The three plantar interosseous muscles are unipennate, as opposed to the bipennate structure of dorsal interosseous muscles, and originate on a single metatarsal bone.

The three muscles originate on the medial aspect of metatarsals III-V.

[1] The muscles then continue distally along the foot and insert in the proximal phalanges III-V.

[1] Since the intersseous muscles cross on the metatarsophalangeal joint, then they act on that specific joint and cause adduction of toes III, IV, and V.[1] Adduction itself is not of extreme importance to the toes, but these muscles work together with the dorsal interosseous muscles in flexion of the foot.

[2] This article incorporates text in the public domain from page 495 of the 20th edition of Gray's Anatomy (1918)