The musculocutaneous nerve arises from the lateral cord of the brachial plexus, opposite the lower border of the pectoralis major, its fibers being derived from C5, C6 and C7.
Structure
The musculocutaneous nerve arises from the lateral cord of the brachial plexus, courses through the anterior part of the arm, and terminates at 2 cm above elbow as lateral cutaneous nerve of the forearm. Musculocutaneous nerve arises from the lateral cord of the brachial plexus with root value of C5 to C7 of the spinal cord. It follows the course of the third part of the axillary artery laterally and enters the frontal aspect of the arm where it penetrates the coracobrachialis muscle. It then passes downwards and laterally between the biceps brachii and the brachialis muscles, to the lateral side of the arm; at 2 cm 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 cutaneous nerve of the forearm. In its course through the arm it innervates the coracobrachialis, biceps brachii, and the greater part of the brachialis. Its terminal branch, the lateral cutaneous nerve of the forearm, supplies the sensation of the lateral side of the forearm from the elbow to the wrist. Besides, the musculocutaneous nerve also gives articular branches to the elbow joint and to the humerus.
Variations
The musculocutaneous nerve presents frequent variations and communications with the median nerve.
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.
Injury to the musculocutaneous nerve can be caused by three mechanisms: repeated microtrauma, indirect trauma or direct trauma on the nerve. Overuse of coracobrachialis, biceps, and brachialis muscles can cause the stretching or compression of musculocutaneous nerve. Those who have it can complain of pain, tingling or reduced sensation over the lateral side of the forearm. This symptom can be reproduced by pressing over the region below the coracoid process. Pain can also be reproduced by flexing the arm against resistance. Other differential diagnoses that can mimick the symptoms of musculocutaneous palsy are: C6 radiculopathy, long head of biceps tendinopathy, pain of the bicipital groove. Electromyography test shows slight neural damage at the biceps and the brachialis muscles with slower motor and sensory conduction over the Erb's point. In indirect trauma, violent abduction and retroposition of the shoulder can stretch and the musculocutaneous nerve lesion. Those with this type of lesion is presented with pain, reduced sensation, and tingling of the lateral part of forearm with reduced strength of elbow flexion. Tinel's sign can be positive. Differential diagnosis includes C5 and C6 nerve root lesions of the brachial plexus where the abduction, external rotation, and elbow flexion is lost. On the other hand, rupture of the biceps can cause the loss of flexion of the elbow without sensory deficits. Rupture of the SHORT HEAD of the biceps can decrease elbow flexion strength, where the brachialis muscle is intact. Rupture of the LONG HEAD of the biceps results in mild weakening of forearm supination as long as the supinator muscle is intact. Electromyography test is negative. In direct trauma, fracture of the humerus, gun shot, glass pieces injuries and more, can cause the musculocutaneous nerve lesion. Iatrogenic nerve injuries are relatively common and in a certain percentage of cases probably inevitable, though an adequate knowledge of the surgical anatomy can help to reduce its frequency. Neurolysis and nerve grafting are the treatment options for the above lesions.
