Ulnar nerve

Arm

From the medial cord, the ulnar nerve passes distally through the axilla, medial to the axillary artery. It descends on the medial aspect of the arm, medial to the brachial artery and the biceps brachii muscle. In the mid-portion of the arm, the nerve pierces the medial intermuscular septum to enter the posterior compartment. Here, the nerve runs anterior to the medial head of triceps brachii muscle and in 70-80% of people, this nerve passes under the arcade of Struthers. This is a thin, aponeurotic band, that extends from the medial head of triceps to the medial intermuscular septum.

The ulnar nerve then passes between the medial epicondyle and olecranon in the groove for ulnar nerve to enter the anterior compartment of the forearm. Posterior to the medial epicondyle, the ulnar nerve is subcutaneous and easily palpable. It is commonly referred to as the "funny bone" in this region. The ulna nerve typically has no branches in the arm. 

Forearm

From the arm, the ulnar nerve runs through the cubital tunnel, superficial to the posterior and oblique bands of the ulnar collateral ligament. This cubital tunnel is bordered by the medial epicondyle, the olecranon and a fibrous band (arcuate ligament) which joins the two heads of flexor carpi ulnaris muscle. Here, the nerve gives off articular branches to the elbow joint. The ulnar nerve continues into the anterior compartment of the forearm by passing between the humeral and ulnar heads of the of flexor carpi ulnaris. 

The ulnar nerve descends on the medial aspect of the forearm, over the flexor digitorum profundus muscle and deep to the flexor carpi ulnaris muscle. Close to the wrist, the nerve emerges lateral to flexor carpi ulnaris with the accompanying ulnar artery, which lies lateral to it. Both structures run superficial to the flexor retinaculum (transverse carpal ligament) to enter the hand just lateral to the pisiform bone and are only covered by fascia and skin.

In the forearm, the ulnar nerve gives rise to two muscular branches: one to the flexor carpi ulnaris and the second to the ulnar (medial) part of the flexor digitorum profundus. The lateral half of the flexor digitorum profundus muscle and the remaining muscles of the anterior compartment of the forearm are supplied by the median nerve. Additionally, the ulna nerve gives rise to two cutaneous branches in the forearm: palmar cutaneous nerve and dorsal cutaneous nerve. These nerves pass into the hand to provide sensory innervation.

Test your knowledge about the arteries, veins and nerves of the elbow and forearm with the following quiz.

Hand

At the wrist, the ulnar nerve and artery enter the hand by coursing through the Guyon’s canal (ulnar canal), a groove between the pisiform and the hook of the hamate, bridged by the palmar carpal ligament. 

Here, the nerve divides into superficial and deep branches to provide both sensory and motor innervation to the hand.

Articular branches

As the ulnar nerve passes between the medial epicondyle and olecranon and through the cubital tunnel, it gives rise to articular branches that provide innervation to the elbow joint.

Muscular branches

Two muscular branches emerge from the ulnar nerve in the proximal forearm. These supply motor innervation to the flexor carpi ulnaris and the medial half of the flexor digitorum profundus.

Palmar cutaneous branch

The palmar cutaneous branch arises from the ulnar nerve around the mid-portion of the forearm. This nerve runs along the ulnar artery to the distal forearm, where it perforates the deep fascia. It provides sensory innervation to the skin at base of the medial palm, which overlies the medial carpal bones.

Dorsal cutaneous branch

The dorsal cutaneous branch of the ulnar nerve emerges in the distal forearm, proximal to the wrist. This nerve courses posteriorly, deep to the flexor carpi ulnaris and perforates the deep fascia to enter the medial aspect of the dorsum of hand. Here, it divides into two or three dorsal digital nerves to supply sensation to the skin on the medial side of the dorsum of hand as well as the proximal regions of the medial 1½ digits (little finger and half of the ring finger).

Superficial branch of ulnar nerve

The superficial branch of the ulnar nerve supplies sensory innervation to the anterior aspect of the ulnar 1½ digits (little finger and half of the ring finger) and medial palmar skin. Additionally, it provides motor innervation to the palmaris brevis muscle in the hypothenar region of the hand. It arises from the ulnar nerve just distal to the pisiform bone and often divides into two palmar digital nerves.

Deep branch of ulnar nerve

The deep branch of the ulnar nerve emerges from the same region of the nerve as the superficial branch. It courses between the muscles of the hypothenar region into the deep aspect of the plam, alongside the deep palmar branch of the ulnar artery. This nerve provides motor innervation to the vast majority of hand muscles. These include: the hypothenar muscles (abductor, flexor, and opponens digiti minimi muscles), all the interossei (3 palmar and 4 dorsal), the medial two lumbricals, the deep head of flexor pollicis brevis and adductor pollicis.

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To summarize, the ulnar nerve supplies motor innervation to the flexor carpi ulnaris, the medial half of the flexor digitorum profundus and all the intrinsic hand muscles with the exception of the LOAF muscles (lateral two lumbricals, opponens pollicis, abductor pollicis brevis and flexor pollicis brevis). 

The ulnar nerve provides sensory supply to the following areas of the hand:

• The skin of the palmar and dorsal aspects of the medial 1½ digits and adjacent palm.
• The skin on the medial side of the dorsum of hand.

Learn more about the ulnar nerve and other major nerves in the upper limb with our video tutorials and quizzes: