Musculocutaneous nerve

Origin and course?

The musculocutaneous nerve (historical eponym: Casserian nerve) arises from the lateral cord (fasciculus lateralis) of the brachial plexus (plexus brachialis).

Its topography in the upper limb includes the following stages:

• On its way from the axilla, the nerve pierces the coracobrachialis muscle (m. coracobrachialis).
• Emerging on the lateral side of this muscle, it passes deep to the long head of the biceps brachii muscle (m. biceps brachii).
• In the arm, the musculocutaneous nerve is located in the anterior fascial compartment. Unlike the radial and ulnar nerves, which pass into 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 from medial to lateral, lying between the biceps brachii and brachialis muscles. At the border with the middle third of the arm, it lies on the deep fascia covering the brachialis muscle (m. brachialis).

Which muscles does it innervate?

The musculocutaneous nerve gives off motor branches to all muscles of the anterior fascial compartment. Specifically, it innervates the following muscles:

• Biceps brachii muscle (m. biceps brachii) — receives fibers from segments C5–C6. Responsible for flexion of the arm and forearm, as well as supination.
• Coracobrachialis muscle (m. coracobrachialis) — innervated by segments C5–C7. Provides elevation and abduction of the arm, as well as flexion and adduction of the shoulder.
• Brachialis muscle (m. brachialis) — receives fibers from C5–C6. It is the primary flexor of the forearm and tensions the joint capsule.

Terminal branch and sensation

At the border with the anterior cubital region, the terminal branch of the musculocutaneous nerve emerges from beneath the biceps brachii.

• The nerve changes its name and is subsequently referred to as the lateral cutaneous nerve of the forearm (n. cutaneus antebrachii lateralis).
• Distally, within the anterior cubital region, it pierces the deep fascia and enters the subcutaneous tissue.
• This branch innervates the skin of the lateral aspect of the forearm, including the region of the thenar eminence.

Clinical significance and reflexes

• Biceps reflex: The effector muscle is the m. biceps brachii, the peripheral nerve is the musculocutaneous nerve (n. musculocutaneous), and the reflex arc center in the spinal cord is located at segments C5, C6.
• Diagnostics: Electromyography (EMG) and nerve conduction velocity (NCV) testing are used to determine the severity of peripheral nerve injury and establish a functional recovery prognosis.

Class and exam questions

What else to study on this topic

• Brachial plexus (plexus brachialis)
• Anterior fascial compartment of the arm
• Biceps brachii muscle (m. biceps brachii)
• Median nerve (n. medianus)

Other topics in the "Anatomical Structures" section

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