The ulnar nerve is formed by fibres from the medial cord. It leaves the axilla to lie on the anterior surface of the medial intermuscular septum until about half way down the arm where it pierces the septum to run between the medial epicondyle and the olecranon. It enters the forearm between the two heads of flexor carpi ulnaris. In the uper forearm the nerve lies between the flexor carpi ulnaris muscles and the flexor digitorum profundus muscles, supplying half of the latter muscle. About halfway down the forearm the ulnar nerve comes to lie close to the ulnar artery, lying on its medial side. Above the wrist the nerve gives off a dorsal branch which passes backwards to supply the dorsal skin of the medial one and one-half digits, and a small palmar cutaneous branch which runs over the flexor retinaculum to supply the medial palm. The nerve passes into the hand over the flexor retinaculum together with the artery. In the hand it divides into superficial and deep branches. The superficial branch supplies palmaris brevis and the skin over the palmar surface of the medial one and one-half digits. The deep branch pierces between abductor digiti minimi and flexor digiti minimi to reach the deep palm where it supplies the interossei, adductor pollicis, the medial lumbricals and opponens digiti minimi.
Injury to the nerve at or above the elbow results in paralysis of the medial half of the flexor digitorum profundus with the loss of flexion of the distal phalanges of the medial two digits. Flexion of the wrist joint will produce abduction due to the paralysis of the flexor carpi ulnaris. The hypothenar eminence muscles will be paralysed and the eminence may be wasted. Since the interossei are paralysed the patient will not be able to hold a sheet of paper between the fingers - loss of abduction and adduction. Adduction of the thumb is lost due to paralysis of the adductor pollicis muscle. The patient gets around this loss by strongly contracting the flexor pollicis longus to bring the terminal phalanx of the thumb against the index finger. The fourth and fifth MCP joints are hyperextended due to the loss of the lumbricals and interossei , while the interphalangeal joints of the same digits are flexed. The picture is that of a 'claw hand'. The sensory loss is to the palm and both palmar and dorsal aspects of the medial one and one-half digits. Injury to the nerve at the wirst spares the flexor carpi ulnaris and the flexor digitorum profundus so that wrist flexion is normal and the fourth and fifth interphalangeal nerves are even more flexed into a claw hand.
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