Which nerve innervates the brachialis muscle?

The brachialis muscle is primarily innervated by the musculocutaneous nerve. This nerve, originating from the lateral cord of the brachial plexus, is responsible for supplying the brachialis, which is a crucial muscle for elbow flexion. The musculocutaneous nerve not only innervates the brachialis but also the biceps brachii and coracobrachialis, contributing to flexion at the elbow joint and supination of the forearm.

In contrast, while the ulnar nerve, radial nerve, and median nerve have significant roles in the upper limb's function, they do not innervate the brachialis muscle directly. The ulnar nerve primarily innervates the intrinsic muscles of the hand and part of the forearm. The radial nerve mainly innervates the muscles in the posterior compartment of the arm and forearm, while the median nerve primarily acts on the flexor muscles of the forearm and some hand muscles. Thus, the musculocutaneous nerve is the correct choice as it directly innervates the brachialis muscle, facilitating its function in elbow flexion.