The ulnar nerve is a nerve that runs near the ulna, one of the two long bones in the forearm. The ulnar collateral ligament of elbow joint is in relation with the ulnar nerve. The nerve is the largest in the human body unprotected by muscle or bone, so injury is common.[1] This nerve is directly connected to the little finger, and the adjacent half of the ring finger, innervating the palmar aspect 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 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".[2] 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".[3] Alternatively, according to the Oxford English Dictionary, it may refer to "the peculiar sensation experienced when it is struck".[4]
Structure
Arm
The ulnar nerve originates from the C8-T1 nerve roots (and occasionally carries C7 fibers which arise from the lateral cord),[5][6] which then form part of the medial cord of the brachial plexus, and descends medial to the brachial artery, up until the insertion point of coracobrachialis muscle (middle 5 cm over the medial border of the humerus). Then, it pierces the medial intermuscular septum and enters the posterior compartment of the arm, accompanied by superior ulnar collateral vessels. It runs at the posteromedial aspects of the humerus, passing behind the medial epicondyle (in the cubital tunnel) at the elbow, where it can be palpated by hand.[7]
Palmar branch of ulnar nerve - arises from the middle part of the forearm and supplies the skin over the hypothenar eminence.[7]
Dorsal branch of ulnar nerve - arises from 7.5 cm above the wrist, winds backwards to supply the skin of the proximal part of the ulnar one and half fingers and the adjoining area between the fingers.[6][7]
Articular branches are given off to the elbow joint.[7]
Deep branch of ulnar nerve - It accompanies the deep branch of the ulnar artery. It passes backwards between the abductor digiti minimi, flexor digiti minimi, and opponens digiti minimi, supplying all the three muscles, and lying on the hook of hamate bone. It then turns laterally, supplying the 3rd and 4th lumbricals and all the palmar interossei muscles and dorsal interossei of the hand. It terminates by supplying the adductor pollicis.[7]
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.[9][10] 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 of the humerus (causing direct ulnar nerve injury), fracture of the lateral epicondyle of the humerus (causing cubitus valgus with tardy ulnar nerve palsy), Driver's Elbow[11]
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.)
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 (and other lesions)[12]
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.
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.
Additional images
Brachial plexus with courses of spinal nerves shown
Cross-section through the middle of upper arm.
Cross-section through the middle of the forearm.
Transverse section across distal ends of radius and ulna.
Transverse section across the wrist and digits.
Ulnar and radial arteries. Deep view.
The right brachial plexus (infraclavicular portion) in the axillary fossa; viewed from below and in front.
Front of right upper extremity, showing surface markings for bones, arteries, and nerves.
Back of right upper extremity, showing surface markings for bones and nerves.
Ulnar nerve
Brachial plexus with characteristic M, ulnar nerve labeled.
Ulnar nerve
Ulnar nerve
Ulnar nerve
Brachial plexus. Deep dissection. Anterolateral view
^Hendrickson, Robert A. (2004). The Facts on File Encyclopedia of Word and Phrase Origins (Facts on File Writer's Library). New York: Checkmark Books. p. 281. ISBN0-8160-5992-6.
Anatomy figure: 07:04-04 at Human Anatomy Online, SUNY Downstate Medical Center - "Anterior view of the nerves, vessels, and superficial tendons that cross the left wrist."
