musculocutaneous nerve

**Origin and roots:** Terminal branch of the lateral cord of the brachial plexus. Root values are C5, C6, and C7.

**Course:** It arises near the inferior border of pectoralis minor and pierces the coracobrachialis muscle. It then descends between the biceps brachii and brachialis, which are in the anterior compartment of the arm. Lateral to the biceps tendon, it emerges as the lateral antebrachial cutaneous nerve.

**Motor function:** It supplies the anterior (flexor) compartment of the arm. - Coracobrachialis: arm flexion and adduction. - Biceps brachii: forearm supination and elbow flexion. Supination is strongest when the elbow is flexed. - Brachialis: the main elbow flexor. It receives dual innervation, with a small lateral part from the radial nerve.

**Sensory function:** The lateral antebrachial cutaneous nerve supplies the skin of the lateral forearm, from the elbow to the wrist.

**Clinical correlations:** - *Injury:* Causes include anterior shoulder dislocation, humeral fracture, and compression, such as in coracobrachialis entrapment or during surgery. - *Findings:* - Weak elbow flexion. Some flexion remains because of the brachioradialis (radial nerve) and part of the brachialis. - Weak supination. Some remains through the supinator (radial nerve). - Loss of the biceps reflex (C5–C6). - Sensory loss over the lateral forearm. - *Weakness with the forearm pronated:* Elbow flexion is weaker in pronation, because the biceps is less effective in that position. The brachioradialis then does more of the work. - *Upper trunk (Erb) palsy:* The musculocutaneous nerve is often involved along with the axillary and suprascapular nerves. This gives the "waiter's tip" posture (adducted, medially rotated arm with extended elbow and pronated forearm). - *Lateral cord lesions:* These affect the musculocutaneous nerve and the lateral root of the median nerve.

**High-yield mnemonic:** "BBC" for Biceps, Brachialis, and Coracobrachialis, the muscles supplied by the musculocutaneous nerve.

**Exam tips:** - The musculocutaneous nerve is the nerve of the anterior arm compartment. The radial nerve is the nerve of the posterior compartment. - The biceps reflex tests C5–C6, with C5 being the main root commonly cited. - It pierces coracobrachialis, which is a classic anatomy question. - Forearm sensory loss is lateral for the musculocutaneous nerve, but medial for the medial antebrachial cutaneous nerve (from the medial cord).