Peripheral nerves supply cutaneous innervation to an area of skin on the surface of the body. For example, the lateral region of the thigh receives its cutaneous innervation from the L2 and L3 nerve roots. However, the sensory neurons from L2 and L3 are distributed to the lateral part of the thigh via the lateral cutaneous nerve of the thigh. The principal cutaneous nerves supplying the skin of the lower limb are illustrated in Figure 34-5B.
The two distributions, the dermatomes and the cutaneous nerves, provide different sensory patterns although they are supplied by the same peripheral nerve root. This is especially evident when examining patients with nerve root injuries. A patient with a lesion of a cutaneous nerve
presents differently than does a patient with a lesion of a spinal root
(dermatomal distribution). If the lateral cutaneous nerve of the thigh is cut, there will be a loss of sensation on the lateral side of the thigh, and the anterior and medial sides of the thigh will remain intact. Therefore, the result is partial loss of the L2 and L3 dermatome. In contrast, if the L2 and L3 nerve roots are cut, the result is loss of sensation of the skin served by the L2 and L3 dermatome, including the anterior and medial thigh.