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Sensory and motor functions are interdependent, as was dramatically illustrated by the early animal experiments of Claude Bernard and Charles Sherrington, in which practically all effective movement of a limb was abolished by eliminating only its sensory innervation (sectioning of posterior roots). Interruption of other sensory pathways and destruction of the parietal cortex also has profound effects on motility. To a large extent, human motor activity depends on a constant influx of sensory impulses (most of them not consciously perceived). Sensory motor integration is therefore necessary for normal nervous system function but disease may affect motor or sensory functions independently. There may be loss or impairment of sensory function, and this can represent the principal manifestation of neurologic disease.
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This chapter deals with general somatic sensation, i.e., afferent impulses that arise in the skin, muscles, or joints. One form of somatic sensation—pain—was discussed in Chap. 8. Because of its overriding clinical importance, pain has been accorded a chapter of its own, but that chapter and this chapter are of one piece. The special senses—vision, hearing, taste, and smell—are considered in the next section (Chaps. 12, 13, 14, and 15), and visceral sensation, most of which does not reach consciousness, is considered with the disorders of the autonomic nervous system (Chap. 26).
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An understanding of sensory disorders depends upon knowledge of functional anatomy. It is necessary to be familiar with the sensory receptors in the skin and deeper structures, the distribution of the peripheral nerves and roots, and the pathways by which sensory impulses are conveyed from the periphery and through the spinal cord and brainstem to the thalamus and cerebral cortex. These aspects of sensory anatomy and physiology were touched upon in Chap. 8 in relation to the perception of pain and are elaborated upon here to include all forms of somatic sensation. An appropriate starting point is Fig. 9-1 that shows the cutaneous distribution of the peripheral nerves.
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All sensation depends on impulses that are excited by stimulation of receptors and conveyed to the central nervous system by afferent (sensory) fibers. Sensory receptors are of two general types: those in the skin, mediating superficial sensation (exteroceptors), and those in the deeper somatic structures (proprioceptors). Skin receptors are particularly numerous and transduce four types of sensory experience: warmth, cold, touch, and pain; these are conventionally referred to as sensations or senses, e.g., tactile sensation or sense of touch. Proprioceptors inform us of the position of our body or parts of our body; of the force, direction, and range of movement of the joints (kinesthetic sense); and a sense of pressure, both painful and painless. Histologically, ...