Leaving the axilla, the nerve accompanies the profunda bracchii vessels and spirals obliquely downwards behind the humerus from medial to lateral, lying for the most part behind the uppermost fibers of the medial head of triceps which separates it from the bone. Only at the lateral edge of the humerus is the nerve in contact with the periosteum of the lower end of the radial groove (often called the spiral groove), so that contrary to popular belief it does not occupy the whole length of the groove. The uppermost fibers of origin of brachialis cover the lowest part of the groove, and in this region the nerve pierces the lateral intermuscular septum to enter the anterior compartment and so reach the cubital fossa where it lies under cover of brachioradialis. Before reaching the humerus the nerve has already given off two of its four branches to triceps (to the long and medial heads) and the posterior cutaneous nerve of arm. Behind the humerus it gives off the branch to the lateral head and another to the medial head which continues down through triceps to supply anconeus. The main nerve also gives origin to the lower lateral cutaneous nerve of the arm and the posterior cutaneous nerve of the forearm. It is characteristic of the radial nerve to give off its branches at a level considerably proximal to the part to be innervated.
The nerve is easily palpated (feel your own!). It can be rolled on the humerus beneath the finger tip one- third of the way down from the deltoid tuberosity to the lateral epicondyle.
The surface marking of the nerve is from the point where the posterior wall of the axilla and arm meet to a point two-thirds of the way along a line from the acromion to the lateral epicondyle, and thence to the front of the epicondyle.
The superficial branch of the radial nerve, the cutaneous continuation of the main nerve, runs from the cubital fossa on the surface of supinator, pronator teres tendon and flexor digitorum superficialis on the lateral side of the forearm under cover of the brachioradialis muscle. While under cover of the latter muscle it lies on the radial side of the radial artery. It leaves the flexor compartment of the forearm by passing backwards under the tendon of brachioradialis a few centimetres above the radial styloid, and breaks up into two or three branches which can be rolled on the surface of the tautened tendon of extensor pollicis longus. They are distributed to the radial two-thirds of the dorsum of the hand and the proximal parts of the dorsal surfaces of thumb, index, middle and half of the ring fingers.