The Anatomy of the Musculocutaneous Nerve

The musculocutaneous nerve is a peripheral nerve in your outer arm. It’s the terminal branch of the lateral cord, which is part of the brachial plexus that extends from your neck to your armpit. The musculocutaneous nerve carries fibers for both motor function (movement) and sensory function (feeling).

Anatomy

With the exception of cranial nerves (in the head), all of the nerves in your body emerge from the spinal cord through the spaces between vertebrae. Nerve roots that come from between the fifth, sixth, and seventh cervical vertebrae (C5, C6, and C7) in your neck merge to form the lateral cord.

The lateral cord then splits off to form:

1. Part of the median nerve
2. The musculocutaneous nerve

The musculocutaneous nerve then continues on and becomes the lateral cutaneous nerve of the forearm, which is also called the lateral antebrachial cutaneous nerve.

Structure

In the upper arm, between your shoulder and elbow joints, you have four muscles:

• Biceps brachii
• Brachialis
• Coracobrachialis
• Triceps brachii

The triceps brachii is in the posterior (back) compartment while the rest are in the anterior (front) compartment. The musculocutaneous nerve sends branches to the three muscles of the anterior compartment.

Branches also connect to the humerus bone and the elbow joint.

In the forearm, between your elbow and wrist, the lateral cutaneous nerve sends branches to the skin of the forward portion (thumb side) of the arm.

Location

The musculocutaneous nerve emerges below the pectoralis minor muscle of the upper chest, near your shoulder and armpit. This region of your body is called the axilla.

The nerve leaves the axilla and enters the arm by traveling through the coracobrachialis muscle where it connects to the humerus bone of the upper arm.

The musculocutaneous nerve then runs down the upper arm between the brachialis and biceps brachii muscles. As it nears the elbow, it pierces connective tissue called the deep fascia and emerges near the biceps tendon and brachioradialis muscle.

From there, it runs into the forearm as the lateral cutaneous nerve, sending off branches until it reaches the wrist just above the thumb joint.

Anatomical Variation

The musculocutaneous nerve has several known anatomical variations. In some people, it may:

• Interact with, adhere to, and exchange fibers with the median nerve
• Pass under the coracobrachialis muscle instead of going through it

Function

The portion of the musculocutaneous nerve that runs through the upper arm provides motor function to the muscles it innervates. After it becomes the lateral cutaneous nerve, it plays only a sensory role.

Motor Function

The three muscles innervated by the musculocutaneous nerve all work together to provide motion to your arms.

The biceps brachii, often just called the biceps, is one of the better-known muscles in the body, thanks to its bulging appearance when you flex your upper arm as well as exercises named for causing that bulge, such as biceps curls. This muscle runs from two connecting points at the shoulder down to your elbow.

The musculocutaneous nerve works with the biceps to flex the arm at the elbow and shoulder to create that classic bodybuilder pose, and it also allows you to rotate your arm so your palm faces up.

The coracobrachialis lies inside your upper arm just below the biceps. It also runs from the shoulder to the elbow. It helps flex your arm at the shoulder and plays a minor role in pulling the arm in toward your body.

The brachialis muscle is the primary muscle involved in flexing your elbow. It starts toward the middle of your upper arm, where it lays beneath the biceps brachii, then passes through the inside of the elbow to connect to the ulna just below the elbow joint.

In providing movement to the brachialis muscle, the musculocutaneous nerve is joined by the radial nerve, which is a branch of the posterior cord. The posterior cord shares some nerve roots with the lateral cord.

Sensory Function

As the lateral cutaneous nerve travels down the forearm toward your wrist, it sends out multiple branches to the skin. (Cutaneous means skin.)

The nerve provides feeling to the forward half (thumb side) of your inner arm from the elbow to the base of the thumb and to the forward portion of your outer arm from elbow to wrist.

Associated Conditions

The musculocutaneous nerve isn’t injured often because it’s well protected in the axilla (near the shoulder and armpit) and elsewhere along its course through the arm. Most often, musculocutaneous nerve dysfunction is caused by injury to the brachial plexus, which contains numerous nerves.

Typically, brachial plexus damage is caused by a penetrating injury to the axilla, such as a stabbing or gunshot wound.

The nerve also can be damaged during shoulder surgeries. Shoulder dislocation doesn’t often cause damage to the musculocutaneous nerve, but it does sometimes happen.

Damage to this nerve can result in problems with the tissues in innervates, including:

• Loss of sensation in the skin on the front side of the forearm
• Weakened flexion at the shoulder and elbow
• Weakened rotation of the arm

The arm movements described above, while weakened, remain possible due to other muscles that aid in these movements, including the pectoralis major and the brachioradialis.

The nerve also can become compressed due to repetitive stress injury, usually in young athletes. Backpacking with a heavy pack can cause compression of the brachial plexus and result in a condition called Erb’s palsy.

Rehabilitation

In cases of musculocutaneous nerve damage, some people experience spontaneous recovery, but even when that happens, it generally takes several months.

Many cases can be treated conservatively, such as with rest, ice, anti-inflammatory medications, and physical therapy. However, if that approach isn’t successful, surgical decompression may become necessary.

In some cases, nerve grafting or nerve transfer may be necessary for restoring function.