Which nerve is responsible for motor control of the sternocleidomastoid and trapezius muscles?

The spinal accessory nerve, also known as cranial nerve XI, plays a crucial role in the motor control of both the sternocleidomastoid and trapezius muscles. The sternocleidomastoid muscle is involved in head rotation and flexion, while the trapezius muscle is responsible for movements of the shoulder girdle and maintaining posture of the upper back. The spinal accessory nerve originates from the spinal cord and has an additional cranial component that contributes to motor innervation.

This selective innervation allows for coordinated movements, such as turning the head and shrugging the shoulders. The functional significance of the spinal accessory nerve in these movements underscores its importance in assessing neuromuscular function related to the muscles it innervates. The other cranial nerves listed do not innervate these specific muscles, as they serve different functions within the nervous system.