[TA] originates in cubital fossa (with superficial branch) as termination of (common) radial nerve; pierces supinator, supplying it and other extensors of forearm. Its terminal portion is the posterior interosseous nerve, which runs between the superficial and deep layers of extensor muscles in the distal third of the forearm.posterior interosseous nerve. SYN: ramus profundus nervi radialis [TA].