Origin and Course
The musculocutaneous nerve (historical eponym: Casserian nerve) arises from the lateral cord (fasciculus lateralis) of the brachial plexus (plexus brachialis).
Its topographical course in the upper extremity includes the following stages:
- Emerging from the axilla, the nerve pierces the coracobrachialis muscle (m. coracobrachialis).
- Passing lateral to this muscle, it travels beneath the long head of the biceps brachii muscle (m. biceps brachii).
- In the arm, the musculocutaneous nerve lies within the anterior fascial compartment. Unlike the radial and ulnar nerves, which pass to the posterior compartment, it remains in the anterior compartment.
- In the middle third of the arm, the nerve runs obliquely from superior to inferior and medial to lateral, positioned between the biceps brachii and brachialis muscles. At the border of the middle and distal thirds of the arm, it pierces the deep fascia covering the brachialis muscle (m. brachialis).
Innervated Muscles
The musculocutaneous nerve gives off motor branches to all muscles of the anterior fascial compartment of the arm. Specifically, it innervates the following muscles:
- Biceps brachii muscle (m. biceps brachii) — receives fibers from segments C5–C6. It is responsible for forearm flexion, shoulder flexion, and forearm supination.
- Coracobrachialis muscle (m. coracobrachialis) — innervated by segments C5–C7. It assists in arm flexion and adduction.
- Brachialis muscle (m. brachialis) — receives fibers from C5–C6. It acts as the primary flexor of the forearm.
Terminal Branch and Sensory Innervation
At the junction with the antecubital fossa, the terminal branch emerges from beneath the biceps brachii muscle.
- The nerve changes its name and continues as the lateral cutaneous nerve of the forearm (n. cutaneus antebrachii lateralis).
- Distally, within the cubital fossa, it pierces the deep fascia into the subcutaneous tissue.
- This branch innervates the skin of the lateral aspect of the forearm down to the wrist.
Clinical Significance and Reflexes
- Biceps reflex: The effector muscle is the m. biceps brachii, the peripheral nerve is the musculocutaneous nerve (n. musculocutaneus), and the spinal reflex arc is integrated at the C5 and C6 spinal cord segments.
- Diagnostics: Electromyography (EMG) and nerve conduction studies (NCS) are used to assess the severity of peripheral nerve injury and estimate functional recovery prognosis.