The Median Nerve and the Anterior Interosseous Nerve
The median nerve, which derives its name from its course along the midline of the forearm, travels from the elbow along this course to the center of the wrist and then into the hand.  In the hand it divides into branches that travel up both sides of the thumb, the index and long fingers, and the radial, or thumb side, of the ring finger.
One of these branches, the anterior interrosseous nerve, takes off at the elbow level.  This nerve runs deeply in the forearm between the two bones of the forearm and supplies the two muscles that flex (bend) the end joint of the thumb and the end joint of the index finger.

Anterior Interosseous Syndrome
Anterior interrosseous syndrome involves pain in the forearm, combined with weakness of these two (finger and thumb) joints, and indicates compression of this nerve at the elbow level.
Such compression can result from an inflammatory process that will resolve spontaneously, but if the problem persists, surgery to release the nerve in the proximal forearm may be necessary.