The L1 and L2 dermatomes can be found in sites adjacent to S2 and S3 dermatomes because of the intervening segments migrating into more distal parts of the lower limbs. The developing lower limbs rotate medially around a longitudinal axis, with a resultant oblique orientation of the dermatomes. As the limb buds for the lower extremities develop, they draw out the nerve roots that correspond with their mesodermal cores and ectodermal coverings. In contrast, an irritative lesion such as a herniated intervertebral disc may produce sharp, radiating pain within the distribution of the affected dermatome. Dermatomal anesthesia requires damage to at least three dorsal roots: the central dorsal root and the roots above and below it. Thus, sectioning or dysfunction of a single dorsal root produces hypoesthesia (diminished sensation), not anesthesia (total loss of sensation) in the region supplied predominantly by that dermatome, as shown in the figure. The nerve roots supplying neighboring dermatomes overlap. The spinal nerve roots are distributed to structures according to their associations with spinal cord segments. A dermatome is the cutaneous area supplied by a single spinal nerve root the cell bodies are located in dorsal root ganglia.
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