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The Functional Organization of Perception and Movement

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The Functional Organization of Perception and Movement 16 The Functional Organization of Perception and Movement Sensory Information Processing Is Illustrated in the In this chapter we outline the neuroanatomi- Somatosensory System cal organization of perception and action. We focus Somatosensory Information from the Trunk and Limbs Is on touch because the somatosensory system is par- Conveyed to the Spinal Cord ticularly well understood and because touch clearly The Primary Sensory Neurons of the Trunk and Limbs illustrates the interaction of sensory and motor Are Clustered in the Dorsal Root Ganglia systems—how information from the body surface The Central Axons of Dorsal Root Ganglion Neurons Are ascends through the sensory relays of the nervous Arranged to Produce a Map of the Body Surface system to the cerebral cortex and is transformed into Each Somatic Submodality Is Processed in a Distinct motor commands that descend to the spinal cord to Subsystem from the Periphery to the Brain produce movements. The Thalamus Is an Essential Link Between Sensory We now have a fairly complete understanding Receptors and the Cerebral Cortex for All Modalities of how the physical energy of a tactile stimulus is Except Olfaction transduced by mechanoreceptors in the skin into Sensory Information Processing Culminates in the electrical activity, and how this activity at different Cerebral Cortex relays in the brain correlates with specifc aspects Voluntary Movement Is Mediated by Direct Connections of the experience of touch. Moreover, because the Between the Cortex and Spinal Cord pathways from one relay to the next are well deline- ated, we can see how sensory information is coded An Overall View at each relay. Trying to comprehend the functional organization of the brain might at frst seem daunting. But as we he human brain identifies objects and car- saw in the last chapter, the organization of the brain ries out actions in ways no current computer is simplifed by three anatomical considerations. First, Tcan even begin to approach. Merely to see—to there are relatively few types of neurons. Each of the look onto the world and recognize a face or facial many thousands of spinal motor neurons or millions expression—entails amazing computational achieve- of neocortical pyramidal cells has a similar structure ments. Indeed, all our perceptual abilities—seeing, and serves a similar function. Second, neurons in the hearing, smelling, tasting, and touching—are analyti- brain and spinal cord are clustered in discrete func- cal triumphs. Similarly, all of our voluntary actions are tional groups called nuclei, which are connected to triumphs of engineering. The brain accomplishes these form functional systems. Third, specifc regions of the computational feats because its information process- cerebral cortex are specialized for sensory, motor, or, as ing units—its nerve cells—are wired together in very we shall learn in detail in Chapters 17 and 18, associa- precise ways. tional functions. Chapter 16 / The Functional Organization of Perception and Movement 357 Sensory Information Processing Is Illustrated In the somatosensory system a light touch and a pain- in the Somatosensory System ful pin prick to the same area of skin are mediated by different pathways in the brain. Complex behaviors, such as using touch alone to differ- entiate a ball from a book, require the integrated action Somatosensory Information from the Trunk and of several nuclei and cortical regions. Information is Limbs Is Conveyed to the Spinal Cord processed in the brain in a hierarchical fashion. Thus information about a stimulus is conveyed through a Sensory information from the trunk and limbs enters succession of subcortical and then cortical regions and the spinal cord, which has a core H-shape region of at each level of processing the information becomes gray matter surrounded by white matter. The gray mat- increasingly complex. In addition, different types of ter on each side of the cord is divided into dorsal (or information, even within a single sensory modality, are posterior) and ventral (or anterior) horns (Figure 16–1). processed in several anatomically discrete pathways. The dorsal horn contains groups of sensory neurons Cell Stain Fiber Stain Dorsal roots Dorsal horn White matter Gray matter Figure 16–1 The major anatomical features of the spinal cord. Top: The left side depicts Ventral a cell stain of the gray matter and the right side horn Ventral roots a fber-stained section. Bottom: The ventral Ventral sulcus horn (green) contains large motor neurons, whereas the dorsal horn (orange) contains To brain smaller neurons. Fibers of the gracile fascicle Gracile fascicle Information stem Ascending axons carry somatosensory information from the lower from body's Cuneate fascicle limbs, whereas fbers of the cuneate fascicle surface carry somatosensory information from the upper body. Fiber bundles of the lateral and ventral columns include both ascending and descending fber bundles. Sensory neurons Motor neurons Innervation of skeletal muscle Lateral columns Ascending and descending axons Ventral columns 358 Part IV / The Neural Basis of Cognition (sensory nuclei) whose axons receive stimulus informa- First, relatively few sensory axons enter the cord at tion from the body’s surface. The ventral horn contains the sacral level. At higher levels (lumbar, thoracic, and groups of motor neurons (motor nuclei) whose axons cervical) the number of sensory axons entering the cord exit the spinal cord and innervate skeletal muscles. increases progressively. Conversely, most descending Unlike the sensory nuclei, the motor nuclei axons from the brain terminate at cervical levels, with form columns that run the length of the spinal cord. progressively fewer descending to lower levels of the Interneurons of various types in the gray matter inhibit spinal cord. Thus the number of fbers in the white the output of the spinal cord neurons. These inhibitory matter is highest at cervical levels (where there are the interneurons thus modulate both sensory informa- highest numbers of both ascending and descending tion fowing toward the brain and motor commands fbers) and lowest at sacral levels. As a result, sacral descending from the brain to the spinal motor neurons. levels of the spinal cord have much less white matter Motor neurons can also adjust the output of other than gray matter, whereas the cervical cord has more motor neurons via the interneurons. white matter than gray matter (Figure 16–2). The white matter surrounding the gray matter con- The second organizational feature is variation in tains bundles of ascending and descending axons that the size of the ventral and dorsal horns. The ventral are divided into dorsal, lateral, and ventral columns. horn is larger at the levels where the motor nerves that The dorsal columns, which lie between the two dorsal innervate the arms and legs exit the spinal cord. The horns of the gray matter, contain only ascending axons number of ventral motor neurons dedicated to a body that carry somatic sensory information to the brain region roughly parallels the dexterity of movements stem (Figure 16–1). The lateral columns include both of that region. Thus a larger number of motor neurons ascending axons and axons descending from the brain is needed to innervate the greater number of muscles stem and neocortex that innervate spinal interneurons and to regulate the greater complexity of movement and motor neurons. The ventral columns also include in the limbs as compared with the trunk. Likewise, ascending and descending axons. The ascending the dorsal horn is larger where sensory nerves from somatic sensory axons in the lateral and ventral col- the limbs enter the cord. Limbs have a greater density umns constitute parallel pathways that convey infor- of sensory receptors to mediate fner tactile discrimi- mation about pain and thermal sensation to higher nation and thus send more fbers to the cord. These levels of the central nervous system. The descending regions of the cord are known as the lumbosacral and axons control axial muscles and posture. cervical enlargements. The spinal cord is divided into four major regions: cervical, thoracic, lumbar, and sacral (Figure 16–2). The Primary Sensory Neurons of the Trunk and These regions are related to the embryological somites Limbs Are Clustered in the Dorsal Root Ganglia from which muscles, bones, and other components of the body develop (see Chapters 52 and 53). Axons The sensory neurons that convey information from the projecting from the spinal cord to body structures that skin, muscles, and joints of the limbs and trunk to the develop at the same segmental level join together in spinal cord are clustered together in dorsal root ganglia the intervertebral foramen with axons entering the spi- within the vertebral column immediately adjacent to nal cord to form spinal nerves. Spinal nerves at the cer- the spinal cord (Figure 16–3). These neurons are pseudo- vical level are involved with sensory perception and unipolar in shape; they have a bifurcated axon with motor function of the back of the head, neck, and arms; central and peripheral branches. The peripheral branch nerves at the thoracic level innervate the upper trunk; terminates in skin, muscle, or other tissue as a free nerve whereas lumbar and sacral spinal nerves innervate the ending or in association with specialized receptors. lower trunk, back, and legs. The central process enters the spinal cord. On entry Each of the four regions of the spinal cord con- the axon forms branches that either terminate within tains several segments; there are 8 cervical segments, the spinal gray matter or ascend to nuclei located near 12 thoracic segments, 5 lumbar segments, and 5 sacral the junction of the spinal cord with the medulla (Figure segments. Although the actual substance of the mature 16–3). These local and ascending branches provide two spinal cord does not look segmented, the segments of functional pathways for somatosensory information the four spinal regions are nonetheless defned by the entering the spinal cord from dorsal root ganglion cells.
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