The ulnar nerve (C, 8; T1) is the main continuation of the medial cord of the brachial plexus (see Plate 3-13).
COURSE IN ARM
Initially, the ulnar nerve lies between the axillary artery and vein; as it enters the arm, it runs on the medial side of the brachial artery. At about the middle of the arm, it pierces the medial intermuscular septum and descends anterior to the medial head of the triceps brachii muscle, alongside the superior ulnar collateral artery. In the lower third of the arm, it inclines posteriorly to reach the interval between the medial humeral epicondyle and the olecranon. As the nerve enters the forearm, it lies in the groove behind the medial epicondyle, between the humeral and ulnar heads of the flexor carpi ulnaris muscle. Above the elbow, the ulnar nerve supplies no constant branches.
COURSE IN FOREARM AND HAND
The ulnar nerve runs downward on the medial side of the forearm, lying first on the ulnar collateral ligament of the elbow joint and then on the flexor digitorum profundus muscle, deep to the flexor carpi ulnaris muscle. At the elbow, the ulnar nerve and artery are separated by a considerable gap, but they are closely apposed in the lower two thirds of the forearm, with the artery on the lateral side. At the flexor carpi ulnaris tendon, the nerve and artery emerge from under its lateral edge and are covered only by skin and fascia. They reach the hand by crossing the anterior surface of the flexor retinaculum lateral to the pisiform, and the nerve splits, under cover of the palmaris brevis muscle, into its superficial and deep terminal branches.
In the forearm and hand, the ulnar nerve gives off articular, muscular, palmar, dorsal, superficial and deep terminal, and vascular branches.
Fine articular branches for the elbow joint arise from the main nerve as it runs posterior to the medial epicondyle; before splitting into its terminal branches, it supplies filaments to the wrist joint.
In the upper forearm, branches are given off to the flexor carpi ulnaris and the medial half of the flexor digitorum profundus muscles. The palmar branch arises 5 to 7 cm above the wrist, descends near the ulnar artery, pierces the deep fascia, and supplies the skin over the hypothenar eminence; it communicates with the medial antebrachial cutaneous nerve and the palmar branch of the median nerve. The dorsal ulnar branch arises 5 to 10 cm above the wrist, passes posteriorly and deep to the flexor carpi ulnaris tendon, pierces the deep fascia, and continues along the dorsomedial side of the wrist. Here, it divides into branches for the areas of skin on the medial side of the back of the hand and fingers. There are usually two or three dorsal digital nerves, one supplying the medial side of the little finger, the second splitting into proper dorsal digital nerves to supply adjacent sides of the little and ring fingers, and the third (when present) supplying contiguous sides of the ring and middle fingers.
The superficial terminal branch supplies the palmaris brevis muscle, innervates the skin on the medial side of the palm, and gives off two palmar digital nerves. The first is the proper palmar digital nerve for the medial side of the little finger; the second, the common palmar digital nerve, communicates with the adjoining common palmar digital branch of the median nerve before dividing into the two proper palmar digital nerves for the adjacent sides of the little and ring fingers. Rarely, the ulnar nerve supplies two and one-half rather than one and one-half digits, and the areas supplied by the median and radial nerves are reciprocally reduced.
The deep terminal branch runs between and supplies the abductor and flexor muscles of the little finger, perforates and supplies the opponens digiti minimi, and then accompanies the deep palmar arterial arch behind the flexor digitorum tendons. In the palm, it gives muscular branches to the third and fourth lumbrical and the interosseous muscles and ends by supplying the adductor pollicis muscle and, sometimes, the deep head of the flexor pollicis brevis muscle.
Variations in the nerve supplies of the palmar muscles are as common as the variations in the cutaneous distribution; they are due to the variety of interconnections between the ulnar and median nerves.