Ulnar nerve

Updated on Jan 10, 2024

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Latin nervus ulnaris TA A14.2.03.040		Dorlands /Elsevier n_05/12566994

From C8, T1 (branch from Medial cord) Innervates flexor carpi ulnaris flexor digitorum profundus lumbrical muscles opponens digiti minimi flexor digiti minimi abductor digiti minimi interossei adductor pollicis MeSH A08.800.800.720.050.850

In human anatomy, the ulnar nerve is a nerve that runs near the ulna bone. The ulnar collateral ligament of elbow joint is in relation with the ulnar nerve. The nerve is the largest unprotected nerve in the human body (meaning unprotected by muscle or bone), so injury is common. This nerve is directly connected to the little finger, and the adjacent half of the ring finger, innervating the palmar side of these fingers, including both front and back of the tips, perhaps as far back as the fingernail beds.

This nerve can cause an electric shock-like sensation by striking the medial epicondyle of the humerus from posteriorly, or inferiorly with the elbow flexed. The ulnar nerve is trapped between the bone and the overlying skin at this point. This is commonly referred to as bumping one's "funny bone". This name is thought to be a pun, based on the sound resemblance between the name of the bone of the upper arm, the "humerus" and the word "humorous". Alternatively, according to the Oxford English Dictionary it may refer to "the peculiar sensation experienced when it is struck".

Structure

The ulnar nerve originates from the C8-T1 nerve roots (and occasionally carries C7 fibres) which form part of the medial cord of the brachial plexus, and descends on the posteromedial aspect of the humerus. It runs inferior to the posteromedial aspects of the humerus, passing behind the medial epicondyle (in the cubital tunnel) at the elbow where it is exposed for several centimetres.

Forearm

It enters the anterior (flexor) compartment of the forearm between the humeral and ulnar heads of flexor carpi ulnaris, lying under the aponeurosis of flexor carpi ulnaris alongside the ulna. There it supplies one and a half muscles (flexor carpi ulnaris and the medial half of flexor digitorum profundus) and courses with the ulnar artery, travelling inferiorly with it deep to the flexor carpi ulnaris muscle.

In the forearm it gives off the following branches:

Muscular branches of ulnar nerve

Palmar branch of ulnar nerve

Dorsal branch of ulnar nerve

Hand

After it travels down the ulna, the ulnar nerve enters the palm of the hand by the Guyon's canal. The ulnar nerve and artery pass superficial to the flexor retinaculum, via the ulnar canal.

Here it gives off the following branches:

Superficial branch of ulnar nerve

Deep branch of ulnar nerve

The course of the ulnar nerve through the wrist contrasts with that of the median nerve, which travels deep to the flexor retinaculum of the hand.

Sensory

The ulnar nerve also provides sensory innervation to the fifth digit and the medial half of the fourth digit, and the corresponding part of the palm:

Palmar branch of ulnar nerve - supplies cutaneous innervation to the anterior skin and nails

Dorsal cutaneous branch of ulnar nerve - supplies cutaneous innervation to the dorsal medial hand and the dorsum of the medial 1.5 fingers

Motor

The ulnar nerve and its branches innervate the following muscles in the forearm and hand:

An Articular branch that passes to the elbow joint while the ulnar nerve is passing between the olecranon and medial epicondyle of the humerus.

In the forearm, via the muscular branches of ulnar nerve:

Flexor carpi ulnaris

Flexor digitorum profundus (medial half)

In the hand, via the deep branch of ulnar nerve:

hypothenar muscles

Opponens digiti minimi

Abductor digiti minimi

Flexor digiti minimi brevis

The third and fourth lumbrical muscles

Dorsal interossei

Palmar interossei

Adductor Pollicis

Flexor pollicis brevis (deep head)

In the hand, via the superficial branch of ulnar nerve:

Palmaris brevis

Clinical significance

The ulnar nerve can suffer injury anywhere between its proximal origin of the brachial plexus all the way to its distal branches in the hand. It is the most commonly injured nerve around the elbow. Although it can be damaged under various circumstances, it is commonly injured by local trauma or physical impingement ("pinched nerve"). Injury of the ulnar nerve at different levels causes specific motor and sensory deficits:

At the elbow

Common mechanisms of injury: Cubital tunnel syndrome, fracture of the medial epicondyle (causing cubitus valgus with tardy ulnar nerve palsy)

Motor deficit:

Weakness in flexion of the hand at the wrist, loss of flexion of ulnar half of digits, or the 4th and 5th digits, loss of ability to cross the digits of the hand. (Note: Motor deficit is absent or very minor in cubital tunnel syndrome as the ulnar nerve is compressed in the cubital tunnel, rather than transected.)

Presence of a claw hand deformity when the hand is at rest, due to hyperextension of the 4th and 5th digits at the metacarpophalangeal joints, and flexion at the interphalangeal joints.

Weakness of adduction of the thumb, which may be assessed by the presence of Froment's sign.

Sensory deficit: Loss of sensation or paresthesiae in ulnar half of the palm and dorsum of hand, and the medial 1½ digits on both palmar and dorsal aspects of the hand

At the wrist

Common mechanism: penetrating wounds, Guyon canal cyst

Motor deficit:

Loss of flexion of ulnar half of digits, or the 4th and 5th digits, loss of ability to cross the digits of the hand.

Presence of a claw hand deformity when the hand is at rest, due to hyperextension of the 4th and 5th digits at the metacarpophalangeal joints, and flexion at the interphalangeal joints.

The claw hand deformity is more prominent with injury at the wrist as opposed to a lesion higher up in the arm, for instance, at the elbow, as the ulnar half of the flexor digitorum profundus is not affected. This pulls the distal interphalangeal joints of the 4th and 5th digit into a more flexed position, producing a more deformed 'claw'. This is known as the ulnar paradox.

Weakness of adduction of the thumb, which may be assessed by the presence of Froment's sign.

Sensory deficit: Loss of sensation or paresthesiae in ulnar half of the palm, and the medial 1½ digits on the palmar aspect of the hand, with dorsal sparing. The dorsal aspect of the hand is unaffected as the posterior cutaneous branch of the ulnar nerve is given off higher up in the forearm and does not reach the wrist.

In severe cases, surgery may be performed to relocate or "release" the nerve to prevent further injury.

References

Ulnar nerve Wikipedia

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