The radial nerve is one of the three major nerves traveling through the human upper limb. It originates from the posterior cord of the brachial plexus, a complex network of nerves branching from the spinal cord near the neck. Its path through the arm and forearm makes it responsible for a wide range of muscle movements and sensations, particularly on the back side of the limb. Understanding its location and function is important, as damage can significantly impair arm and hand use.
Tracing the Radial Nerve’s Anatomical Route
The radial nerve begins high in the armpit, or axilla, as a terminal branch of the posterior cord of the brachial plexus. From this origin, it immediately travels down the arm, entering the posterior compartment of the upper arm deep within the tissue, running alongside the deep brachial artery.
The nerve then wraps around the shaft of the humerus within a shallow channel called the radial groove or spiral groove. This close proximity to the bone makes it vulnerable to injury, particularly in cases of mid-shaft humeral fractures. After spiraling laterally, the nerve pierces the lateral intermuscular septum to move toward the front of the elbow.
Near the elbow joint, the nerve divides into two major terminal branches as it passes anterior to the lateral epicondyle. The deep branch, often called the posterior interosseous nerve (PIN), wraps around the neck of the radius bone through the supinator muscle to enter the posterior forearm. The superficial branch is purely sensory and descends down the forearm, positioned deep to the brachioradialis muscle. This branch continues toward the wrist, crossing the anatomical snuffbox to provide sensation to the back of the hand.
Primary Motor and Sensory Roles
The radial nerve’s primary motor function is controlling the extensor muscles of the upper limb, enabling the ability to straighten the elbow, wrist, and fingers. In the upper arm, it innervates all three heads of the triceps brachii muscle, which extends the forearm at the elbow joint. It also supplies the anconeus muscle, which assists with elbow extension.
In the forearm, the deep branch controls the muscles that extend the wrist and fingers, allowing movements like extending the thumb and opening the hand. These muscles include the extensor carpi radialis longus and brevis, which are strong wrist extensors. The nerve also innervates the supinator muscle, assisting in turning the palm upward.
The sensory component is carried by the superficial branch, which provides sensation to specific areas of the skin. It supplies the skin over the posterior aspect of the arm and forearm. Its terminal distribution is to the dorsal side of the hand, covering the skin over the thumb, the index finger, half of the middle finger, and the web space between them.
Common Signs of Radial Nerve Injury
Damage to the radial nerve causes a characteristic set of symptoms, the most recognizable motor deficit being “wrist drop.” This condition is the inability to actively extend the wrist and fingers due to paralysis of the supplied extensor muscles. A person with wrist drop often finds their hand hanging limply and struggles with grasping objects because they cannot position the wrist properly.
Sensory changes are commonly reported, often manifesting as numbness, tingling, or a burning pain, known as paresthesia. These symptoms occur in the specific areas of the hand and forearm that the nerve supplies, such as the back of the hand near the thumb. The exact symptoms depend on where along the nerve’s path the injury has occurred.
Common causes of radial nerve injury include direct trauma, such as a fracture of the humerus shaft, due to the nerve’s close contact with the bone in the radial groove. Compression injuries are frequent, sometimes known as “Saturday night palsy.” This results from prolonged pressure on the nerve in the axilla or upper arm, often from sleeping in an awkward position. Improper use of crutches or sustained pressure from a tight wristband can also lead to nerve compression.