1. arises from the medial cord of the brachial plexus conveying fibers mainly from the C8 and T1 nerves; it passes down the arm, behind the medial epicondyle of the humerus, and down the ulnar side of the anterior compartment of the forearm to the hand; it gives off muscular branches in the forearm to the flexor carpi ulnaris muscle and the ulnar portion of flexor digitorum profundus and supplies the hypothenar, interosseous, medial lumbricals, adductor pollicis, and deep head of flexor hallucis brevis, and the intrinsic muscles of the hand and the skin of the small finger and medial side of the ring finger and adjacent portions of the palm of the hand. The ulnar nerve is most vulnerable to injury where it passes subcutaneously behind the medial epicondyle of the humerus. Mild injury here produces the so-called crazy bone sensation. An ulnar nerve lesion here causes loss of flexion of metacarpophalangeal joints and of extension at the interphalangeal joints (“claw hand”).
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