Each cutaneous afferent fiber connects to several receptors of the same type (touch, pain, or temperature), which are irregularly distributed in the skin and account for what had been termed “sensory spots” as alluded to above and in the preceding chapter. Proprioceptive fibers subserve pressure sense and, with endings in articular structures, the sense of position and movement; they enable one to discriminate the form, size, texture, and weight of objects. However, sensations of tickle, itch, and wetness are believed to arise from combinations of several types of receptors. Itch, for example, is a distinctive sensation that can be separated on clinical and neurophysiologic grounds from touch and from pain. It is transmitted by specific C fibers, not by touch mechanisms; regions of analgesia no longer can be stimulated to itch but areas of anesthesia retain this sensation. The pathophysiology of itching has been discussed by Greaves and Wall and is reviewed further by Yosipovitch and colleagues.
The sensory nerves are incorporated into plexi (lumbosacral, brachial plexus) and then divide into the dorsal roots. All the sensory neurons have their cell bodies in the dorsal root ganglia. The peripheral extensions of these cells are the sensory nerves; the central projections of these same cells form the posterior (dorsal) roots and enter the spinal cord. Each dorsal root contains all the fibers from skin, muscles, connective tissue, ligaments, tendons, joints, bones, and viscera that lie within the distribution of a single body segment (somite). This segmental innervation has been amply demonstrated in humans and animals by observing the effects of lesions that involve one or two spinal nerves, such as (1) herpes zoster, which also causes visible vesicles in the corresponding area of skin; (2) the effects of a prolapsed intervertebral disc, which causes hypalgesia in a single root zone; and (3) surgical section of several roots on each side of an intact root. Maps of the dermatomes derived from these several types of data are shown in Figs. 8-1, 8-3 and 8-4. It should be noted that there is considerable overlap between adjacent dermatomal segments, more so for touch than for pain. By contrast, there is less overlap between adjacent peripheral nerves and almost none between the divisions of the trigeminal nerve. Also, the maps differ somewhat according to the methods used in constructing them. In contrast to most dermatomal charts, those of Keegan and Garrett (based on the injection of local anesthetic into single dorsal root ganglia) show bands of hypalgesia to be continuous longitudinally from the periphery to the spine (see Fig. 8-4). The distribution of pain fibers from deep structures, although not exactly corresponding to that of pain fibers from the skin, also follows a segmental pattern. In Chap. 7 it was commented that the areas of projection of referred pain from the visceral organs and musculoskeletal structures roughly correspond to the overlying dermatomes but they have distinctive patterns, termed sclerotomes (see Fig. 7-5).
Posterior Root Entry Zone, Dorsal Horns,