Musculocutaneous Nerve

Introduction

Introduction to the Musculocutaneous Nerve

The musculocutaneous nerve is a mixed peripheral nerve of the upper limb, arising from the lateral cord of the brachial plexus (C5-C7). It is the primary nerve of the anterior compartment of the arm, supplying all three flexor muscles: coracobrachialis, biceps brachii, and brachialis. After innervating these muscles, it pierces the deep fascia lateral to the biceps tendon and continues as the lateral antebrachial cutaneous nerve, providing sensory innervation to the lateral (radial) aspect of the forearm. Its name reflects its dual function: motor to muscles and cutaneous to the forearm. The musculocutaneous nerve is rarely injured in isolation, but injury causes weakness of elbow flexion and supination, along with sensory loss over the lateral forearm.

Study

Origin and Course

The musculocutaneous nerve originates from the lateral cord of the brachial plexus (C5, C6, C7). It enters the arm by passing through the coracobrachialis muscle (which it innervates), then runs obliquely between the biceps brachii (anteriorly) and brachialis (posteriorly). It supplies both of these muscles. At the elbow, the nerve emerges lateral to the biceps tendon, pierces the deep fascia, and becomes the lateral antebrachial cutaneous nerve, which descends along the radial side of the forearm to supply skin as far as the wrist.

Branches and Innervation

Muscular branches: - Coracobrachialis: The first branch, given off just before or as the nerve pierces the muscle. - Biceps brachii (short and long heads): Usually a single branch or two. - Brachialis: The nerve supplies the brachialis (except a small lateral portion supplied by the radial nerve). Cutaneous branch: - Lateral antebrachial cutaneous nerve: Terminal sensory continuation, supplies skin over the lateral (radial) half of the forearm, from elbow to wrist (palmar and dorsal aspects). Articular branches: Small twigs to the elbow joint (inconsistent).

Clinical Relevance

Isolated musculocutaneous nerve injury is rare due to its deep location. Causes include: - Penetrating trauma to the axilla or arm. - Iatrogenic injury during shoulder surgery (e.g., coracoid process procedures) or biceps tenodesis. - Stretch injury from anterior shoulder dislocation. - Compression in the arm (e.g., by hypertrophied coracobrachialis or fracture callus). Clinical findings: - Weakness of elbow flexion (biceps and brachialis) – patient may compensate with brachioradialis (radial nerve) and pronator teres (median), but flexion is weak, especially with supinated forearm. - Weakness of supination (biceps is a powerful supinator) – pronation may be unopposed. - Sensory loss over lateral forearm (no hand involvement – hand sensation is intact because hand is supplied by median, ulnar, and radial nerves). - Biceps reflex (C5-C6) is diminished or absent. The musculocutaneous nerve may be involved in brachial plexus injuries (e.g., upper trunk C5-C6 lesions, Erb's palsy) along with other nerves.

Summary

Key Takeaways – Musculocutaneous Nerve in the Arm

Origin: Lateral cord of brachial plexus (C5-C7). Motor: Coracobrachialis, biceps brachii, brachialis (all anterior compartment muscles of arm). Sensory: Lateral antebrachial cutaneous nerve – lateral forearm (no hand). Course: Pierces coracobrachialis → between biceps and brachialis → becomes cutaneous lateral to biceps tendon. Injury: Weak elbow flexion (especially supinated flexion), weak supination, loss of biceps reflex, sensory loss over lateral forearm.