Muscles of the hand

Muscles of Hand
The intrinsic muscles of the hand are located in five compartment : Fig. 6.75A
• Thenar muscles in the thenar compartment: abductor pollicis brevis, flexor pollicis brevis, and opponens pollicis.
• Adductor pollicis in the adductor compartment.
• Hypothenar muscles in the hypothenar compartment: abductor digiti minimi, flexor digiti minimi brevis, and opponens digiti minimi.
• Short muscles of the hand, the lumbricals, in the central compartment with the long flexor tendons.
• The interossei in separate interosseous compartments between the metacarpals.
THENAR MUSCLES
The thenar muscles form the thenar eminence on the lateral surface of the palm and are chiefly responsible for opposition of the thumb. Normal movement of the thumb is important for the precise activities of the hand. The high degree of freedom of movements of the thumb results from the 1st metacarpal being independent, with mobile joints at both ends.
• Extension: extensor pollicis longus, extensor pollicis brevis, and abductor pollicis longus.
• Flexion: flexor pollicis longus and flexor pollicis brevis.
• Abduction: abductor pollicis longus and abductor pollicis brevis.
• Adduction: adductor pollicis and 1st dorsal interosseous.
• Opposition: opponens pollicis. This movement occurs at the carpometacarpal joint and results in a “cupping” of the palm. Bringing the tip of the thumb into contact with the 5th finger or any of the other fingers involves considerably more movement than can be produced by the opponens pollicis alone.
Abductor Pollicis Brevis.
The abductor pollicis brevis (APB), the short abductor of the thumb, forms the anterolateral part of the thenar eminence. In addition to abducting the thumb, the APB assists the opponens pollicis during the early stages of opposition by rotating its proximal phalanx slightly medially.


Flexor Pollicis Brevis.
The flexor pollicis brevis (FPB), the short flexor of the thumb, is located medial to the APB. Its two bellies, located on opposite sides of the tendon of the FPL, share (with each other and often with the APB) a common, sesamoid-containing tendon at their distal attachment. The bellies usually differ in their innervation: The larger superficial head of the FPB is innervated by the recurrent branch of the median nerve, whereas the smaller deep head is usually innervated by the deep palmar branch of the ulnar nerve. The FPB flexes the thumb at the carpometacarpal and metacarpophalangeal joints and aids in opposition of the thumb.

Opponens Pollicis.
The opponens pollicis is a quadrangular muscle that lies deep to the APB and lateral to the FPB. The opponens pollicis opposes the thumb, the most important thumb movement. It flexes and rotates the 1st metacarpal medially at the carpometacarpal joint during opposition; this movement occurs when picking up an object. During opposition, the tip of the thumb is brought into contact with the pad of the little finger.

ADDUCTOR POLLICIS
The adductor pollicis is located in the adductor compartment of the hand. The fan-shaped muscle has two heads of origin, which are separated by the radial artery as it enters the palm to form the deep palmar arch. Its tendon usually contains a sesamoid bone. The adductor pollicis adducts the thumb, moving the thumb to the palm of the hand, thereby giving power to the grip.

HYPOTHENAR MUSCLES
The hypothenar muscles (abductor digiti minimi, flexor digiti minimi brevis, and opponens digiti minimi) produce the hypothenar eminence on the medial side of the palm and move the little finger .These muscles are in the hypothenar compartment with the 5th metacarpal .
Abductor Digiti Minimi.
The abductor digiti minimi is the most superficial of the three muscles forming the hypothenar eminence. The abductor digiti minimi abducts the 5th finger and helps flex its proximal phalanx.
Flexor Digiti Minimi Brevis.
The flexor digiti minimi brevis is variable in size; it lies lateral to the abductor digiti minimi. The flexor digiti minimi brevis flexes the proximal phalanx of the 5th finger at the metacarpophalangeal joint.
Opponens Digiti Minimi.
The opponens digiti minimi is a quadrangular muscle that lies deep to the abductor and flexor muscles of the 5th finger. The opponens digiti minimi draws the 5th metacarpal anteriorly and rotates it laterally, thereby deepening the hollow of the palm and bringing the 5th finger into opposition with the thumb. Like the opponens pollicis, the opponens digiti minimi acts exclusively at the carpometacarpal joint.
Palmaris Brevis.
The palmaris brevis is a small, thin muscle in the subcutaneous tissue of the hypothenar eminence ; it is not in the hypothenar compartment. The palmaris brevis wrinkles the skin of the hypothenar eminence and deepens the hollow of the palm, thereby aiding the palmar grip. The palmaris brevis covers and protects the ulnar nerve and artery. It is attached proximally to the medial border of the palmar aponeurosis and to the skin on the medial border of the hand.

SHORT MUSCLES OF HAND
The short muscles of the hand are the lumbricals and interossei.

Lumbricals.
The four slender lumbrical muscles were named because of their worm-like form (L. lumbricus, earthworm. The lumbricals flex the fingers at the metacarpophalangeal joints and extend the interphalangeal joints.

Interossei.
The four dorsal interosseous muscles (dorsal interossei) are located between the metacarpals; the three palmar interosseous muscles (palmar interossei) are on the palmar surfaces of the metacarpals in the interosseous compartment of the hand .The 1st dorsal interosseous muscle is easy to palpate; oppose the thumb firmly against the index finger and it can be easily felt. Some authors describe four palmar interossei; in so doing, they are including the deep head of the FPB because of its similar innervation and placement on the thumb. The four dorsal interossei abduct the fingers, and the three palmar interossei adduct them .
A mnemonic device is to make acronyms of dorsal abduct (DAB) and palmar adduct (PAD). Acting together, the dorsal and palmar interossei and the lumbricals produce flexion at the metacarpophalangeal joints and extension of the interphalangeal joints (the so-called Z-movement). This occurs because of their attachment to the lateral bands of the extensor expansions .
Understanding the Z-movement is useful because it is the opposite of claw hand, which occurs in ulnar paralysis when the interossei and the 3rd and 4th lumbricals are incapable of acting together to produce the Z-movement .