Ulnar nerve

Nervus ulnaris

Also known as: funny bone nerve, nerve of fine finger movement

A branch of the medial cord (C8–T1, often with C7). It passes behind the medial epicondyle (the "funny bone") and supplies flexor carpi ulnaris, the medial half of FDP and most of the intrinsic hand muscles. Its lesions produce a claw hand and loss of fine finger movement.

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Overview

RootsC8–T1 (± C7)

Palpable atBehind medial epicondyle

Wrist passageGuyon's canal (superficial to flexor retinaculum)

Hand muscles suppliedAll intrinsics except LOAF

The ulnar nerve descends medial to the brachial artery and gives no branches in the arm. At mid-arm it pierces the medial intermuscular septum and passes into the posterior compartment. It lies superficially in the groove behind the medial epicondyle, where it can be rolled under the finger, then passes beneath the cubital tunnel retinaculum (Osborne ligament). It enters the forearm between the two heads of flexor carpi ulnaris and descends on FDP under FCU, with the ulnar artery on its lateral side in the distal two-thirds.

About 5 cm above the wrist it gives the dorsal cutaneous branch, which supplies the dorsal medial hand and 1½ digits, and a palmar cutaneous branch. It then passes superficial to the flexor retinaculum, lateral to the pisiform, through Guyon's canal (the ulnar canal), and divides there:
• the superficial branch supplies palmaris brevis and the skin of the medial 1½ digits;
• the deep branch supplies the hypothenar muscles, all the interossei, lumbricals 3 and 4, adductor pollicis and the deep head of flexor pollicis brevis.

As the nerve of the small muscles responsible for fine finger movement, the ulnar nerve is functionally the "musician's nerve".

Location: Medial arm → behind the medial epicondyle → anteromedial forearm under FCU → Guyon's canal → medial hand.

Anatomy

Innervation: FCU, the medial half of FDP, and all the intrinsic hand muscles except the LOAF muscles (median). Skin of the medial 1½ digits and the medial palm and dorsum.
Nerve roots: C8, T1 (often C7) — medial cord.
Course: Medial cord → medial to the axillary and brachial arteries → pierces the medial intermuscular septum → posterior to the medial epicondyle (cubital tunnel) → between the heads of FCU → on FDP under FCU, medial to the ulnar artery → superficial to the flexor retinaculum, lateral to the pisiform (Guyon's canal) → superficial and deep branches.
Branches: - Forearm: flexor carpi ulnaris and the medial half of FDP (ring and little fingers).
• Cutaneous: the palmar cutaneous branch and the dorsal cutaneous branch, which arises proximal to the wrist and is spared in Guyon canal lesions.
• Superficial branch: palmaris brevis and the palmar skin of the medial 1½ digits.
• Deep branch: hypothenar muscles (abductor, flexor and opponens digiti minimi), palmar and dorsal interossei, lumbricals 3–4, adductor pollicis and the deep head of FPB.

Function

Ulnar deviation and flexion of the wrist (FCU)
Flexion of the DIP joints of the ring and little fingers (FDP)
Abduction and adduction of the fingers (interossei); MCP flexion with IP extension (lumbricals, interossei)
Thumb adduction (adductor pollicis), essential for key and power pinch
Sensation of the medial 1½ digits

Clinical significance

Claw hand and the ulnar paradox

Loss of the medial lumbricals and the interossei produces a claw of the ring and little fingers: hyperextended MCP joints with flexed IP joints. In a distal lesion at the wrist (Guyon's canal, lacerations, "cyclist's palsy"), the FDP to these fingers is intact and actively flexes the DIP joints, so the claw is more marked. In a proximal lesion at the elbow, FDP is also paralysed and the claw is milder. This is the ulnar paradox.

Cubital tunnel syndrome and elbow injury

The second most common entrapment neuropathy. It is caused by prolonged elbow flexion, leaning on the elbow, osteoarthritis, or fracture of the medial epicondyle (a late "tardy" ulnar palsy after childhood supracondylar or lateral condyle fractures). Signs:
• paraesthesia of the medial 1½ fingers, including the dorsum, unlike a Guyon canal lesion;
• wasting of the first dorsal interosseous and guttering between the metacarpals;
• weak finger abduction (DAB) and adduction (PAD);
• Froment sign: when gripping paper between the thumb and index finger, the patient flexes the thumb IP joint with FPL (median) to compensate for a weak adductor pollicis;
• Wartenberg sign: persistent abduction of the little finger.

Mnemonics

Interossei actions

"PAD and DAB": Palmar interossei ADduct, Dorsal interossei ABduct. Both are supplied by the ulnar nerve.

Compiled from standard anatomy and physiology references — see the Nervous System sources. For education only; not medical advice.