Final answer:
The function of the accessory nerve (CN XI) is to control the neck and shoulder muscles, primarily the sternocleidomastoid and trapezius muscles, enabling movements such as flexing and rotating the head, and shrugging the shoulders.
Step-by-step explanation:
The function of the accessory nerve (CN XI) is Option 1: Control of neck and shoulder muscles. The accessory nerve, also known as the spinal accessory nerve, is responsible for innervating the sternocleidomastoid and trapezius muscles. These muscles are essential for movements such as the flexion, extension, and rotation of the head and neck, as well as shrugging the shoulders. When testing for strength or flexibility, observing these movements can provide insight into the function of the accessory nerve.
Movement of the head and neck using the sternocleidomastoid and trapezius muscles is controlled by the accessory nerve. The motor input for these muscles is provided by both the accessory nerve and additional input from the cervical spinal nerves. Therefore, the accessory nerve plays a vital role in daily activities that involve moving the head or stabilizing the shoulders.