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  • Sensory Information Processing Is Illustrated in the Somatosensory System

    • Somatosensory Information from the Trunk and Limbs Is Conveyed to the Spinal Cord

    • The Primary Sensory Neurons of the Trunk and Limbs Are Clustered in the Dorsal Root Ganglia

    • The Central Axons of Dorsal Root Ganglion Neurons Are Arranged to Produce a Map of the Body Surface

    • Each Somatic Submodality Is Processed in a Distinct Subsystem from the Periphery to the Brain

  • The Thalamus Is an Essential Link Between Sensory Receptors and the Cerebral Cortex for All Modalities Except Olfaction

  • Sensory Information Processing Culminates in the Cerebral Cortex

  • Voluntary Movement Is Mediated by Direct Connections Between the Cortex and Spinal Cord

  • An Overall View


The human brain identifies objects and carries out actions in ways no current computer can even begin to approach. Merely to see—to look onto the world and recognize a face or facial expression—entails amazing computational achievements. Indeed, all our perceptual abilities—seeing, hearing, smelling, tasting, and touching—are analytical triumphs. Similarly, all of our voluntary actions are triumphs of engineering. The brain accomplishes these computational feats because its information processing units—its nerve cells—are wired together in very precise ways.


In this chapter we outline the neuroanatomical organization of perception and action. We focus on touch because the somatosensory system is particularly well understood and because touch clearly illustrates the interaction of sensory and motor systems—how information from the body surface ascends through the sensory relays of the nervous system to the cerebral cortex and is transformed into motor commands that descend to the spinal cord to produce movements.


We now have a fairly complete understanding of how the physical energy of a tactile stimulus is transduced by mechanoreceptors in the skin into electrical activity, and how this activity at different relays in the brain correlates with specific aspects of the experience of touch. Moreover, because the pathways from one relay to the next are well delineated, we can see how sensory information is coded at each relay.


Trying to comprehend the functional organization of the brain might at first seem daunting. But as we saw in the last chapter, the organization of the brain is simplified by three anatomical considerations. First, there are relatively few types of neurons. Each of the many thousands of spinal motor neurons or millions of neocortical pyramidal cells has a similar structure and serves a similar function. Second, neurons in the brain and spinal cord are clustered in discrete functional groups called nuclei, which are connected to form functional systems. Third, specific regions of the cerebral cortex are specialized for sensory, motor, or, as we shall learn in detail in Chapters 17 and 18, associational functions.


Sensory Information Processing Is Illustrated in the Somatosensory System


Complex behaviors, such as using touch alone to differentiate a ball from a book, require the integrated action of several ...

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