Harvey's Garden

Hand muscles

3 min read

Innervation Summary

  • Median Nerve: Innervates most of the thenar muscles and the lateral two lumbricals.
    • Mnemonic: “LOAF” or “OAF + Lateral 2 Lumbricals”
      • Lumbricals 1 & 2
      • Opponens pollicis
      • Abductor pollicis brevis
      • Flexor pollicis brevis (superficial head)
  • Ulnar Nerve: Innervates all other intrinsic hand muscles (“everything else”).
    • Hypothenar muscles, all interossei, medial two lumbricals, and adductor pollicis.

Muscle Groups & Functions

Thenar Muscles (Median N.)

  • Forms the thenar eminence (thumb pad). Responsible for thumb opposition.
  • Muscles (OAF):
    • Opponens pollicis: Opposes the thumb by rotating it medially.
    • Abductor pollicis brevis: Abducts the thumb.
    • Flexor pollicis brevis: Flexes the thumb’s MCP joint.

Hypothenar Muscles (Ulnar N.)

  • Forms the hypothenar eminence (pinky side). Controls the 5th digit.
  • Muscles (OAF):
    • Opponens digiti minimi: Opposes the 5th digit.
    • Abductor digiti minimi: Abducts the 5th digit.
    • Flexor digiti minimi: Flexes the 5th digit’s MCP joint.

Lumbricals

  • Action: Flex MCP joints and Extend IP (interphalangeal) joints.
  • Innervation:
    • 1st & 2nd (index, middle fingers): Median N.
    • 3rd & 4th (ring, little fingers): Ulnar N.

Interossei (Ulnar N.)

  • Dorsal Interossei (DAB): ABduct the fingers. There are 4.
  • Palmar Interossei (PAD): ADduct the fingers. There are 3.
  • Both groups assist lumbricals in flexing MCPs and extending IPs.

Other Intrinsic Muscles

  • Adductor Pollicis (Ulnar N.): The most powerful adductor of the thumb.
  • Palmaris Brevis (Ulnar N.): Wrinkles the skin of the hypothenar eminence, aiding grip.

Clinical Correlations & Buzzwords

  • Median Nerve Injury:

    • “Ape Hand” Deformity: Wasting of the thenar eminence, leading to loss of opposition. The thumb falls back in line with the other fingers.
    • “Hand of Benediction” / “Pope’s Blessing”: Seen when a patient with a proximal median nerve lesion tries to make a fist. They are unable to flex the 1st, 2nd, and 3rd digits, while the 4th and 5th digits flex (due to intact ulnar nerve).
    • Anterior Interosseous Nerve (AIN) Syndrome: A pure motor neuropathy; patient cannot make the “OK” sign (loss of flexion at thumb IP and index DIP).

  • Ulnar Nerve Injury:

    • “Ulnar Claw” / “Claw Hand”: Seen at rest in patients with a distal ulnar nerve lesion. Paralysis of the medial two lumbricals causes hyperextension of the 4th and 5th MCPs and flexion of the IPs.
    • “Ulnar Paradox”: A proximal ulnar nerve lesion (e.g., at the elbow) results in a less prominent claw deformity than a distal lesion. This is because the ulnar-innervated part of the flexor digitorum profundus is also paralyzed, reducing flexion of the IP joints.
    • Wasting of interosseous muscles: Leads to visible gutters between extensor tendons on the dorsum of the hand.
    • Froment’s Sign: Patient is asked to hold paper between thumb and index finger. Weakness of the ulnar-innervated adductor pollicis causes the patient to flex their thumb IP joint (using the median-innervated FPL) to compensate.

Graph View