Ulnar Nerve in Forearm

Introduction

Introduction to the Ulnar Nerve in the Forearm

The ulnar nerve is the principal motor supply to the intrinsic muscles of the hand and provides motor innervation to two muscles in the forearm: flexor carpi ulnaris and the medial half of flexor digitorum profundus. It also conveys sensory fibres from the medial one and a half digits and the corresponding part of the palm and dorsum of the hand. Its vulnerable course behind the medial epicondyle and through the cubital tunnel makes it susceptible to compression neuropathy. In the distal forearm, it runs in close relation to the ulnar artery through Guyon’s canal, where it is at risk in penetrating wrist injuries.

This module covers the origin, course, relations, branches, anatomical variations, and clinical syndromes associated with the ulnar nerve in the forearm.

Study

1. Origin and Entry into the Forearm

The ulnar nerve arises from the medial cord of the brachial plexus, receiving fibres from the C8 and T1 roots, with a variable contribution from C7. It descends in the arm medial to the brachial artery, then pierces the medial intermuscular septum to enter the posterior compartment. It passes posterior to the medial epicondyle of the humerus in the cubital tunnel (a fibro‑osseous canal formed by the medial epicondyle and the cubital tunnel retinaculum or Osborne’s ligament).

The nerve enters the forearm by passing between the humeral and ulnar heads of the flexor carpi ulnaris (FCU), which it immediately supplies. This intramuscular course provides a potential site of compression.

2. Course and Relations in the Forearm

After emerging from FCU, the ulnar nerve descends on the anterior surface of the flexor digitorum profundus (FDP), deep to the flexor carpi ulnaris. It remains on the FDP throughout the forearm, covered by the FCU muscle belly.

In the proximal two‑thirds, the ulnar nerve is separated from the ulnar artery, which lies lateral to it. In the distal third, the artery joins the nerve and lies lateral to it. At the wrist, both structures pass into the palm through Guyon’s canal (ulnar tunnel), bounded by the pisiform medially, the hook of hamate laterally, and the palmar carpal ligament and palmaris brevis superficially. In the canal, the nerve lies medial to the artery.

3. Branches in the Forearm

The ulnar nerve gives off the following branches in the forearm:

Muscular branches: To flexor carpi ulnaris (given off as the nerve enters the muscle) and to the medial half of flexor digitorum profundus (the tendons to the ring and little fingers). These branches arise in the proximal forearm.

Palmar cutaneous branch: Arises in the mid‑forearm, descends superficial to the ulnar artery and flexor retinaculum, supplying the skin over the hypothenar eminence. It does not pass through Guyon’s canal.

Dorsal cutaneous branch: Arises 5–10 cm proximal to the wrist, winds around the ulnar border of the forearm deep to the FCU tendon, and supplies the skin over the dorsum of the medial one and a half digits (excluding the nail beds, which are supplied by the median nerve). This is an important sensory branch for distinguishing ulnar nerve lesions at the wrist (where it is spared) from those at the elbow.

Articular branches: Small filaments given to the elbow joint and the wrist joint.

Terminal branches: In Guyon’s canal, the ulnar nerve divides into a superficial branch (sensory, supplying the palmar aspect of the little and ulnar half of the ring finger, and motor to palmaris brevis) and a deep branch (motor, supplying the hypothenar muscles, all interossei, the third and fourth lumbricals, adductor pollicis, and the deep head of flexor pollicis brevis).

4. Anatomical Variations

Martin–Gruber anastomosis: In about 15–20% of individuals, motor fibres from the median nerve or anterior interosseous nerve cross to the ulnar nerve in the forearm. This can lead to intrinsic hand muscles being innervated by the median nerve, affecting the presentation of nerve injuries.

Cubital tunnel variants: The roof of the cubital tunnel may be thickened or absent; an accessory muscle, the anconeus epitrochlearis, may compress the nerve. Subluxation of the nerve over the medial epicondyle can also occur.

High division: The ulnar nerve may divide early in the forearm, with separate superficial and deep branches.

5. Clinical Considerations

Cubital tunnel syndrome: The most common ulnar nerve entrapment, occurring at the elbow. Causes intermittent paraesthesia in the ulnar digits, weakness of grip, and later wasting of the interossei and hypothenar muscles. Provocative tests include Tinel’s sign at the cubital tunnel and the elbow flexion compression test.

Ulnar nerve laceration at the wrist: A deep cut in the proximal palm or wrist may sever the nerve in Guyon’s canal. This spares the dorsal cutaneous branch (already given off) and the palmar cutaneous branch, so sensation over the dorsum of the hand and hypothenar eminence is preserved. Motor loss includes all intrinsic hand muscles supplied by the ulnar nerve, leading to a ‘claw hand’ deformity (more prominent in the ring and little fingers due to loss of the ulnar‑innervated lumbricals).

Claw hand (main en griffe): Characterised by hyperextension at the metacarpophalangeal joints and flexion at the interphalangeal joints of the ring and little fingers, due to unopposed action of the long extensors and flexors in the absence of intrinsic muscle action.

Froment’s sign: When asked to pinch a piece of paper between thumb and index finger, the patient flexes the interphalangeal joint of the thumb (using flexor pollicis longus, innervated by median nerve) because the adductor pollicis (ulnar nerve) is paralysed. This is a classic test for ulnar nerve motor dysfunction.

Wartenberg’s sign: Inability to adduct the extended little finger due to paralysis of the palmar interosseous muscle, so the little finger remains abducted.

Summary

Summary of the Ulnar Nerve in the Forearm

The ulnar nerve (C8, T1) provides motor supply to the flexor carpi ulnaris and the medial half of flexor digitorum profundus in the forearm, and sensory innervation to the palmar and dorsal aspects of the medial one and a half digits. Its vulnerable course around the medial epicondyle and between the heads of FCU makes it susceptible to compression at the elbow (cubital tunnel syndrome). In the distal forearm, it accompanies the ulnar artery into Guyon’s canal, where lesions spare dorsal sensation but cause profound intrinsic hand motor loss. Understanding its branching pattern is essential for precise localisation of nerve injuries.