Palmar interossei muscles

The palmar Interossei muscles are a group of three / four smaller, unipennate intrinsic muscles of the hand. They are located within the metacarpal spaces on the palmar side, lying between the metacarpal bones. These muscles are distinct from the larger, bipennate dorsal interossei.

Each palmar interosseus muscle typically arises from the entire length of the metacarpal bone of one finger. It then inserts onto the side of the base of the first (proximal) phalanx and the dorsal extensor expansion of that same finger.

There are four palmar interossei muscles, though their presence and specific actions vary slightly:

Pollical palmar interosseus muscle

This palmar interosseous muscle is found in the first metacarpal space (i.e., between the thumb and index finger). It is often rudimentary or considered part of either the adductor pollicis or the flexor pollicis brevis. When present, it originates from the medial (ulnar) side of the palmar surface of the first metacarpal bone (thumb) and inserts into both the base of the proximal phalanx of the thumb and its extensor hood. Its action is to adduct the thumb towards the index finger.

First palmar interosseus muscle

This muscle is located in the second metacarpal space. It arises from the ulnar side of the second metacarpal bone. Its tendon inserts into the ulnar side of the base of the proximal phalanx of the index finger and its extensor hood, enabling adduction of the index finger towards the middle finger.

Second and third palmar interossei

The second and third palmar interosseus muscles are located in the third and fourth metacarpal spaces. They arise from the radial sides of the fourth and fifth metacarpal bones, and insert into the same sides (i.e., radial sides) of the base of proximal phalanges of the ring and little fingers, adducting these fingers towards the middle finger.

In essence, the palmar interossei muscles are responsible for adducting the fingers (moving them towards the middle finger). In addition to the pollical palmar interosseus muscle, the thumb has its dedicated adductor muscle called the adductor pollicis. The palmar interossei also assist in flexion at the metacarpophalangeal joints and extension at the interphalangeal joints via their insertion into the extensor expansion.