Palmar interossei muscles

Origin and insertion

Palmar interossei are unipennate muscles of the palmar surface of the hand. A pennate muscle is a muscle that attaches obliquely to its tendon. The uni- prefix means that its muscle fibers converge to attach on one side of the tendon. These muscles consist of four, sometimes three muscles, numbered 1–4 from the lateral to the medial side.

• 1st palmar interosseous: when present, it arises on the ulnar side of the base of the 1st metacarpal bone. It inserts into a sesamoid bone on the ulnar side of the proximal phalanx of the thumb, as well as on its extensor expansion.
• 2nd palmar interosseous: arises from the entire length of the 2nd metacarpal bone and is inserted into the ipsilateral extensor expansion of the index finger. 
• 3rd palmar interosseous: arises from the radial side of the 4th metacarpal bone, and inserts together with the third lumbrical into the radial base of proximal phalanx of ring finger and on its extensor expansion.
• 4th palmar interosseous: arises from the radial side of the 5th metacarpal bone, and inserts with the fourth lumbrical into the base of the proximal phalanx of thelittle finger and on its extensor expansion.

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Relations

Palmar interossei muscles are smaller than the dorsal interossei and are situated on the palmar surface of the hand in between the metacarpal bones.

The 1st palmar interosseous courses between the first dorsal interosseous posteriorly and the oblique head of adductor pollicis anteriorly.

The 2nd palmar interosseous is also overlapped by the oblique head of adductor pollicis, while the 3rd and 4th palmar interossei are overlapped by the tendons of the flexor digitorum profundus intended for the ring and little finger.

Innervation

Both palmar interossei and dorsal interossei are innervated by the deep branch of ulnar nerve, derived from roots C8 and T1.

Blood supply

Palmar interossei muscles receive arterial blood supply from branches of the deep palmar arch, princeps pollicis artery, radialis indicis artery, palmar metacarpal arteries, proximal and distal perforating arteries and common and proper palmar arteries.

Function

The main function of palmar interossei is to adduct the fingers in a longitudinal axis, which means the movement of the fingers towards the middle finger. 

Specifically, the 1st palmar interosseous pulls the index finger medially, whereas the 2nd and 3rd pull the ring and little fingers laterally.

 Functionally, the palmar interossei are antagonists to the dorsal interossei in the metacarpophalangeal joint.

Do you want a method that prevents you from mixing up the functions of the interossei muscles? Then use the following mnemonic!

PAD DAB ('Use your hand to dab with a pad')

• Palmar interossei - ADduct
• Dorsal interossei - ABduct

In addition, palmar interossei greatly contribute to flexion at metacarpophalangeal joint, and extension at the interphalangeal joints of the 2nd, 4th and 5th finger.