The cervical plexus is formed from the anterior rami of cervical nerves C1-C4. It has cutaneous branches that innervate the skin of the head and neck, and muscular/communicating branches. The phrenic nerve from C3-C5 innervates the diaphragm. The document outlines the branches of the cervical plexus in detail, including the lesser occipital nerve, great auricular nerve, and supraclavicular nerves. It describes the muscular and communicating branches that innervate neck muscles and connect with other nerves.