What muscles are innervated by the musculocutaneous nerve?
Feb 25, 2023
Source: Cainiu Health
Dr. Lin Yunfei
Introduction
The muscles primarily innervated by the musculocutaneous nerve include the coracobrachialis, biceps brachii, and brachialis. In addition, the musculocutaneous nerve also supplies the lateral cutaneous nerve of the forearm. Injury to the musculocutaneous nerve due to surgery or trauma can result in corresponding motor deficits in these muscles and sensory disturbances in the forearm, causing numbness. This may subsequently affect shoulder flexion and adduction, as well as supination and flexion of the elbow joint.
The muscles innervated by the musculocutaneous nerve primarily include the coracobrachialis, biceps brachii, and brachialis.
1. Coracobrachialis
The coracobrachialis lies deep to the short head of the biceps brachii and works together with the biceps to flex and adduct the shoulder. It also contributes to shoulder extension when the shoulder joint is maximally flexed. The coracobrachialis plays a key role in shoulder stability by helping to maintain alignment between the humeral head and the glenoid cavity during various ranges of motion. Additionally, during walking, it helps coordinate forward swinging of the arm.
2. Biceps Brachii
The biceps brachii is a major muscle of the anterior forearm and is innervated by the musculocutaneous nerve. This nerve originates from cervical spinal segments C5–C6, emerges from the lateral cord of the brachial plexus, passes downward through the coracobrachialis, and then descends between the biceps brachii and brachialis muscles, supplying all three. The primary function of the biceps brachii is elbow flexion, which is achieved through relaxation of its antagonist, the triceps brachii. The biceps reflex is a deep tendon reflex and is a common reflex test in the upper limb.
3. Brachialis
The brachialis is located superior to the elbow joint, lying beneath the biceps brachii. It receives dual innervation from both the radial nerve and the musculocutaneous nerve. Its main action is flexion of the elbow joint. As one of the primary flexors of the forearm, the brachialis works together with the biceps brachii and coracobrachialis to antagonize the triceps brachii.
In addition, the musculocutaneous nerve also provides sensory innervation to the lateral skin of the forearm. Injury to this nerve due to surgery or trauma can result in motor deficits in the muscles it supplies, as well as sensory disturbances such as numbness in the lateral forearm. This may impair shoulder flexion and adduction, as well as supination and flexion of the elbow joint.
