Final answer:
The musculocutaneous nerve stimulation causes flexion of the elbow, with the biceps brachii and other muscles contributing to this action. The triceps brachii is the muscle that extends the forearm and the wrist flexors originate from the medial epicondyle of the humerus.
Step-by-step explanation:
Stimulation of the musculocutaneous nerve primarily causes flexion of the elbow. This nerve innervates several muscles that are responsible for this action, including the biceps brachii, brachialis, and coracobrachialis. When considering the muscles of the forearm and their actions, the biceps brachii is a primary forearm flexor, while the triceps brachii is the principal extensor of the forearm; deltoid and brachialis are not mainly responsible for these actions.