Which cranial nerve innervates the sternocleidomastoid and trapezius muscles?

The accessory nerve, also known as cranial nerve XI, is responsible for the innervation of the sternocleidomastoid and trapezius muscles. This nerve emerges from the upper spinal cord and has both cranial and spinal roots; however, it is primarily the motor function that enables neck and shoulder movement.

The sternocleidomastoid muscle is involved in rotating the head and flexing the neck, while the trapezius muscle aids in shoulder elevation and scapular movement. As such, the accessory nerve plays a crucial role in controlling these muscles, allowing for actions like turning the head and shrugging the shoulders.

The other cranial nerves mentioned do not serve this function. The hypoglossal nerve is primarily responsible for tongue movement, the vagus nerve has extensive autonomic functions throughout the body including the heart and digestive tract, and the glossopharyngeal nerve is involved in taste and some aspects of swallowing, but none of these cranial nerves innervate the sternocleidomastoid or trapezius muscles.