The triceps brachii is a substantial, three-headed muscle located on the back of the upper arm, occupying the entire posterior compartment. It is composed of a long head, a lateral head, and a medial head. Like all skeletal muscles, the triceps requires innervation—a direct connection from the nervous system—to contract and perform its primary function of moving the forearm.
Identifying the Triceps Innervation
The nerve responsible for stimulating the triceps brachii muscle is the Radial Nerve. This nerve is one of the five major nerves that branch off the brachial plexus, a network originating from the C5 through T1 spinal nerve roots. The radial nerve is a continuation of the posterior cord and acts as the exclusive motor supply for all three heads of the triceps.
The Radial Nerve’s Anatomical Pathway
The radial nerve begins high in the arm, originating from the posterior cord of the brachial plexus within the axilla. As it descends, it wraps around the back of the humerus, traveling through a shallow, oblique depression known as the radial groove or spiral groove. This passage is a landmark because the nerve lies in direct contact with the bone for a significant length of its course.
Traveling alongside the deep brachial artery, the radial nerve winds laterally around the mid-shaft of the humerus, passing between the medial and lateral heads of the triceps muscle. This intimate relationship makes the nerve vulnerable to injury, particularly from humeral shaft fractures that can compress or sever it.
The radial nerve is also susceptible to external compression, which can lead to temporary paralysis. The branches supplying the long and lateral heads often arise proximal to the spiral groove, offering them some protection. However, the branches to the medial head typically arise while the nerve is still within the groove, meaning this head is more consistently affected by mid-shaft trauma.
Function of the Triceps Muscle
The triceps brachii muscle is the principal extensor of the forearm at the elbow joint, meaning its primary job is to straighten the arm. When the radial nerve stimulates the muscle, the triceps contracts. The three heads—long, lateral, and medial—work together, but they each have distinct roles and origins.
The long head originates from the scapula, crossing both the shoulder and elbow joints. Due to this dual attachment, the long head contributes to both elbow extension and the adduction and extension of the arm at the shoulder. The lateral head originates superior to the radial groove and is typically the strongest head, recruited for movements that require high-intensity force.
The medial head originates inferior to the radial groove and provides the force for forearm extension at various speeds. All three heads converge into a single tendon that inserts onto the olecranon process of the ulna, allowing for pushing movements and stabilizing the elbow.
Clinical Impact of Nerve Damage
Damage to the radial nerve significantly affects the muscle groups it controls, including the triceps. Common injury mechanisms include prolonged compression, leading to transient radial nerve palsy, or fractures of the humerus. Fractures in the middle third, where the nerve is bound to the spiral groove, are a common cause of direct trauma.
If the radial nerve is damaged high up in the arm, such as in the axilla or spiral groove, the resulting loss of triceps function leads to an inability to actively straighten the elbow. An injury at this level may also cause the loss of the triceps reflex. If the injury occurs lower down, the triceps branches may be spared, allowing elbow extension, but the patient will likely experience “wrist drop.”
Treatment for radial nerve damage varies based on the severity of the injury. Mild compression injuries often resolve spontaneously with conservative management, including rest, anti-inflammatory medications, and a splint or brace. More severe injuries may require surgical intervention, such as nerve repair or transfer, to restore function. Physical therapy is an important part of the recovery process, helping to maintain range of motion and muscle strength while awaiting nerve regeneration.