Ulnar nerve

The ulnar nerve provides motor innervation to most of the intrinsic muscles of the hand. These include the hypothenar muscles (abductor digiti minimi, flexor digiti minimi, opponens digiti minimi), all seven interossei (both dorsal and palmar), the medial two lumbricals (for the ring and little fingers), and the adductor pollicis. The ulnar nerve also supplies the flexor carpi ulnaris and the medial half of flexor digitorum profundus in the forearm.

The ulnar nerve provides sensory innervation to the medial one and a half digits—the little finger (digit 5) and the ulnar half of the ring finger (digit 4)—along with the corresponding area of the palm (hypothenar eminence). On the dorsum of the hand, the dorsal cutaneous branch of the ulnar nerve (which branches above the wrist) supplies the dorsal aspect of the little finger, ring finger, and the ulnar half of the long finger.

Cubital tunnel syndrome is entrapment or compression of the ulnar nerve at the elbow, specifically as it passes through the cubital tunnel—a fibro-osseous channel posterior to the medial epicondyle of the humerus. It is the second most common peripheral nerve entrapment (after carpal tunnel syndrome). Patients experience numbness and tingling in the ring and little fingers, particularly with elbow flexion, and may develop weakness of the intrinsic hand muscles with prolonged compression.

Claw hand deformity (main en griffe) occurs after ulnar nerve injury at or above the wrist. The interossei and medial lumbricals normally flex the metacarpophalangeal (MCP) joints and extend the interphalangeal (IP) joints. With ulnar nerve loss, unopposed long finger flexors cause IP joint flexion while the extensors hyperextend the MCP joints, producing a claw posture of the ring and little fingers. The index and middle fingers are less affected because their lumbricals are innervated by the median nerve.

The 'funny bone' sensation—a tingling, buzzing, or electric shock feeling radiating into the ring and little fingers—is caused by direct compression or impact to the ulnar nerve as it passes posterior to the medial epicondyle of the humerus. At this point, the nerve is superficial and relatively unprotected. Despite the common term, the sensation is caused by nerve compression, not by stimulating the humerus (the 'humorous' bone etymology notwithstanding).