The ulnar nerve is the nerve that runs down the length of the arm, beginning in the brachial plexus and terminating in the pinky and ring finger. Nerves are cord-like structures in the body that deliver signals between the nervous system and other organs so that the body can sense and react to stimuli, such as pressure or temperature. They are divided into two groups by function: the sensory, or afferent, group and the motor, or efferent, group. The ulnar nerve supplies both sensation and motion.
The ulnar nerve stem from the medial cord of the brachial plexus, a nerve grouping near the shoulder and travels down the inner arm alongside the humerus, the bone of the upper arm. The ulnar nerve then passes through the cubital tunnel in the elbow, the hinge joint between the upper arm and forearm. Here, the nerve earns its claim to fame as the cause of the pain felt when one hits the funny bone. For a few centimeters past the elbow, the ulnar nerve is unprotected by bone or muscle, exposing it to injuries that cause the strange, painful sensation felt when the elbow is hit.
After the elbow, the ulnar nerve passes under the flexor carpi ulnaris, a muscle of the forearm, and runs along the ulna, after which it is named. The ulna is one of the two bones in the forearm, terminating at the wrist on the side of the pinky. The ulnar nerve travels alongside the ulnar artery and innervates the flexor carpi ulnaris and half of the flexor digitorum profundus, another muscle of the forearm. Innervate means to supply nerves, and therefore sensation or motion, to a tissue. In the forearm, the ulnar nerve branches off into the muscular branches, dorsal branch, and the palmar branch in order to innervate different parts of the arm and hand.
The nerve then passes between the pisiform and hamate bones of the wrist and into the palm of the hand through Guyon’s canal. Here, the ulnar nerve separates again into the superficial and deep branches. The branches give sensation and motion to the palm to the pinky side of the palm and to the pinky finger and half of the ring finger.
Ulnar entrapment is a fairly common condition in which the nerve becomes compressed, or pinched. This usually occurs in the elbow, causing cubital tunnel syndrome, but may also arise in the wrist, where it is called Guyon’s canal stenosis. A patient with cubital tunnel syndrome may experience a prolonged sensation of hitting the funny bone, with symptoms of numbness and tingling in the pinky, ring finger, and the back of that half of the hand. The condition usually progresses so that the symptoms last for longer periods of time. At late stages, a patient may experience a curling of the pinky and ring finger called the ulnar claw.
If the compression occurs at the wrist, the ulnar claw may be more severe, but the back of the hand will retain normal sensation. Cubital tunnel syndrome and Guyon’s canal stenosis can often be cured by altering sleeping positions, avoiding holding the arm in one position for long periods of time, and improving posture. If the underlying cause lies in an abnormality or disease in the wrist, elbow, brachial plexus, or spine, the condition may be treated with surgery. Compression of the ulnar nerve should not be confused with carpal tunnel syndrome, which results from compression of the median nerve and causes numbness or tingling in the thumb, index, middle, and half of the ring finger.