The spinal cord is segmental in character, and the spinal nerves are distributed to structures developed from the associated segments, or metameres (see Plate 2-1). In the trunk, the correspondence between neural and bodily segments is clearly apparent because they are arranged in consecutive encircling bands. In the limbs, however, due to plexus formation and interchange of nerve fibers in the nerves supplying them, the segmental distribution is obscured, although the arrangement is explicable embryologically. As the limb buds develop, they draw out parts of certain segments, together with their mesodermal cores, ectodermal coverings, and corresponding segmental nerves and vessels. Thus the more proximal dermatomes are elongated strips situated along the preaxial (outer) sides of the limbs, and the more distal ones are situated along their postaxial (medial sides). The oblique disposition in the lower limbs is due to the fact that during development, the limbs rotate medially around a longitudinal axis.
The fifth cervical to first thoracic metameres and the first lumbar to third sacral metameres contribute, respectively, to the formation of the upper and lower limbs. This is reflected in their nerve supplies and explains why in the neck, trunk, and upper limb the C5 and T1 dermatomes are in parts contiguous, and why in the trunk, perineum, and lower limbs the L1 and L2 dermatomes are in places adjacent to those of S2 and S3—the intervening segments have migrated into the more distal parts of the limbs.
The nerves supplying neighboring dermatomes overlap, and thus division of one dorsal nerve root produces hypoesthesia rather than anesthesia. To effect complete cutaneous anesthesia in any area, at least three adjoining spinal nerves or their dorsal roots must be blocked or divided. The exception to this general rule is that section of the dorsal root of C2 produces an area of complete anesthesia in the occipital region of the scalp. The degree of nerve overlap varies for different sensations, being greater for touch than for pain and temperature. The segmental muscular supplies also overlap so that most of the larger muscles (especially those in the limbs) are innervated by fibers from several ventral nerve roots. Therefore paresis occurs if only one or two roots are affected, but paralysis results if more roots are damaged or destroyed.
Knowledge of the dermatomes enables the clinician to locate lesions affecting the spinal cord or spinal nerves, and the dermatomes of the hand and foot deserve special attention.