter is released. The glia include four types of macroglia. Oligodendrocytes and Schwann cells form the myelin sheath in the central and peripheral nervous systems, respectively. Astrocytes serve as structural and metabolic support for neurons. Ependymal cells line the ventricular system. The glia also consist of the microglia, which are phagocytic.

Regional Anatomy of the Nervous System

The nervous system contains two separate divisions, the peripheral nervous system and the central nervous system (Figure 1–4). Each system may be further subdivided. The autonomic division of the peripheral nervous system controls the glands and smooth muscle of the viscera and blood vessels, whereas the somatic division provides the sensory innervation of body tissues and the motor innervation of skeletal muscle. There are seven separate components of the central nervous system (Figures 1–4 and 1–6, 1–7, 1–8, 1–9, 1–10, 1–11, 1–12): (1) spinal cord, (2) medulla, (3) pons, (4) cerebellum, (5) midbrain, (6) diencephalon, which contains the hypothalamus and thalamus, and (7) cerebral hemispheres, which contain the basal ganglia, amygdala, hippocampal formation, and cerebral cortex. The external surface of the cerebral cortex is characterized by gyri (convolutions), sulci (grooves), and fissures (particularly deep grooves) (Figure 1–14). The cerebral cortex consists of four lobes: frontal, parietal, temporal, and occipital. The insular cortex is buried beneath the frontal, parietal, and temporal lobes. The corpus callosum, a commissure, interconnects each of the lobes. Three sets of structures lie beneath the cortical surface: the hippocampal formation, the amygdala, and the basal ganglia. The limbic system comprises a diverse set of cortical and subcortical structures. The olfactory bulbs lie on the orbital surface of the frontal lobes.

Ventricular System

Cavities comprising the ventricular system are filled with cerebrospinal fluid and are located within the central nervous system (Figure 1–13). One of the two lateral ventricles is located in each of the cerebral hemispheres, the third ventricle is located in the diencephalon, and the fourth ventricle is between the brain stem (pons and medulla) and the cerebellum. The central canal is the component of the ventricular system in the spinal cord. The interventricular foramina connect the two lateral ventricles with the third ventricle. The cerebral aqueduct is in the midbrain and connects the third and fourth ventricles.

Meninges

The central nervous system is covered by three meningeal layers, from outermost to innermost: dura mater, arachnoid mater, and pia mater (Figure 1–15). Arachnoid mater and pia mater are separated by the subarachnoid space, which also contains cerebrospinal fluid. Two prominent flaps in the dura separate brain structures: falx cerebri and the tentorium cerebelli (Figure 1–15). Also located within the dura are the dural sinuses, low-pressure blood vessels (Figure 1–15).

Axes and Planes of Section

The central nervous system is oriented along two major axes (Figure 1–16): the rostrocaudal axis, which is also termed the longitudinal axis, and the dorsoventral axis, which is perpendicular to the longitudinal axis. Sections through the central nervous system are cut in relation to the rostrocaudal axis (Figure 1–17). Horizontal sections are cut parallel to the rostrocaudal axis, from one side to the other. Transverse, or coronal, sections are cut perpendicular to the rostrocaudal axis, between the dorsal and ventral surfaces. Sagittal sections are cut parallel to the longitudinal axis and the midline, also between the dorsal and ventral surfaces.