The musculocutaneous nerve arises from the lateral cord of the brachial plexus, opposite the lower border of the Pectoralis minor, its fibers being derived from the fifth, sixth, and seventh cervical nerves.
It penetrates the Coracobrachialis muscle and passes obliquely between the Biceps brachii and the Brachialis, to the lateral side of the arm; a little above the elbow it pierces the deep fascia lateral to the tendon of the Biceps brachii and is continued into the forearm as the lateral antebrachial cutaneous nerve.
In its course through the arm it innervates the Coracobrachialis, Biceps brachii, and the greater part of the Brachialis.
- The branch to the Coracobrachialis is given off from the nerve close to its origin, and in some instances as a separate filament from the lateral cord of the plexus; it is derived from the seventh, cervical nerve.
- The branches to the Biceps brachii and Brachialis are given off after the musculocutaneous has pierced the Coracobrachialis; that supplying the Brachialis gives a filament to the elbow-joint.
- The nerve also sends a small branch to the bone, which enters the nutrient foramen with the accompanying artery.
The musculocutaneous nerve presents frequent irregularities.
It may adhere for some distance to the median and then pass outward, beneath the Biceps brachii, instead of through the Coracobrachialis.
Some of the fibers of the median may run for some distance in the musculocutaneous and then leave it to join their proper trunk; less frequently the reverse is the case, and the median sends a branch to join the musculocutaneous.
The nerve may pass under the Coracobrachialis or through the Biceps brachii.
Occasionally it gives a filament to the Pronator teres, and it supplies the dorsal surface of the thumb when the superficial branch of the radial nerve is absent.
Although rare, the musculocutaneous n. can be affected through compression due to hypertrophy or entrapment between the biceps aponeurosis & brachialis fascia or it may be injured through stretch as occurs in dislocations & sometimes in surgery.
Isolated injury, causes weakness of elbow flexion & supination of the forearm.
A discrete sensory disturbance is present on the radial side of the forearm.
The nerve is usually involved in an upper brachial plexus palsy
Injury can occur before entering the coracobrachialis due to dislocation or apparently due to stretch due to throwing injury
Heavy backpacks can cause damage to the upper trunk of the brachial plexus – dysfunction can be severe & prolonged with similar injury as occurs with Erb's palsy from breech deliveries. Early detection is important – the combination of time, avoidance of wearing a backpack, and strengthening of the shoulder muscles will probably be effective.
Distal to the coracobrachialis, the MC cause appears to be weight lifting – either through compression due to hypertrophy or entrapment between the biceps & brachialis, the nerve may lead to a painless loss of muscle strength in flexion & supination of the forearm. Initial treatment should include avoidance of biceps curls or other biceps exercises.