The arm's nervous system innervates — or supplies organs or muscles with nerves — portions of the upper arm, forearm, and hand. There are four main nerves of the arm and they wrap around the bones. The ulnar and median nerves originate in the brachial plexus, which is a bundle of nerve fibers extending from the spinal cord to the arm and ending in the hand. Radial nerves span the brachial plexus and forearm. Musculocutaneous nerves stem from the brachial plexus, through the bicep, and into the forearm.
Named after the ulna bone, the ulnar nerve reaches as far as the little finger. Despite passing through the upper arm, the ulnar nerve does not provide nerves to that area. The forearm and hand, however, both have ulnar nerves. Additionally, the ulnar nerve provides the sense of touch to the ring and little finger in what is known as cutaneous innervation. Injury to the ulnar nerves of the arm brings about a loss of grip and can also inhibit someone’s ability to fully extend their fingers. The result is an ailment known as clawing where someone’s hand or fingers appear curved.
Median nerves are also nerves of the arm that traverse down the arm and into the hand. Just like the ulnar nerves, median nerves do not innervate the upper arm but do supply the muscles in the forearm and hand with nerves. The median nerves terminate in the thumb, index, and middle fingers and provide feeling to those fingers as well. A limited range of motion in the thumb or impaired mobility in the index and middle fingers are the most likely consequences of injuring this nerve.
The radial nerves are those that endow the upper arm with nerves. Like the aforementioned nerves, radial nerves start in the brachial plexus and continue to their positions in the arm — in this case the triceps muscle. Radial nerves also innervate the muscles in the forearm. Although these nerves conclude in the forearm, radial nerves also provide cutaneous innervation to the back of the hand. Damage to the radial nerves can result in loss of muscle in the triceps and cause loss of grip.
Musculocutaneous nerves are disbursed primarily in the upper arm. These nerves of the arm are connected to the biceps muscle and provide feeling to that area. Of the four primary arm nerves, musculocutaneous nerves are the most susceptible to injury due to their attachment to the frequently utilized biceps muscles.