The brachial plexus is a group of nerves that stem from the spinal cord in the neck and travel all the way down the arm. These nerves control the muscles of the shoulder, elbow, wrist and hand. They also provide feeling in the arm.
It starts with the five “roots” at the neck. The second level is called “trunks,” which continue toward the shoulder then divide into the third layer of two nerves called the anterior division and the posterior division. The nerves in the fourth layer are called “cords,” and the final layer is comprised of the “branches” that feed the shoulder and arm. See the image below for details.