What innervates the thenar muscles?

Innervation of the Thenar Muscles

The thenar muscles are primarily innervated by the median nerve, specifically through its recurrent branch (also called the muscular thenar branch) which supplies the abductor pollicis brevis, opponens pollicis, and the superficial head of the flexor pollicis brevis, while the deep head of the flexor pollicis brevis and the adductor pollicis receive innervation from the deep branch of the ulnar nerve. 1, 2

Detailed Innervation Pattern

Median Nerve Innervation

• Recurrent Branch (Muscular Thenar Branch): This branch typically arises from the median nerve distal to the transverse carpal ligament and follows a characteristic recurrent course 1

  • Supplies:
    • Abductor pollicis brevis
    • Opponens pollicis
    • Superficial head of flexor pollicis brevis

• Direct Branch: In addition to the recurrent branch, a direct branch from the median nerve may also contribute to thenar muscle innervation, particularly to the superficial head of flexor pollicis brevis 2

Ulnar Nerve Innervation

• Deep Terminal Branch: This branch of the ulnar nerve supplies:
  • Deep head of flexor pollicis brevis
  • Adductor pollicis (both transverse and oblique heads)
  • First palmar interosseous muscle (when present) 2

Anatomical Variations

Several important anatomical variations exist in thenar muscle innervation:

1. Double Muscular Thenar Branches: Cases have been documented where two muscular thenar branches originate from the first common palmar digital nerve, with the proximal branch supplying the abductor pollicis brevis and opponens pollicis, while the distal branch innervates the superficial head of flexor pollicis brevis 1

2. Median-Ulnar Nerve Anastomosis: Anastomosis between the median and ulnar nerves occurs in approximately 91% of cases, typically under the tendon of the flexor pollicis longus 2

3. Martin-Gruber Anastomosis: In cases with this forearm anastomosis between median and ulnar nerves, there can be remarkable expansion of the ulnar nerve's innervation territory within the thenar muscles 2

4. Articular Innervation: The thenar branch of the median nerve also contributes to the innervation of the trapeziometacarpal joint, which has implications for denervation procedures for osteoarthritis 3

Clinical Significance

Understanding thenar muscle innervation is crucial for:

1. Carpal Tunnel Syndrome Assessment: Compound muscle action potentials (CMAPs) recorded at the thenar region can help assess the severity of median nerve compression. The median/ulnar CMAP amplitude ratio is particularly useful, with ratios below 0.5 suggesting severe loss of motor units in the thenar muscles 4

2. Surgical Planning: Knowledge of potential variations is essential during surgical procedures like carpal tunnel release to avoid iatrogenic injury to the recurrent branch 1

3. Nerve Grafting Procedures: The thenar branch of the median nerve has been used in nerve grafting procedures to enhance ulnar nerve recovery in cases of ulnar nerve transection, showing promising results in preventing irreversible atrophy of intrinsic hand muscles 5

4. Electrodiagnostic Studies: Proper interpretation of electromyography and nerve conduction studies requires understanding the normal innervation patterns and common variations 4

The boundary between median and ulnar nerve innervation territories typically coincides with the position of the flexor pollicis longus tendon, which serves as an important anatomical landmark 2.

Understanding these innervation patterns is essential for accurate diagnosis and treatment of various hand pathologies, including compression neuropathies, traumatic nerve injuries, and degenerative joint conditions.