The Organization of the Central Nervous System
IN THE EARLIER CHAPTERS OF THIS book we emphasized that modern neuroscience is based importantly on two tenets. First, the brain is organized into functionally specific areas, and second, neurons in different parts of the vertebrate nervous system, indeed in all nervous systems, are quite similar. What distinguishes one functionally distinct brain region from another, and one brain from the next, are the number and types of neurons in each and how they are interconnected through development. The specific patterns of interconnection and the resulting functional organization of neural circuits in distinct brain regions underlie the individuation of behavior.
All behavior, from simple reflex responses to complex mental acts, is the product of signaling between appropriately interconnected neurons. Consider the simple act of hitting a tennis ball (Figure 15-1). Visual information about the motion of the approaching ball is analyzed in the visual system. This information is combined with proprioceptive information about the position of the arms, legs, and trunk to calculate the movement necessary to intercept the ball. Once the swing is initiated, many minor adjustments of the motor program are made based on a steady stream of sensory information about the trajectory of the approaching ball. Finally, this entire act is accessible to consciousness, and thus may elicit memories and emotions. Of course, as the swing is being executed, the brain is also engaged in maintaining the player’s heart rate, respiration, and other autonomic functions that are typically outside the awareness of the player.
Figure 15-1 A simple behavior is mediated by many parts of the brain.
A. A tennis player watching an approaching ball uses the visual cortex to judge the size, direction, and velocity of the ball. The premotor cortex develops a motor program to return the ball. The amygdala acts in conjunction with other brain regions to adjust the heart rate, respiration, and other homeostatic mechanisms and also activates the hypothalamus to motivate the player to hit well.
B. To execute the shot the player must use all of the structures illustrated in part A as well as others. The motor cortex sends signals to the spinal cord that activate and inhibit many muscles in the arms and legs. The basal ganglia become involved in initiating motor patterns and perhaps recalling learned movements to hit the ball properly. The cerebellum adjusts movements based on proprioceptive information from peripheral sensory receptors. The posterior parietal cortex provides the player with a sense of where his body is located in space and where his racket arm is located with respect to his body. Throughout the movement, brain stem neurons regulate heart rate, respiration, and arousal. The hippocampus is not involved in hitting the ball but is involved in storing the memory of the return so that the player can brag about it later.
Thus, to understand the neural control of any behavior it is necessary to break down that behavior into key components, to then identify the regions of the brain responsible for each component, and to analyze the neural connections between those regions. Although the anatomy of the brain and its interconnections appear complex, brain anatomy is easier to grasp if one understands the relatively simple set of principles that underlie the fundamental organization of the nervous system. In this and the next five chapters we examine the relationship between anatomy and behavior in a series of progressively more complex examples.
In this chapter and the next we review the major anatomical components of the central nervous system by outlining the organization of the major functional systems that recruit these components. In Chapters 17 and 18 we examine how complex cognitive functions are constructed from the interaction of cortical association areas. In Chapter 19 we describe how in higher cortical association areas perception—the product of the sensory systems—and action—the product of the motor systems—work in parallel. Finally, in Chapter 20 we explore how complex cognitive functions of humans, including attention and consciousness, can be studied by means of brain imaging.
The location and orientation of components of the central nervous system within the body are described with reference to three axes: the rostral-caudal, dorsalventral, and medial-lateral axes (Figure 15-2).
Figure 15-2 The central nervous system is described along three major axes. (Adapted, with permission, from Martin 2003.)
A. Rostral means toward the nose and caudal toward the tail. Dorsal means toward the back of the animal and ventral toward the belly. In lower mammals the orientations of these two axes are maintained through development into adult life. In humans and other higher primates the longitudinal axis flexes in the brain stem by approximately 110°. Because of this flexure the same positional terms have different meanings when referring to structures below and above the flexure. Below the flexure, in the spinal cord, rostral means toward the head, caudal means toward the coccyx (the lower end of the spinal column), ventral (anterior) means toward the belly, and dorsal (posterior) means toward the back. Above the flexure, rostral means toward the nose, caudal means toward the back of the head, ventral means toward the jaw, and dorsal means toward the top of the head. The term superior is often used synonymously with dorsal, and inferior means the same as ventral.
B. Medial means toward the middle of the brain and lateral toward the side.
C. When brains are sectioned for analysis, slices are typically made in one of three cardinal planes: horizontal, coronal, or sagittal.
The spinal cord is the most caudal part of the central nervous system and in many respects the simplest part. It extends from the base of the skull to the first lumbar vertebra. The spinal cord receives sensory information from the skin, joints, and muscles of the trunk and limbs, and contains the motor neurons responsible for both voluntary and reflex movements. Along its length the spinal cord varies in size and shape, depending on whether the emerging motor nerves innervate the limbs or trunk; it is thicker at levels that innervate the arms and legs.
The spinal cord is divided into a core of central gray matter and surrounding white matter. The gray matter, which contains nerve cell bodies, is typically divided into dorsal and ventral horns (so-called because the gray matter appears H-shaped in transverse sections). The dorsal horn contains an orderly arrangement of sensory relay neurons that receive input from the periphery, whereas the ventral horn contains groups of motor neurons and interneurons that regulate motor neuronal firing patterns. The axons of motor neurons innervate specific muscles. The white matter is made up in part of rostral-caudal (longitudinal) ascending and descending tracts of myelinated axons. The ascending pathways carry sensory information to the brain, while the descending pathways carry motor commands and modulatory signals from the brain to the muscles.
The nerve fibers to and from the spinal cord are bundled in 31 spinal nerves, each of which has a sensory and a motor division. The sensory division (the dorsal root) carries information from muscles and skin into the spinal cord and terminates in the dorsal aspect of the cord. Different classes of axons within the dorsal roots convey pain, temperature, touch, and visceral sensory information. The motor division (the ventral root) emerges from the ventral aspect of the cord and comprises the axons of motor neurons that innervate muscles. Ventral roots from certain levels of the spinal cord also include sympathetic and parasympathetic axons. The motor neurons of the spinal cord comprise the “final common pathway” through which all higher brain levels controlling motor activity must act.
The brain, which lies rostral to the spinal cord, is composed of six regions: the medulla, pons, midbrain, cerebellum, diencephalon, and cerebral hemispheres or telencephalon (Figure 15-3). Each of these divisions is found in both hemispheres of the brain with slight bilateral differences. Each of the six divisions is further subdivided into several anatomically and functionally distinct areas.
Figure 15-3 The major divisions of the central nervous system. Drawings show a lateral view of the left side of the brain and the medial surface of the right side of the brain. (Adapted, with permission, from Nieuwenhuys, Voogd, and van Huijzen 1988.)
The three divisions of the central nervous system immediately rostral to the spinal cord—the medulla, pons, and midbrain—are collectively termed the brain stem.
The medulla, the most caudal portion of the brain stem, is a direct extension of the spinal cord and resembles the spinal cord both in organization and function. Neuronal groups in the medulla participate in regulating blood pressure and respiration. The medulla also contains neuronal groups that are early components of pathways that mediate taste, hearing, and maintenance of balance as well as the control of neck and facial muscles.
The pons lies rostral to the medulla and protrudes from the ventral surface of the brain stem. The ventral portion of the pons contains the pontine nuclei, groups of neurons that relay information about movement and sensation from the cerebral cortex to the cerebellum. The dorsal portion of the pons contains structures involved in respiration, taste, and sleep.
The midbrain, the smallest part of the brain stem, lies rostral to the pons. Nuclei in the midbrain provide important linkages between components of the motor system, particularly the cerebellum, basal ganglia, and cerebral hemispheres. For example, the substantia nigra provides important input to a portion of the basal ganglia that regulates voluntary movements. The substantia nigra is the focus of intense interest as damage to its dopaminergic neurons is responsible for the pronounced motor disturbances that are characteristic of Parkinson disease (see Chapter 43). The midbrain also contains components of the auditory and visual systems. Finally, several regions of the midbrain give rise to pathways that are connected to the extraocular muscles that control eye movements.
The brain stem has five distinct functions. First, just as the spinal cord mediates sensation and motor control of the trunk and limbs, the brain stem mediates sensation and motor control of the head, neck, and face. The sensory input and motor output of the brain stem is carried by 12 cranial nerves that are functionally analogous to the 31 spinal nerves. Second, the brain stem is the site of entry for information from several specialized senses, such as hearing, balance, and taste. Third, specialized neurons in the brain stem mediate parasympathetic reflexes, such as decreases in cardiac output and blood pressure, increased peristalsis of the gut, and constriction of the pupils. Fourth, the brain stem contains ascending and descending pathways that carry sensory and motor information to other divisions of the central nervous system. Fifth, a relatively diffuse network of neurons distributed throughout the core of the brain stem, known as the reticular formation, receives a summary of much of the incoming sensory information and is important in regulating alertness and arousal.
The cerebellum lies over the pons and is divided into several lobes by distinct fissures. The cerebellum is important for maintaining posture and coordinating head, eye, and arm movements, and is also involved in minute regulation of motor output and learning motor skills. Until recently, the cerebellum was considered a purely motor structure, but new anatomical information about its interconnections with the cerebral cortex and functional imaging studies have shown that it is also involved in language and other cognitive functions. The cerebellum contains far more neurons than any other single subdivision of the brain, including the cerebral hemispheres. Its internal circuitry, however, is well understood because relatively few types of neurons are involved. The cerebellum receives information about somatic sensation from the spinal cord, information about balance from the vestibular organs of the inner ear, and motor and sensory information from various areas of the cerebral cortex via the pontine nuclei.
The diencephalon contains two major subdivisions: the thalamus and hypothalamus. The thalamus is an essential link in the pathway of sensory information from the periphery (other than olfactory receptors in the nose) to sensory regions of the cerebral hemispheres. It once was thought to act only as a relay station for sensory information traveling to the neocortex, but now it is clear that it also determines which sensory information reaches the neocortex. The thalamus also interconnects the cerebellum and basal ganglia with regions of the cerebral cortex concerned with movement and cognition. Like the reticular formation, the diencephalon also has regions that are thought to influence levels of attention and consciousness.
The hypothalamus lies ventral to the thalamus and regulates homeostasis and several reproductive behaviors. For example, it plays an important role in somatic growth, eating, drinking, and maternal behavior by regulating the hormonal secretions of the pituitary gland. The hypothalamus also influences behavior through its extensive afferent and efferent connections with practically every region of the central nervous system. It is an essential component of the motivational systems of the brain, initiating and maintaining behaviors the organism finds aversive or rewarding. Finally, one group of neurons in the hypothalamus, the suprachiasmatic nucleus, regulates circadian rhythms, cyclical behaviors that are entrained to the daily light-dark cycle.
The cerebral hemispheres are the largest part of the human brain. They consist of the cerebral cortex, the underlying white matter, and three deep-lying structures: the basal ganglia, amygdala, and hippocampal formation. The cerebral hemispheres have perceptual, motor, and cognitive functions, including memory and emotion. The two hemispheres are interconnected by the corpus callosum, which is visible on the medial surface of the hemispheres. The corpus callosum is the largest of the commissures (large bundles of axons that mainly link similar regions of the left and right sides of the brain). The amygdala is concerned with the expression of emotion, the hippocampus with memory formation, and the basal ganglia with the control of movement and aspects of motor learning.
While the spinal cord, brain stem, and diencephalon mediate many life-sustaining functions, it is the cerebral cortex—the thin outer layer of the cerebral hemispheres—that is responsible for much of the planning and execution of actions in everyday life. The cerebral cortex is divided into four major lobes—frontal, parietal, temporal, and occipital—named after the overlying cranial bones (Figure 15-4). Each lobe includes many distinct functional subregions. The temporal lobe, for example, has distinct regions with auditory, visual, or memory functions.
Figure 15-4 The major lobes and some prominent sulci of the human cerebral cortex. A lateral view of the left side of the brain is shown at left and a medial view of the right side of the brain at right. (Reproduced, with permission, from Martin 2003.)
Two additional regions of the cerebral cortex are the cingulate cortex, which surrounds the dorsal surface of the corpus callosum and is involved in the regulation of emotion and cognition, and the insular cortex (insula), which is not visible on the surface owing to the overgrowth of the frontal, parietal, and temporal lobes (Figure 15-5) and is concerned with emotion and the regulation of homeostasis. The overhanging portion of the cerebral cortex that buries the insula within the lateral sulcus is called the operculum.
Figure 15-5 Structures in the middle of the cerebral hemispheres. These include the basal ganglia (caudate nucleus and globus pallidus) and insular cortex. Large cavities in the brain called ventricles are filled with cerebrospinal fluid. (Adapted, with permission, from England and Wakely 1991.)