• Critical function provided is of elbow flexion.
• Innervates the coracobrachialis, biceps brachii, and brachialis muscles, and provides sensation to the lateral cutaneous aspect of the forearm.
• Originates from roots C5, C6, C7.
Physical examination of the musculocutaneous nerve includes both motor and sensory examination. The motor examination involves testing the coracobrachialis, biceps brachii and brachialis muscles. Of the three, the biceps brachii and brachialis are the easiest to test — by testing elbow flexion.
Biceps brachiiBrachialisCoracobrachialisLateral antebrachial cutaneous nerve
• Both the biceps and the brachialis muscles are strong flexors of the elbow. To differentiate between the two, the forearm can be held in pronation and supination during elbow flexion. In forearm supination, with resisted elbow flexion, the biceps muscle belly and tendon are clearly palpable. In forearm pronation and resisted elbow flexion, the brachialis muscle belly can be palpated underneath/on either side of the biceps tendon. The brachioradialis muscle (innervated by the radial nerve, and helps with elbow flexion with the forearm in neutral rotation position [thumb pointing upward]) and can be palpated radial/lateral to the biceps and brachialis muscles.
• Coracobrachialis is useful for shoulder flexion when the elbow is flexed (this position shortens the biceps muscle, diminishing its contribution to shoulder flexion).
• The sensory examination involves testing sensation over the lateral cutaneous aspect of the forearm innervated by the lateral antebrachial cutaneous nerve.
• Loss of musculocutaneous nerve leads to near-complete loss of elbow flexion in almost all patients. The rare patient maintains some ability to flex the elbow using the brachioradialis muscle innervated by the radial nerve.
• Loss of musculocutaneous nerve function is frequently seen in upper brachial plexus injury patterns. Reinnervation of the critical function, elbow flexion, is important to restoring normal upper extremity function. Without elbow flexion, patients cannot position the arm in space for eating, grooming, and other basic activities of daily living.
• In upper brachial plexus injury patterns, when the denervation time permits (3 to 6 months or less is ideal), and where shoulder and elbow function is absent but hand function is preserved, restoration of the elbow flexion can be accomplished by the ‘double fascicular nerve transfer’ where expendable median and ulnar nerve motor branches are transferred to the nerves to brachialis and biceps. In chronic injuries, free functional muscle transfer or other muscle or tendon transfers are necessary to restore elbow flexion.
• Roots: C5, C6, C7.
• Nerve: Lower subscapular nerve.
• Muscles Innervated: coracobrachialis, biceps brachii, brachialis.
• Sensory Distribution Innervated: Lateral antebrachial cutaneous.
• Innervation Route:• C6, C7 → lateral cord → musculocutaneous nerve → coracobrachialis.
• C5, C6 → lateral cord → musculocutaneous nerve → biceps brachii.
• C5, C6 → lateral cord → musculocutaneous nerve → brachialis.
• C5, C6, C7 → lateral cord → musculocutaneous nerve → lateral antebrachial cutaneous nerve.