Palmar interossei muscles
The palmar interossei are three muscles located in the metacarpus. They arise from the metacarpal bones of the index, ring and little finger and are innervated by the deep branch of the ulnar nerve. Together, those muscles work together to close your spread fingers.
The origin surfaces of these muscles are facing towards the middle finger:
- Palmar interosseous muscle I: ulnar side of the index finger
- Palmar interosseous muscle II: radial side of the ring finger
- Palmar interosseous muscle III: radial side of the little finger
From there, the muscles insert at the dorsal aponeurosis and the base of the proximal phalanx of their respective finger. The middle finger itself does not have its own palmar interosseous.
The nerve innervation is provided by the deep branch of the ulnar nerve (C8-Th1).
The most important function of the palmar interossei is the closing of spread fingers, which means the movement of the fingers towards the middle finger (adduction in the metacarpophalangeal joints [MCP]).
In detail, the first pulls the index finger medially whereas the second and third pull the ring and little fingers laterally. Functionally, the palmar interossei act antagonistically to the dorsal interossei in the MCP.
(mnemonic: DAB = Dorsals ABduct, PAD = Palmars ADduct).
However both muscle groups perform common functions, as they flex the fingers in the MCP and extend in the proximal and distal interphalangeal joints (PIP and DIP). An analogous muscle group can be found in the foot which fulfills identical movements in the toes (plantar interossei muscles).
An unexplored fourth muscle
Current literature assumes that there are three palmar interossei. Nevertheless, in 1858 the German anatomist Henle has described a fourth palmar interosseous between the thumb and the index finger (palmar interosseous of Henle).
At first, it was assumed that the fibers he found were not an independent muscle, but instead part of either the adductor pollicis or flexor pollicis brevis. However, newest studies have shown that most people actually do have one additional short hand muscle between the thumb and the index finger. At this point, it is not clarified how this unknown muscle should be classified and which function it fulfills.