The posterior interosseous nerve is a branch of the radial nerve in the forearm. It originates from the C7 and C8 nerve roots of the brachial plexus and innervates the muscles that extend the wrist and fingers, as well as some of the muscles that supinate the forearm. It runs between the two bones of the forearm, the radius and the ulna, and can be susceptible to compression or injury at various points along its course. Compression or damage to the posterior interosseous nerve can result in posterior interosseus nerve syndrome, which can cause weakness or paralysis of the muscles it innervates, as well as sensory changes in the affected area.