Which nerve controls the trapezius and sternocleidomastoid muscles?

The spinal accessory nerve, also known as cranial nerve XI, is responsible for innervating the trapezius and sternocleidomastoid muscles. The trapezius muscle plays a critical role in moving and stabilizing the shoulder, as well as extending and rotating the neck. The sternocleidomastoid muscle is essential for neck movement and head rotation, and it is involved in various actions including flexing the neck.

Cranial nerve XI has both cranial and spinal components. The cranial part originates from the medulla oblongata and joins with the spinal part that arises from the upper cervical segments of the spinal cord. This combined structure exits the skull through the jugular foramen and supplies these specific muscles.

The other nerves mentioned do not innervate the trapezius and sternocleidomastoid muscles. The hypoglossal nerve primarily controls the muscles of the tongue, the facial nerve is mainly responsible for facial expression and taste sensations, and the glossopharyngeal nerve innervates structures in the throat and plays a role in taste and swallowing. None of these are involved in the motor control of the muscles in question, thus affirming the spinal accessory nerve as the correct choice.