The radial nerve originates from the posterior cord of the brachial plexus, carrying fibers primarily from the C5 through T1 spinal nerve roots. It provides both motor function (movement control) and sensory function (sensation) to the posterior compartments of the arm, forearm, and hand. The nerve is responsible for straightening the elbow, wrist, and fingers.
Anatomical Pathway and Major Branches
The radial nerve begins in the axilla before traveling down the posterior aspect of the arm. It spirals around the shaft of the humerus bone within the radial groove. This anatomical location makes the nerve susceptible to injury from mid-shaft humerus fractures or prolonged pressure.
Near the elbow joint, the nerve divides into its two major terminal branches. The deep branch is predominantly a motor nerve that continues as the posterior interosseous nerve (PIN). The superficial branch is primarily a sensory nerve, responsible for skin sensation in the hand.
Motor Control in the Upper Arm
In the upper arm, the radial nerve is the sole nerve supply to the muscles that straighten the elbow joint. It provides motor branches to the Triceps Brachii muscle, which is the primary extensor of the forearm.
The Anconeus, a small, triangular muscle near the elbow, also receives its signal from the radial nerve and assists in elbow extension. Before the nerve splits, it innervates two muscles that originate in the arm: the Brachioradialis, which helps to flex the elbow, and the Extensor Carpi Radialis Longus, which is an extensor of the wrist.
Motor Control in the Forearm and Wrist
The deep branch of the radial nerve controls the extensor muscles located in the posterior compartment of the forearm. After piercing the Supinator muscle, the deep branch becomes the posterior interosseous nerve (PIN), which innervates most of the remaining extensors.
Wrist and Forearm Muscles
The nerve controls the wrist extensors, which pull the hand back and stabilize the wrist. The deep branch also innervates the Supinator muscle, which rotates the forearm to turn the palm up. The muscles controlled by the deep branch include:
- Extensor Carpi Radialis Brevis
- Extensor Carpi Ulnaris
- Supinator
Finger and Thumb Extensors
The majority of the muscles that extend the digits are under the control of the posterior interosseous nerve. This includes the muscles that extend the four fingers and the specialized muscles that control the thumb:
- Extensor Digitorum (extends the four fingers)
- Extensor Digiti Minimi (extends the little finger)
- Extensor Indicis (independently extends the index finger)
- Abductor Pollicis Longus (moves the thumb away from the palm)
- Extensor Pollicis Longus (straightens the thumb)
- Extensor Pollicis Brevis (straightens the thumb)
Sensory Distribution and Clinical Relevance
The superficial radial nerve is purely sensory, providing feeling to a specific area of the skin on the back of the hand. This includes the skin over the thumb, the index and middle fingers, and the radial half of the ring finger, stopping short of the actual fingertips.
Damage to the radial nerve, often due to compression against the humerus—a condition sometimes called “Saturday night palsy”—results in a characteristic clinical finding. Because the motor fibers controlling the extensors are affected, the person loses the ability to actively straighten the wrist and fingers. This functional deficit is known as “wrist drop,” where the hand hangs limply and the fingers remain flexed due to the unopposed action of the forearm flexor muscles.