Which cranial nerve is primarily responsible for motor function to the sternocleidomastoid and trapezius muscles?

The cranial nerve responsible for motor function to the sternocleidomastoid and trapezius muscles is the spinal accessory nerve. This nerve, also known as cranial nerve XI, innervates these muscles, which are crucial for head and shoulder movement. The sternocleidomastoid muscle assists in rotating and flexing the head, while the trapezius muscle is involved in elevating the shoulders and extending the neck. Therefore, damage or dysfunction of the spinal accessory nerve can lead to weakness or paralysis in these muscle groups, impacting a person's ability to perform movements like turning the head or shrugging the shoulders effectively.

The other cranial nerves listed do not serve this function. The vagus nerve primarily oversees autonomic functions and innervation of the heart and digestive tract. The hypoglossal nerve is responsible for motor control of the tongue, critical for speech and swallowing. The abducens nerve controls lateral eye movement but does not play a role in the innervation of the muscles mentioned. Understanding the specific roles of these cranial nerves helps clarify the motor functions attributed to the spinal accessory nerve.