The peripheral afferent pain fibers of both A-d and C types have their cell bodies in the dorsal root ganglia; central extensions of these nerve cells project, via the dorsal root, to the dorsal horn of the spinal cord (or, in the case of cranial pain afferents, to the spinal trigeminal nucleus, the medullary analogue of the dorsal horn). The pain afferents occupy mainly the lateral part of the root entry zone. Within the spinal cord, many of the thinnest fibers (C fibers) form a discrete bundle, the tract of Lissauer (Fig. 7-1A). Transection of the tract of Lissauer produces ipsilateral segmental analgesia, demonstrating that it is predominantly a pain pathway, but it does contain deep sensory and propriospinal fibers as well. Although it is customary to speak of a lateral and medial division of the posterior spinal nerve root (the former contains small pain fibers and the latter, large myelinated fibers), the separation into discrete functional bundles is not complete, and in humans the two groups of fibers cannot be differentially interrupted by selective rhizotomy.
Dermatomal Distribution of Pain Fibers (See Fig. 8-1)
The sensory unit consists of the sensory nerve cell in the dorsal root ganglion, its central and peripheral extensions, and its cutaneous and visceral endings. The topography of the units maintains a consistent distribution throughout the sensory system from the periphery to the sensory cortex. The discrete segmental distribution of the sensory units permits the construction of clinically useful sensory maps (see Fig. 8-1). This aspect of sensory anatomy is elaborated in the next chapter, which includes maps of the sensory dermatomes and cutaneous nerves. However, as a means of quick orientation to the topography of peripheral pain pathways, it is useful to remember that the facial structures and anterior cranium lie in the fields of the trigeminal nerves; the back of the head, second cervical; the neck, third cervical; the epaulet area, fourth cervical; the deltoid area, fifth cervical; the radial forearm and thumb, sixth cervical; the index and middle fingers, seventh cervical; the little finger and ulnar border of hand and forearm, eighth cervical–first thoracic; the nipple, fourth to fifth thoracic; the umbilicus, tenth thoracic; the groin, first lumbar; the medial side of the knee, third lumbar; the medial malleolus, the fourth lumbar; the great toe, fifth lumbar; the little toe, first sacral; the back of the thigh and lateral foot, second sacral; and the genitoanal zones, third, fourth, and fifth sacral segments.
The distribution of pain fibers from deep structures, although not fully corresponding to those from the skin, also follows a roughly segmental pattern. The first to fourth thoracic nerve roots are the important sensory pathways for the heart and lungs; the sixth to eighth thoracic, for the upper abdominal organs; the lower thoracic and upper lumbar, for the lower abdominal viscera; and lower pelvic organs such as the bladder and rectum, the second through fourth sacral roots. These areas of pain projection from visceral structures roughly correspond to the areas of skin innervation by the same roots, with some exceptions, such as the testicles, because of routing of sensory nerves to organs that migrate caudally with development.
Neurologically relevant maps of pain projection from the bones, ligaments, and adjacent musculoskeletal structures have been termed sclerotomes; they differ slightly in their distribution from the dermatomes. A further discussion of referred pain and a figure comparing sclerotomes and dermatomes is given later in the chapter.
The Dorsal Horn