Muscles Innervated by the Ulnar Nerve
The ulnar nerve innervates all interosseous muscles (both dorsal and palmar), the medial two lumbricals, the hypothenar muscles (abductor digiti minimi, flexor digiti minimi brevis, and opponens digiti minimi), the adductor pollicis, and the deep head of the flexor pollicis brevis in the hand, plus the flexor carpi ulnaris and the medial half of the flexor digitorum profundus in the forearm.
Forearm Muscles
The ulnar nerve innervates two muscles in the forearm:
- Flexor carpi ulnaris (FCU): This is consistently the first muscle to receive innervation from the ulnar nerve, with branches arising proximally in the forearm 1.
- Flexor digitorum profundus (FDP) - medial half: The ulnar nerve supplies the ulnar portion of the FDP, which controls flexion of the distal interphalangeal joints of the ring and small fingers 1, 2.
The muscular branches to these forearm muscles typically arise within the proximal third of the forearm, with the FCU receiving one to two branches (occasionally three or four), and the FDP usually receiving a single branch (occasionally two) 1.
Hand Muscles - Intrinsic Musculature
Interosseous Muscles
- All dorsal interossei (4 muscles): These muscles abduct the fingers away from the midline of the hand 3.
- All palmar interossei (3-4 muscles): These muscles adduct the fingers toward the midline 3.
The interosseous muscles serve as the cornerstone of hand function, providing the foundation for intrinsic and extrinsic hand movements, and contributing approximately 50% of grip strength despite their small individual size 3, 2.
Lumbricals
- Medial two lumbricals (3rd and 4th): These lumbricals to the ring and small fingers are innervated by the ulnar nerve, while the lateral two lumbricals (to the index and middle fingers) are innervated by the median nerve 2.
Hypothenar Muscles
All three hypothenar muscles receive ulnar nerve innervation 4:
- Abductor digiti minimi
- Flexor digiti minimi brevis
- Opponens digiti minimi: This muscle typically receives a separate nerve branch, while the abductor and flexor digiti minimi are often fused and their supplying nerves may form an intrahypothenar loop 4.
Thenar Muscles (Partial)
Within the thenar eminence, the ulnar nerve innervates 4:
- Adductor pollicis: Supplied by terminal branches of the deep branch of the ulnar nerve
- Deep head of the flexor pollicis brevis: Also supplied by the deep branch, though this muscle may have dual innervation from both median and ulnar nerves 4
The superficial head of the flexor pollicis brevis, opponens pollicis, and abductor pollicis brevis are typically innervated by the median nerve 4.
Clinical Significance
Low ulnar nerve palsy (distal lesion) results in sensory loss, digital clawing (particularly of the ring and small fingers), thumb deformity, abduction of the small finger, and asynchronous finger motion, while preserving the ulnar-innervated forearm muscles 2.
High ulnar nerve palsy (proximal lesion) presents with all the above features plus paralysis of the flexor carpi ulnaris and the medial half of the flexor digitorum profundus 2.
Anatomical Variations
Rare anatomical variations exist, including the "all ulnar hand" where all intrinsic hand muscles receive exclusive ulnar innervation (both motor and sensory), though this is extremely rare 5. Additionally, Martin-Gruber anastomosis (median-to-ulnar nerve communication in the forearm) occurs in some individuals and can alter the typical innervation pattern 1.