Final answer:
The anterior neck muscles, skin of the neck, and certain head and shoulder regions are innervated by the cervical plexus and the brachial plexus, with contributions from the accessory nerve to the sternocleidomastoid and trapezius muscles.
Step-by-step explanation:
The innervations of the anterior neck muscles, the skin of the neck, and parts of the head and shoulders are branches primarily of the cervical plexus and the brachial plexus. The cervical plexus consists of axons from spinal nerves C1 through C5 and serves the posterior head, neck, and diaphragm (via the phrenic nerve). On the other hand, the brachial plexus, formed by spinal nerves C4 through T1, supplies nerves to the arms and some regions of the shoulders and neck including contributions to the accessory nerve, which innervates the sternocleidomastoid and trapezius muscles. These muscles are involved in head movement and elevation of the scapula and clavicle.