The radial nerve is a major peripheral nerve of the upper limb, originating as a terminal continuation of the posterior cord of the brachial plexus (C5 through T1). It serves as the primary conduit for both movement and sensation in the arm, forearm, and hand. Its function is to coordinate the extension of the elbow, wrist, and fingers, while also providing tactile information from the skin.
The Path of the Radial Nerve
The nerve begins its journey high up in the axilla (armpit) immediately after originating from the posterior cord. It descends through the upper arm, passing behind the axillary artery and entering the posterior compartment. Here, it follows a distinctive path, spiraling around the shaft of the humerus in the radial groove.
This intimate relationship with the humerus shaft makes the nerve vulnerable to injury from mid-shaft fractures. As it descends, it is accompanied by the deep brachial artery (profunda brachii artery). The nerve then pierces the lateral intermuscular septum to move from the back of the arm to the front, just above the elbow.
Upon reaching the elbow joint (cubital fossa), the radial nerve divides into its two principal terminal branches: the predominantly motor Deep Branch and the entirely sensory Superficial Branch. The Deep Branch immediately enters the posterior forearm compartment by passing through the supinator muscle, where it is renamed the Posterior Interosseous Nerve (PIN). The Superficial Branch continues down the forearm under the brachioradialis muscle, eventually becoming the sensory nerve of the hand.
Motor Control: Muscles of Extension
The radial nerve is the sole nerve responsible for innervating the extensor muscles of the arm and forearm, controlling all elbow, wrist, and finger extension. Before reaching the forearm, branches supply the three heads of the Triceps Brachii muscle (the primary elbow extensor). It also innervates the Anconeus muscle, which assists the triceps in elbow extension and helps stabilize the joint.
In the proximal forearm, before the nerve divides, it supplies three muscles of the mobile wad: the Brachioradialis (a powerful elbow flexor), the Extensor Carpi Radialis Longus (ECRL) (which extends and abducts the wrist), and the Extensor Carpi Radialis Brevis (ECRB). The ECRB is often innervated by a direct branch from the Deep Branch.
The Deep Branch, or Posterior Interosseous Nerve (PIN), supplies the remaining muscles of the posterior forearm compartment. These muscles are essential for finer movements of the wrist, thumb, and fingers, supporting a strong grip and release. The PIN first innervates the Supinator muscle, which rotates the forearm to turn the palm upward.
Beyond the supinator, the PIN supplies the extensors of the fingers and thumb. These include the Extensor Digitorum (ED), which extends all four fingers, and the Extensor Digiti Minimi (EDM), which extends the little finger. Wrist stabilization is further supported by the Extensor Carpi Ulnaris (ECU), which extends and adducts the wrist.
The PIN also innervates the muscles controlling the thumb and index finger. These include the Abductor Pollicis Longus (APL) (moving the thumb away from the palm), the Extensor Pollicis Brevis (EPB) and Extensor Pollicis Longus (EPL) (straightening the thumb), and the Extensor Indicis (EI) (allowing independent extension of the index finger).
Sensory Coverage: Skin Regions
The radial nerve’s sensory function covers regions of the skin along the back of the arm, forearm, and hand through several cutaneous branches. The Posterior Cutaneous Nerve of the Arm provides sensation to the back of the upper arm. The Posterior Cutaneous Nerve of the Forearm supplies a strip of skin along the entire length of the back of the forearm.
The majority of the radial nerve’s sensory contribution to the hand is provided by the Superficial Radial Nerve, the sensory-only terminal branch. This nerve emerges from beneath the brachioradialis tendon near the wrist and crosses the anatomical snuffbox to reach the back of the hand. It provides sensation to the dorsolateral aspect of the hand, including the back of the thumb and the index finger.
The Superficial Radial Nerve covers the dorsal surface of the lateral three and a half digits (the thumb, index finger, middle finger, and the lateral half of the ring finger). This sensory innervation stops short of the fingertips. The skin over the nail beds and distal phalanges is supplied by branches of the median and ulnar nerves, meaning radial nerve damage typically spares the tips of these fingers.