The nervous system controls every muscle in the body through innervation, allowing for voluntary movement. The biceps brachii is a prominent muscle in the anterior upper arm responsible for powerful forearm movements. Understanding which nerve provides the necessary signals to the biceps is essential to comprehending arm strength and function. This article explores the specific nerve that innervates the biceps brachii, its origin, and the functional deficits that occur if it is damaged.
Defining the Biceps Brachii
The biceps brachii is a spindle-shaped muscle located in the anterior compartment of the upper arm, identified by its two heads. The long head begins at the supraglenoid tubercle of the scapula, while the short head originates from the coracoid process. These two heads merge into a single muscle belly. The primary attachment point for the biceps is the radial tuberosity on the radius bone, along with the bicipital aponeurosis. The muscle’s main actions are powerful supination (outward rotation) of the forearm and flexion (bending) of the elbow joint.
The Musculocutaneous Nerve
The Musculocutaneous Nerve (MCN) provides the sole motor supply to the biceps brachii muscle. It is classified as a mixed nerve because it contains both motor fibers, which control muscle contraction, and sensory fibers, which relay sensation from the skin. The MCN delivers the motor signals needed for elbow flexion and forearm supination.
The MCN also supplies motor innervation to the coracobrachialis and the brachialis, the two other muscles in the anterior compartment of the arm. Since the brachialis is a strong elbow flexor, the motor function of the entire anterior upper arm is dependent on the MCN. After the nerve gives off its motor branches in the arm, it continues into the forearm as a sensory nerve.
Where the Nerve Originates
The Musculocutaneous Nerve is a terminal branch of the brachial plexus, a network of nerves near the shoulder. This plexus is formed by the ventral rami of spinal nerves C5 through T1. The MCN carries nerve fibers primarily from the cervical spinal roots C5, C6, and C7.
The nerve emerges from the lateral cord of the brachial plexus. From its origin in the axilla (armpit), the nerve pierces the coracobrachialis muscle. It then travels downward, situated between the biceps brachii and the underlying brachialis muscle, before continuing into the forearm.
Consequences of Nerve Injury
Damage to the Musculocutaneous Nerve can result from direct trauma, compression, or excessive stretching, such as during a shoulder dislocation. Injury leads to a distinct pattern of motor and sensory deficits.
The most significant motor deficit is pronounced weakness in elbow flexion, noticeable when attempting to bend the arm against resistance. This occurs because the biceps brachii and brachialis muscles, the primary flexors of the elbow, are compromised. A person with this injury will also experience a substantial loss of forearm supination power, as the biceps is the strongest supinator. The biceps reflex may also be diminished or absent.
The sensory component of the MCN continues past the elbow as the lateral cutaneous nerve of the forearm. A secondary consequence of MCN injury is altered sensation, such as numbness or tingling, along the outer side of the forearm.