The radial nerve is a major pathway of communication, carrying both movement instructions and sensory information between the brain and the upper limb. It is one of the five primary nerves that branch off the brachial plexus, a complex network of nerves originating in the neck. Understanding its precise route is key, as its path dictates which muscles and skin regions it influences in the arm, wrist, and hand.
Origin and Initial Path in the Upper Arm
The radial nerve begins as a continuation of the posterior cord of the brachial plexus, situated deep within the armpit, or axilla. From this starting point, it travels downward behind the axillary artery and enters the posterior compartment of the arm beneath the teres major muscle. It is the largest nerve originating from the brachial plexus.
Its most recognizable feature in the upper arm is its oblique course as it winds around the back of the humerus, the long bone of the upper arm. This winding path takes it through a shallow depression known as the spiral groove, also called the radial groove. In this location, the nerve lies in direct contact with the bone, alongside the deep brachial artery, meaning mid-shaft fractures of the humerus can easily compromise the nerve’s function.
Near the middle of the arm, the nerve pierces the lateral intermuscular septum to shift from the posterior compartment to the anterior compartment. It descends between the brachialis and the brachioradialis muscles as it heads toward the elbow joint.
Navigating the Elbow Joint and Branching
The radial nerve passes anteriorly to the lateral epicondyle of the humerus, the bony prominence on the outer side of the elbow. It enters the cubital fossa, the triangular area at the front of the elbow. Near the radiohumeral joint line, the radial nerve divides into its two major terminal branches.
This division creates the Deep Branch, which is primarily motor, and the Superficial Branch, which is almost entirely sensory. The Deep Branch immediately takes a sharp turn, traveling around the neck of the radius bone. The Deep Branch is sometimes referred to as the Posterior Interosseous Nerve. The Superficial Branch continues its course down the forearm.
Distribution in the Forearm and Hand
The Superficial Branch descends through the forearm, running deep to the brachioradialis muscle. For a portion of its journey, it travels close to the radial artery in the middle third of the forearm. About seven centimeters above the wrist, this branch emerges from beneath the brachioradialis tendon and crosses the tendons of the anatomical snuffbox. It then divides into digital cutaneous nerves that supply sensation to the skin of the hand.
The Deep Branch, focused on movement, travels through a slit in the supinator muscle. After winding around the radius bone to enter the posterior compartment of the forearm, it is referred to as the Posterior Interosseous Nerve (PIN). The path through the supinator at the fibrous arch known as the Arcade of Frohse is a common site for compression. The PIN continues downward between the deep and superficial layers of the extensor muscles, providing motor signals for extending the wrist and fingers.
Key Motor and Sensory Responsibilities
The radial nerve controls the muscles responsible for the extension of the elbow, wrist, and fingers. In the upper arm, it innervates the three heads of the powerful triceps brachii muscle, enabling the straightening of the elbow. It also supplies muscles like the brachioradialis and extensor carpi radialis longus, which assist in elbow flexion and wrist extension.
Its primary motor function powers the twelve muscles in the posterior compartment of the forearm, which are responsible for extending the wrist, thumb, and fingers. A loss of radial nerve function often results in the inability to lift the wrist and fingers, a condition sometimes called “wrist drop.”
The sensory component provides feeling to the skin on the back of the arm and forearm. Specifically, it provides sensation to the back of the hand, covering the thumb, index finger, middle finger, and the radial side of the ring finger.