The brachial plexus is a group of spinal nerves that run from the lower neck through the upper shoulder to the arm. Injury occurs when these nerves are stretched or torn during a traumatic injury. Sports, falls, and car accidents are the most common causes in adults. Infants can experience damage during childbirth. Injury causes pain, numbness, and weakness in the arm and shoulder. Many patients with this disorder will heal without specific treatment.