The transverse cutaneous nerve of the neck arises from the ventral rami of C2 and C3.  It passes horizontally forward around the posterior border of the sternocleidomastoid muscle and at its midpoint, crosses deep to the external jugular vein to reach the anterior border.  Here it pierces the deep cervical fascia and divides into ascending and descending branches deep to platysma.

The ascending branch ascends to the submandibular region to form a plexus with the cervical branch of the facial nerve (VII cranial nerve) deep to platysma.  From this plexus small branches arise, which pierce platysma to terminate in the skin of the upper anterior neck.  The descending branch pierces the platysma muscle to terminate in the antero-lateral skin of the neck, as far as the sternum.

Supply
The transverse cutaneous nerve of the neck supplies the skin of the upper anterior neck and the antero-lateral skin of the neck, as far as the sternum.