The preceding chapter dealt with pain and its pathways and mechanisms. There are, of course, several other somatosensory experiences that also utilize specialized end organs, pathways, and neurophysiologic mechanisms; these include touch, vibration and joint position senses, appreciation of deep pressure, as well as integrated sensory experiences that depend on cortical functions and are the subject of the current chapter. The separation between these two broad somatosensory systems is logical in so far as each depends on distinctive tracts in the peripheral nerves, spinal cord, and brain. In clinical practice, however, they are tested in parallel and give complementary information regarding the localization and nature of a lesion. Because the peripheral nervous system is organized in a segmental pattern, the superficial representation of all sensation, nociceptive and non-nociceptive, follows the dermatomal and peripheral nerve map shown in Fig. 8-1.
The cutaneous fields of peripheral nerves. (Reproduced by permission from Haymaker W, Woodhall B: Peripheral Nerve Injuries, 2nd ed. Philadelphia, Saunders, 1953.)
The sphere of tactile sensory experiences is every bit as rich as that originating in vision and audition. The receptors that translate mechanical forces on the skin are varied and incredibly well suited to allow the brain to distinguish between subtle experiences, from the texture of water to the coarseness of sand between the fingers. Furthermore, 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 also may be loss or impairment of sensory function, and this can represent the principal manifestation of neurologic disease.
ANATOMIC AND PHYSIOLOGIC CONSIDERATIONS
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. Proprioceptors provide information on the position of the body or parts of the body; of the force, direction, and range of movement of the joints (kinesthetic sense); and a sense of pressure, both painful and painless. Histologically, a wide variety of sensory receptors have been described, from simple, free terminals to highly branched and encapsulated structures, ...