Learning Objective
Understand the anatomy, function, and innervation of the intrinsic muscles of the hand, including thenar, hypothenar, lumbrical, interossei, and other palmar muscles, and their role in fine motor control.
Muscles of the Hand
The muscles controlling hand movements are divided into two groups:
- Extrinsic muscles: Located in the forearm (anterior and posterior compartments). They produce a forceful grip and gross movements of the hand and fingers.
- Intrinsic muscles: Located within the hand itself. They control fine motor movements and the precise manipulation of objects.
This section focuses on the intrinsic muscles of the hand – their attachments, actions, innervation, and clinical significance.
Thenar Muscles
The thenar muscles form the thenar eminence at the base of the thumb and enable fine thumb movements. All are primarily innervated by the median nerve.
1. Opponens Pollicis
- Attachments: Trapezium tubercle + flexor retinaculum → lateral margin of 1st metacarpal
- Actions: Opposes the thumb by medially rotating and flexing the metacarpal
- Innervation: Recurrent branch of the median nerve
2. Abductor Pollicis Brevis
- Attachments: Tubercle of scaphoid & trapezium + flexor retinaculum → lateral side of proximal phalanx of thumb
- Actions: Abducts the thumb
- Innervation: Recurrent branch of the median nerve
3. Flexor Pollicis Brevis
- Attachments: Trapezium tubercle + flexor retinaculum → base of proximal phalanx of thumb
- Actions: Flexes the metacarpophalangeal joint of the thumb
- Innervation: Superficial head: median nerve; Deep head: deep branch of ulnar nerve
Hypothenar Muscles
The hypothenar muscles form the hypothenar eminence at the base of the little finger and are innervated by the ulnar nerve.
1. Opponens Digiti Minimi
- Attachments: Hook of hamate + flexor retinaculum → medial margin of 5th metacarpal
- Actions: Rotates metacarpal V toward the palm, producing opposition
- Innervation: Ulnar nerve
2. Abductor Digiti Minimi
- Attachments: Pisiform + tendon of flexor carpi ulnaris → base of proximal phalanx of little finger
- Actions: Abducts the little finger
- Innervation: Ulnar nerve
3. Flexor Digiti Minimi Brevis
- Attachments: Hook of hamate + flexor retinaculum → base of proximal phalanx of little finger
- Actions: Flexes the metacarpophalangeal joint of the little finger
- Innervation: Ulnar nerve
Lumbricals
Four lumbricals link flexor digitorum profundus tendons to extensor hoods, allowing coordinated finger movements.
- Actions: Flexion at metacarpophalangeal (MCP) joints and extension at interphalangeal (IP) joints.
- Innervation: Lateral two (index & middle): median nerve; Medial two (ring & little): ulnar nerve
- Clinical Relevance: Denervation causes ulnar claw (medial lumbricals) or hand of benediction (median nerve).
Interossei
Located between metacarpals, the interossei assist the lumbricals in fine finger movements.
Dorsal Interossei
- Four muscles, abduct fingers (“DAB”)
- Attachments: Sides of metacarpals → extensor hood & proximal phalanx
- Actions: Abduction of fingers; flex MCP and extend IP joints
- Innervation: Ulnar nerve
Palmar Interossei
- Three muscles, adduct fingers (“PAD”)
- Attachments: Sides of metacarpals → extensor hood & proximal phalanx
- Actions: Adduction of fingers; flex MCP and extend IP joints
- Innervation: Ulnar nerve
Other Muscles in the Palm
1. Palmaris Brevis
- Attachments: Palmar aponeurosis + flexor retinaculum → dermis of medial hand
- Actions: Wrinkles the skin of the hypothenar eminence, deepens the palm curvature, and improves grip
- Innervation: Ulnar nerve
2. Adductor Pollicis
- Attachments: Transverse head: metacarpal III; Oblique head: capitate & bases of metacarpals II–III → base of proximal phalanx of thumb
- Actions: Adducts the thumb
- Innervation: Ulnar nerve
