The biceps brachii muscle is located on the front of the upper arm, between the shoulder and the elbow. This muscle has two distinct heads, which originate from the shoulder blade and merge into a single muscle belly before attaching to the forearm. The biceps brachii plays a role in various arm movements, particularly in bending the elbow and rotating the forearm. For any muscle to perform its actions, it requires “innervation,” which refers to the supply of nerves that carry signals from the brain and spinal cord to the muscle.
The Musculocutaneous Nerve
The primary nerve innervating the biceps brachii is the musculocutaneous nerve. It is a branch of the brachial plexus, a network of nerves originating from the spinal cord in the neck and shoulder region. Specifically, the musculocutaneous nerve typically arises from the C5, C6, and C7 spinal nerves.
As part of the brachial plexus, the musculocutaneous nerve emerges from the lateral cord. After its origin, the musculocutaneous nerve travels through the upper arm, supplying the biceps brachii and other muscles in the anterior arm, including the coracobrachialis and brachialis muscles. This nerve also has a sensory component, providing sensation to the forearm.
How the Nerve Activates Your Biceps
The musculocutaneous nerve follows a pathway from its origin to the biceps brachii muscle. After leaving the brachial plexus, the nerve passes through the coracobrachialis muscle, then continues between the biceps brachii and the brachialis muscles. Along this path, the musculocutaneous nerve sends out smaller branches directly into the muscle fibers of the biceps brachii. These branches carry electrical impulses from the nervous system to the muscle.
When you decide to move your arm, your brain sends an electrochemical signal, known as an action potential, down a motor neuron. This signal travels along the musculocutaneous nerve until it reaches the neuromuscular junction, which is the specialized connection point between the nerve and the muscle fiber. At this junction, the nerve releases a neurotransmitter, acetylcholine, into a tiny gap. This neurotransmitter then binds to receptors on the muscle fiber’s membrane, triggering muscle contraction.
The precise innervation provided by the musculocutaneous nerve enables the biceps brachii to perform its primary actions: flexing the elbow and supinating the forearm. Elbow flexion involves bending the arm at the elbow, such as when lifting an object towards the body. Forearm supination is the action of rotating the forearm so that the palm faces upward or forward, like when turning a doorknob. These movements are only possible because the musculocutaneous nerve effectively transmits signals to the biceps, coordinating its contraction and relaxation.
When Biceps Innervation is Compromised
If the musculocutaneous nerve is damaged or its function becomes impaired, the consequences can significantly affect arm movement and sensation. Common causes of such damage include direct trauma, such as lacerations or fractures, or compression, which can occur from prolonged pressure or entrapment within surrounding tissues. Issues related to the brachial plexus, from which the musculocutaneous nerve originates, can also lead to its impairment.
When the musculocutaneous nerve’s ability to transmit signals is compromised, individuals may experience specific symptoms. Weakness in elbow flexion, making it difficult to bend the arm, is a common motor symptom because the biceps brachii receives its motor supply from this nerve. Difficulty with forearm supination, or rotating the palm upwards, is another typical sign. In some cases, there might also be sensory loss or altered sensation, such as numbness or tingling, in the outer forearm, as the musculocutaneous nerve also has a sensory branch that supplies this area. These symptoms can impact daily activities, highlighting the importance of proper nerve function for arm mobility and sensation.