The musculocutaneous nerve is a major peripheral nerve of the upper limb. It originates from the lateral cord of the brachial plexus (C5, C6, and C7) and provides motor function to the anterior muscles of the arm and sensory function to the lateral forearm. In this article, we shall explore the anatomy of the musculocutaneous nerve including its anatomical course, motor and sensory functions, and clinical correlations.
The musculocutaneous nerve exits from the axilla at the inferior border of the pectoralis minor muscle. It then pierces the coracobrachialis muscle near its point of insertion on the humerus and gives a branch to the muscle. From there, the musculocutaneous nerve travels down the flexor compartment of the upper arm, which is located superficial to the brachialis but deep to the biceps brachii muscle. It innervates both of these muscles and provides branches to the humerus and elbow. The nerve then passes through the deep fascia lateral to the biceps brachii and emerges lateral to the biceps tendon and the brachioradialis. It continues into the forearm as the lateral cutaneous nerve and supplies sensory innervation to the skin on the lateral aspect of the forearm.
The musculocutaneous nerve provides motor function to the anterior muscles of the arm- the biceps brachii, brachialis, and coracobrachialis muscles. These muscles flex the upper arm at the shoulder and elbow. In addition, the biceps brachii also supinates the forearm. Sensory function is provided by the lateral cutaneous nerve of the forearm. The nerve initially enters the deep forearm, but then pierces the deep fascia to become subcutaneous. In this region, it can be found near the cephalic vein and provides innervation to the skin on the anterolateral aspect of the forearm.
Injuries to the musculocutaneous nerve are rare given its well-protected location in the axilla. Common causes of injury include penetrating trauma to the axilla, such as a stab wound, or iatrogenic injury resulting from heavy retraction during the deltopectoral approach to the shoulder.
In the event of an injury, the motor functions of the musculocutaneous nerve will be affected- the coracobrachialis, biceps brachii, and brachialis muscles will experience weakened flexion at the shoulder and elbow. However, the pectoralis major and brachioradialis muscles can still enable flexion of the upper arm and forearm, respectively. Supination of the forearm may be weak, but can still be achieved with the help of the brachioradialis muscle. Additionally, there may be a loss of sensation over the lateral side of the forearm.
The anatomical course of the musculocutaneous nerve is known to vary. It may interact with the median nerve, adhere to the nerve and exchange fibres, or pass under the coracobrachialis instead of through it. It may also travel through the biceps brachii muscle instead of over it.
The musculocutaneous nerve is an important peripheral nerve of the upper limb as it provides both motor and sensory functions. A thorough understanding of its anatomy and functions is essential for clinical professionals to be able to recognise and treat any possible injuries or disorders affecting it.
Injuries to the musculocutaneous nerve are relatively uncommon due to its well-protected placement in the axilla. However, two leading causes of injury are penetrating trauma to the axilla, such as knife or gunshot wounds, and iatrogenic injury resulting from excessive force during the deltopectoral approach to the shoulder.
When the musculocutaneous nerve is damaged, several motor functions are affected. These include the coracobrachialis, biceps brachii, and brachialis muscles, which can weaken the ability to flex at the shoulder and elbow. However, some of these movements can still be accomplished, albeit at reduced strength, by the pectoralis major and brachioradialis, respectively. Damage to the musculocutaneous nerve also weakens the supinator of the forearm, yet the brachioradialis can still often perform this motion.
Sensory functions are also affected by musculocutaneous nerve injury, leading to a loss of sensation along the lateral side of the forearm.
The musculocutaneous nerve is the terminal branch of the lateral cord of the brachial plexus and can be found in the axilla. It runs through the coracobrachialis, biceps brachii, and brachialis muscles; then passes deep to the brachial artery and median nerve. It ultimately reaches the radial side of the forearm and innervates the lateral half of the flexor compartment.
The musculocutaneous nerve is responsible for both motor and sensory functions. Motor functions include the contribution of the coracobrachialis, biceps brachii, and brachialis muscles. The musculocutaneous nerve supplies these muscles with their motor innervation, allowing for shoulder flexion, elbow flexion, and forearm supination. Sensory functions include supplying sensation to the lateral side of the forearm.
Following injury to the musculocutaneous nerve, patients present with weakness of shoulder flexion and elbow flexion. Weakness of forearm supination is also possible. Along with this, loss of sensation over the lateral side of the forearm may also be present.
Diagnostic imaging such as magnetic resonance imaging (MRI) and computed tomography (CT) scan can be performed in order to assess the extent of the injury and rule out any associated soft tissue or bone damage.
Treatment of musculocutaneous nerve injury depends on the location, extent, and cause of the injury. Generally, conservative measures such as rest, ice, compression, and elevation (RICE) can be used to reduce pain and inflammation and promote healing. Physical therapy may also be indicated in order to help restore strength, range of motion, and overall function.
In cases of severe nerve damage, surgical intervention may be necessary. The most common type of surgery for musculocutaneous nerve injury is neurolysis, which involves releasing the damaged nerve from surrounding tissues and scar tissue that may be constricting it. Other surgical options include nerve grafting and nerve transfers.
The prognosis of musculocutaneous nerve injury largely depends on the extent of the damage and the treatment received. Generally, most patients can make a full recovery with proper treatment. However, those with severe nerve damage may not be able to restore full function and may be left with some degree of residual motor and sensory deficits.
In order to prevent musculocutaneous nerve injury, it is important to take precautions when engaging in activities or occupations that may put one at risk. It is also important to be aware of the signs and symptoms of musculocutaneous nerve injury and seek medical attention if any are present.
It is also important for healthcare practitioners to be aware of the musculocutaneous nerve and take all necessary precautions to minimize the risk of iatrogenic injury. This includes avoiding excessive force and retraction during surgery and other medical procedures.
Injury to the musculocutaneous nerve is not a common occurrence, but can have devastating effects if not addressed properly. By understanding the anatomy, clinical presentation, diagnostic evaluation, treatment options, prognosis, and prevention of musculocutaneous nerve injury, healthcare practitioners can better ensure that their patients receive the best care possible.
