Why is there no C1 dermatome?

The term dermatome has two different meanings. One of the meanings refers to the areas of the skin innervated by a spinal nerve. The second meaning refers to the external portion of an embryonic somite that develops into the connective tissue of the skin. With regard to the first meaning, dermatomes are defined on the basis of sensory innervation. In this sense, they represent the distribution of the sensory nerve axons originating from the neurons in the dorsal root ganglia associated with each spinal cord segment.

Most spinal nerves consist of both motor and sensory axons. The sensory component of each spinal nerve supplies sensory innervation to a particular area of the skin defining what is called a dermatome. The dermatomes are established early in embryonic development when the spinal nerves grow out to innervate their designated body surface area. Cervical spinal nerve C1 consists exclusively of motor axons that innervate the geniohyoid and infrahyoid muscles via the (XIIth) cranial nerve, and the suboccipital muscles. Since C1 lacks sensory axons there is no C1 dermatome. The sensory axons of segment C1 are distributed with the axons of spinal segment C2.