Spinal nerves derive from the neural tube and the neural crest during embryonic development. They become associated with adjacent segmental muscle masses to which they supply both afferent and efferent innervation. Segmental muscle masses are called somites.A dermatomes is a skin segment innervated by the cutaneous sensory branch of a single spinal or cranial nerve. All spinal nerves except the first cervical nerve (CI) supply innervation to dermatomes. Cranial nerves supply muscles of the head in a similar manner. Cutaneous nerves supply the skin segments or dermatomes. The trigeminal nerve (CN V ) supplies the scalp and skin of the face. Cutaneous branches of spinal nerves supply adjacent dermatomes. The distribution plan of spinal nerves to somites and dermatomes is established by the fourth week of gestation ( embryonic development). As development proceeds, dermatomes of the trunk remain uniform and regula, and the spinal nerves are able to keep a direct relationship-- spinal nerve to dermatome. However, skin dermatome development becomes irregular with time, and spinal nerve relation to dermatomes become less obvious.
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In adults, some trunk dermatome overlap extensively on both sides.Therefore, it is not clear which nerve belongs to which dermatome, or somite. This poses a problem for anesthetists who often have to anesthetize three adjacent segments to be sure to block a given spinal nerve. Nevertheless, in patients with spinal cord injury, it is often possible for neurologists to detect the damaged nerve and the injured area of the spinal cord by checking which dermatomes respond abnormally to stimuli.