Which nerve is primarily responsible for motor innervation to the flexor pollicis longus?

The median nerve is responsible for the motor innervation of the flexor pollicis longus muscle, which is essential for flexing the thumb. This nerve originates from the brachial plexus, directly supplying the majority of the flexor muscles in the forearm. The flexor pollicis longus, specifically, is one of the deep muscles in the anterior compartment of the forearm. Its primary action is to flex the thumb at the interphalangeal joint.

The involvement of the median nerve in innervating this muscle highlights its critical role in thumb function and dexterity. Damage or lesions affecting the median nerve can result in difficulties in thumb opposition and flexion, underscoring the importance of this nerve in hand movements.

Other nerves, such as the ulnar nerve, primarily facilitate innervation to the intrinsic muscles of the hand but do not significantly innervate the flexor pollicis longus. The musculocutaneous nerve is primarily involved with the anterior compartment muscles of the arm, while the radial nerve innervates primarily the extensor muscles in the forearm. Therefore, the median nerve is the correct answer as it directly provides the necessary motor function for the flexor pollicis longus.