What nerve contracts the biceps brachii muscle? A. Axillary B. Radial C. Ulnar D. Median E. Musculocutaneous
The correct answer and explanation is:
The correct answer is: E. Musculocutaneous nerve
Explanation:
The biceps brachii muscle is primarily responsible for flexing the elbow and supinating the forearm (turning the palm upward). To perform these actions, the muscle needs to receive motor innervation from a specific nerve.
The musculocutaneous nerve arises from the lateral cord of the brachial plexus, containing fibers mainly from the C5, C6, and C7 spinal nerve roots. This nerve is the primary motor nerve that innervates the biceps brachii muscle, as well as the brachialis and coracobrachialis muscles. The musculocutaneous nerve enters the anterior compartment of the arm and directly supplies the biceps brachii, allowing it to contract and perform its actions.
Let’s briefly look at why the other nerve options are incorrect:
- A. Axillary nerve: This nerve innervates the deltoid and teres minor muscles, involved mainly in shoulder abduction and rotation, not elbow flexion.
- B. Radial nerve: This nerve supplies the extensor muscles of the forearm responsible for wrist and finger extension. It innervates the triceps brachii (which extends the elbow) but not the biceps brachii.
- C. Ulnar nerve: This nerve primarily supplies intrinsic muscles of the hand and some forearm muscles but does not innervate any muscles in the arm like the biceps.
- D. Median nerve: This nerve innervates most of the flexor muscles in the forearm and some hand muscles, but not the biceps brachii.
Additional context:
In clinical examination, damage to the musculocutaneous nerve leads to weakness or loss of elbow flexion and weakened forearm supination due to paralysis of the biceps brachii and brachialis. This can occur in trauma or neuropathies affecting the lateral cord of the brachial plexus.
Understanding the nerve supply of muscles is critical for diagnosing neurological injuries and guiding rehabilitation in physical therapy or surgery. The musculocutaneous nerve’s role is pivotal in upper limb movement, specifically in bringing the hand closer to the shoulder via elbow flexion.