Gross Anatomy Block 2 Practice Test

The key idea is how the flexor digitorum profundus is organized and what joints its tendons act on. The profundus has two portions that go to the fingers: a lateral part to digits 2 and 3, and a medial part to digits 4 and 5. The medial portion is supplied by the ulnar nerve and its tendons insert on the distal phalanges of the ring and little fingers. Those tendons cross the distal interphalangeal joints, so when they contract they bend, or flex, the distal phalanges at those joints. That makes the action on digits 4 and 5 at the DIP joints the correct description. In contrast, flexion at the proximal interphalangeal joints is mainly a function of the flexor digitorum superficialis, and the lateral portion of the profundus would be responsible for digits 2 and 3 at the DIP. Extending the digits would be the action of antagonistic muscles, not the profundus.