RADIAL NERVE ANATOMY

The radial nerve is the largest branch of the brachial plexus. The radial nerve is derived primarily from the C5, C6, C7, C8 and T1.
The radial nerve enters the arm from axilla along the medial side of the humerus to reach the spiral groove. From the axilla to the spiral groove, the radial nerve gives off motor branches to triceps and anconeus. It also receives the posterior cutaneous nerve of the arm, the posterior cutaneous nerve of the forearm and the lower lateral cutaneous nerve of the arm in this region.
At the spiral groove, the radial nerve is in contact with the humerus as the nerve travels laterally, and then it pierces the lateral intermuscular septum. Here, it is bordered medially by the brachialis (Br) muscle, and laterally (from proximal to distal) by the brachioradialis (BR), the extensor carpi radialis longus (ECRL), and the extensor carpi radialis brevis (ECRB). All these muscles receive motor supply from radial nerve. The radial nerve then crosses the elbow joint anterior to the lateral epicondyle of the humerus. At the elbow the radial nerve divides into a motor nerve, the posterior interosseus nerve; and a sensory nerve, the superficial radial nerve.

Posterior Interosseus Nerve
The posterior interosseus nerve enters and innervates the supinator (Sup) muscle. The nerve then gives motor branches to - extensor digitorum communis (EDC), extensor digiti minimi (EDM), extensor carpi ulnaris (ECU), abductor pollicis longus (APL), extensor pollicis longus (EPL), extensor pollicis brevis (EPB), and extensor indicis proprius (EIP).

Superficial Radial Nerve (SRN)
At the elbow, the superficial radial nerve stays superficial to the supinator and proceeds anterolaterally, deep to the BR muscle. Approximately at the junction of the proximal two thirds and the distal one third of the forearm (approx 8 cm from tip of radial styloid), the SRN becomes superficial and crosses over to the posterior aspect of the distal radial forearm, passing superficial to the tendons of the anatomical snuffbox (APL, EPL, and EPB) and traversing the wrist over the extensor retinaculum. It supplies cutaneous sensation to the dorsal surface of the lateral hand, as shown in the figure.

Reference:

1. Richard S Snell, Clinical Anatomy: Lippincott Williams & Wilkins, 7th edition
2. Preston DC. Distal Median Neuropathies. In: Entrapment and other focal neuropathies; Neurologic Clinics: WB Saunders company, August 1999
3. http://www.wheelessonline.com/ortho/the_superficial_branch_of_the_radial_nerve_an_anatomic_study_with_surgical_implications