The dermatomal innervation is from multiple peripheral nerves that carry the same spinal levels (Figure 29-5C). For example, the spinal root of C6 provides cutaneous innervation to the lateral part of the forearm and hand. In other words, all sensory neurons originating in this region will terminate at the C6 spinal nerve level (Figure 29-5D).
The two sensory distributions (i.e., cutaneous and dermatomal) provide different sensory patterns to the upper limb although the same nerve root supplies both distributions. This is especially evident when examining a patient with nerve root injuries. A lesion to a cutaneous nerve
will present differently than will a lesion to a spinal root
(dermatomal distribution). If the lateral cutaneous nerve of the forearm is damaged, a loss of sensation will occur on only part of the lateral side of the forearm, whereas sensation to the side of the hand will remain intact. Therefore, the result is partial loss of the C6 dermatome. In contrast, if the C6 nerve root is severed, the result is loss of all of the sensory innervation of the skin served by the C6 dermatome in the forearm and hand.