The posterior interosseous nerve of the forearm is a continuation of the deep radial nerve as it passes through the cubital fossa. It descends on the interosseous membrane in the posterior compartment of the forearm where it provides motor innervation of the extensor muscles of the hand and wrist. It is mainly made up from fibres originating from C7-C8 (these nerve roots are responsible for wrist and finger extension).
Because of its location (close to the membrane), it can be easily compressed and is usually the first to be affected in a compartment syndrome, but more commonly occurs as an eponymous syndrome secondary to inflammation, trauma, vascular problems etc etc. It ONLY causes motor problems- a wrist/finger drop (failure of extension).