What is the primary function of the accessory nerve (CN XI)?

The primary function of the accessory nerve (CN XI) is to facilitate head movement and shoulder elevation. This cranial nerve innervates two key muscles: the sternocleidomastoid and the trapezius. The sternocleidomastoid muscle allows for the rotation and flexion of the head, while the trapezius muscle is responsible for the elevation of the shoulders and retraction of the scapula. Thus, the accessory nerve plays a crucial role in movements involving head positioning and shoulder shrugging, which are essential for a range of daily activities and overall body posture.

The other choices do not relate to the function of the accessory nerve. For instance, vision and light perception are primarily governed by the optic nerve (CN II), while facial expressions are controlled by the facial nerve (CN VII). Additionally, the autonomic regulation of heart rate is managed through the vagus nerve (CN X). Therefore, the accessory nerve is specifically associated with head and shoulder movements, making that the correct answer.