3.6 : Hand Intrinsic Muscles (Thenar, Hypothenar, Lumbricals & Interossei)

Hand Thenar Group - Four muscles

Abductor pollicis brevis, Flexor pollicis brevis, Opponens pollicis, and Adductor pollicis

Hand Hypothenar Group - Four muscles

Abductor digiti minimi, Flexor digiti minimi brevis, Opponens digiti minimi, and Palmaris brevis

Hand Intrinsic Compartment - Three muscle groups

Lumbricals, Dorsal interossei, and Palmar interossei

Abductor pollicis brevis - Origin

Tubercle of the scaphoid and flexor retinaculum

Abductor pollicis brevis - Insertion

Lateral side of the base of the proximal phalanx of the thumb (via lateral sesamoid bone)

Abductor pollicis brevis - Innervation

Median nerve

Abductor pollicis brevis - Action

Abducts the thumb and assists thumb opposition

Opponens pollicis - Origin

Trapezium and flexor retinaculum

Opponens pollicis - Insertion

First metacarpal bone

Opponens pollicis - Innervation

Median nerve

Opponens pollicis - Action

Opposition of the thumb; draws 1st metacarpal anteriorly and medially toward the little finger

Flexor pollicis brevis - Superficial head origin

Flexor retinaculum and tubercle of trapezium

Flexor pollicis brevis - Deep head origin

Trapezoid, capitate, and base of 1st metacarpal

Flexor pollicis brevis - Insertion

Lateral sesamoid bone and base of proximal phalanx of thumb

Flexor pollicis brevis - Dual innervation (exceptional feature)

Superficial head: Median nerve; Deep head: Ulnar nerve

Flexor pollicis brevis - Action

Flexes the thumb at the MCP joint and assists thumb opposition

Adductor pollicis - Oblique head origin

Capitate, bases of 2nd/3rd metacarpals, and carpal ligaments

Adductor pollicis - Transverse head origin

Shaft of 3rd metacarpal

Adductor pollicis - Insertion

Medial sesamoid bone and base of proximal phalanx of thumb

Adductor pollicis - Innervation

Ulnar nerve (C8-T1)

Adductor pollicis - Action

Adducts the thumb toward palm and assists thumb opposition

Abductor digiti minimi - Origin

Pisiform bone and flexor retinaculum

Abductor digiti minimi - Insertion

Base of the proximal phalanx (1st phalanx) of the little finger

Abductor digiti minimi - Innervation

Ulnar nerve

Abductor digiti minimi - Action

Abducts the little finger relative to hand axis; assists flexion of MCP joint of little finger

Flexor digiti minimi brevis - Origin

Hook of the hamate and flexor retinaculum

Flexor digiti minimi brevis - Insertion

Base of the proximal phalanx of the little finger

Flexor digiti minimi brevis - Innervation

Ulnar nerve

Flexor digiti minimi brevis - Action

Flexion of the proximal phalanx (1st phalanx) of the little finger

Opponens digiti minimi - Origin

Hook of the hamate and flexor retinaculum

Opponens digiti minimi - Insertion

Medial border and surface of the 5th metacarpal bone

Opponens digiti minimi - Innervation

Ulnar nerve

Opponens digiti minimi - Action

Opposes the little finger; draws 5th metacarpal anteriorly and laterally, deepening palm hollow

Palmaris brevis - Origin

Flexor retinaculum and hypothenar fascia

Palmaris brevis - Insertion

Skin along the medial border of the hand over hypothenar eminence

Palmaris brevis - Innervation

Ulnar nerve

Palmaris brevis - Action

Tenses the skin of the palm over hypothenar eminence; assists grip

Lumbricals - Origin

Radial side of the tendons of flexor digitorum profundus

Lumbricals - Insertion

Into the dorsal aponeurosis/expansion with tendon of extensor digitorum

Lumbricals - Dual innervation

Fingers 2-3 (1st and 2nd lumbricals): Median nerve; Fingers 4-5 (3rd and 4th lumbricals): Ulnar nerve

Lumbricals - Action

Flexion of MCP joints; extension of proximal and distal IP joints; abduction toward hand axis (especially 2nd and 3rd)

Lumbricals - Pennate morphology (1st-2nd vs 3rd-4th)

1st and 2nd lumbricals are unipennate; 3rd and 4th lumbricals are bipennate

Dorsal interossei - Origin

Interosseous spaces 1 to 4 (adjacent sides of metacarpals 1-5)

Dorsal interossei - Insertion

Fuses with extensor digitorum tendon/dorsal hood and base of proximal phalanx

Dorsal interossei - Innervation

Ulnar nerve

Dorsal interossei - Action

Abduct fingers relative to axis of the hand (3rd digit); flex MCP joints; extend IP joints

Hand Interossei - DAB mnemonic meaning

Dorsal interossei ABduct digits (4 bipennate muscles)

Palmar interossei - Origin

Palmar surfaces of the 2nd, 4th, and 5th metacarpal bones

Palmar interossei - Insertion

Bases of proximal phalanges and extensor expansions of digits 2, 4, and 5

Palmar interossei - Innervation

Ulnar nerve

Palmar interossei - Action

Adduct fingers toward axis of the hand (3rd digit); flex MCP joints; extend IP joints

Hand Interossei - PAD mnemonic meaning

Palmar interossei ADduct digits

Palmar interossei - Anatomical exception (1st palmar interosseous)

1st palmar interosseous is rudimentary (when present, considered part of adductor pollicis or flexor pollicis brevis)