Musculocutaneous nerve

At a glance

English

Musculocutaneous nerve

Latin (TA2)

Nervus musculocutaneus

System

Nerve

Region

Upper limb

Side

Paired, left and right

About the musculocutaneous nerve

The musculocutaneous nerve leaves the lateral cord of the plexus in the axilla, around the lower border of pectoralis minor, carrying fibres from C5, C6 and C7. It runs at first lateral to the axillary artery's third part, and then turns into the arm by passing through coracobrachialis, which it supplies on the way. Piercing coracobrachialis is its most distinctive feature and helps identify it at operation.

Below coracobrachialis the nerve runs downwards and laterally in the plane between biceps brachii in front and brachialis behind, supplying both. A little above the elbow it emerges at the lateral edge of the biceps tendon, perforates the deep fascia and carries on below the elbow as the lateral antebrachial cutaneous nerve. That terminal branch supplies skin along the outer forearm between elbow and wrist. The elbow joint and the humerus also receive small twigs.

Variation is common. The nerve may run beneath coracobrachialis instead of through it, or may stay attached to the median nerve for some distance before separating. Fibres can pass between the two nerves in either direction, and occasionally the musculocutaneous nerve is missing and the median nerve supplies its muscles. Rarely it gives a twig to pronator teres, or takes over the skin on the back of the thumb if the superficial radial nerve is missing.

Because it serves the main flexors of the elbow and biceps, the chief supinator, damage to the nerve weakens elbow flexion and supination, with numbness on the outer forearm and a reduced biceps reflex. Isolated injury is uncommon but can follow forceful abduction and backward movement of the shoulder, heavy repetitive use of the arm flexors, humeral fractures, penetrating wounds or surgery on the front of the shoulder. Brachialis and brachioradialis, the latter supplied by the radial nerve, can still bend the elbow partly. A C6 root problem and biceps tendon rupture are the main conditions to tell apart from it.

Course and supply

Arises from

Lateral cord of the plexus (C5 to C7), in the axilla near the lower border of pectoralis minor.

Ends in

Becomes the lateral antebrachial cutaneous nerve above the elbow, lateral to the biceps tendon.

Supplies

Motor to coracobrachialis, biceps brachii and most of brachialis, for shoulder flexion, elbow flexion and supination; sensory to the lateral forearm and to the elbow joint.