M01.03.023 Hand

The muscles acting on the hand are divided into extrinsic and intrinsic groups:

Extrinsic muscles: Located in the forearm, they control crude hand movements and forceful grip.
Intrinsic muscles: Located within the hand, responsible for fine motor functions.

This guide focuses on the intrinsic muscles of the hand, detailing their attachments, actions, and innervation.

1. Thenar Muscles

The thenar muscles are responsible for the fine movements of the thumb and form the thenar eminence at the base of the thumb.

Muscle	Attachments	Actions	Innervation
Opponens Pollicis	Tubercle of trapezium and flexor retinaculum → Lateral margin of 1st metacarpal	Opposition of thumb (medial rotation)	Median nerve (recurrent branch)
Abductor Pollicis Brevis	Tubercles of scaphoid and trapezium, flexor retinaculum → Lateral side of proximal phalanx of thumb	Abduction of thumb	Median nerve (recurrent branch)
Flexor Pollicis Brevis	Tubercle of trapezium, flexor retinaculum → Base of proximal phalanx of thumb	Flexes MCP joint of thumb	Median nerve (superficial part), Ulnar nerve (deep part)

The hypothenar muscles control movements of the little finger and form the hypothenar eminence at the base of the little finger.

Muscle	Attachments	Actions	Innervation
Opponens Digiti Minimi	Hook of hamate, flexor retinaculum → Medial margin of 5th metacarpal	Opposes the little finger	Ulnar nerve
Abductor Digiti Minimi	Pisiform, tendon of flexor carpi ulnaris → Base of proximal phalanx of little finger	Abduction of little finger	Ulnar nerve
Flexor Digiti Minimi Brevis	Hook of hamate, flexor retinaculum → Base of proximal phalanx of little finger	Flexes MCP joint of little finger	Ulnar nerve

The lumbricals are unique muscles linking extensor and flexor tendons, essential for finger movement.

Muscle Group	Attachments	Actions	Innervation
Lateral Lumbricals (2)	Tendons of flexor digitorum profundus → Extensor hood (index and middle fingers)	Flex MCP joint, extend IP joints	Median nerve
Medial Lumbricals (2)	Tendons of flexor digitorum profundus → Extensor hood (ring and little fingers)	Flex MCP joint, extend IP joints	Ulnar nerve

These muscles are involved in fine finger movements like abduction and adduction.

Muscle Group	Attachments	Actions	Innervation
Dorsal Interossei (4)	Adjacent sides of metacarpals → Extensor hood and proximal phalanges	Abduction of digits, assist lumbricals	Ulnar nerve
Palmar Interossei (3)	Palmar surfaces of metacarpals → Extensor hood and proximal phalanges	Adduction of digits, assist lumbricals	Ulnar nerve

Muscle	Attachments	Actions	Innervation
Palmaris Brevis	Palmar aponeurosis, flexor retinaculum → Dermis of skin on the medial margin of the hand	Wrinkles skin of hypothenar eminence, aids grip	Ulnar nerve
Adductor Pollicis	Capitate, metacarpals II and III → Base of proximal phalanx of thumb	Adduction of thumb	Ulnar nerve

Thenar vs. Hypothenar: Thenar muscles act on the thumb; hypothenar muscles act on the little finger.
Nerve Supply:
- Thenar muscles: Median nerve (except the deep part of flexor pollicis brevis).
- Hypothenar muscles: Ulnar nerve.
- Lumbricals: Median (lateral two) and Ulnar (medial two).
- Interossei: Ulnar nerve.
Clinical Relevance:
- Ulnar nerve injury: Impairs interossei and medial lumbricals, leading to claw hand.
- Median nerve injury: Affects thenar muscles, leading to “ape hand.”