The brachial plexus is part of the somatic division of the nervous system. It sends and receives signals between the muscles and skin of the arm and the spinal cord. It consists of several bundles of nerves that exit the neck and stretch across the armpit into the arm.
Beginning at five of the cervical vertebrae, the brachial plexus nerves join to form three bundles. They then branch off to provide signals to different areas, including muscles in the shoulder and the front and back of the arm, and extend down to the hand. Injuries to the brachial plexus can cause a loss of feeling or function of the arm and hand.