Cranial nerve XI, also known as the accessory nerve, is primarily responsible for controlling certain muscles in the neck and shoulders. It has two main parts: the spinal part, which originates from the spinal cord, and the cranial part, which arises from the brainstem. This nerve innervates the sternocleidomastoid and trapezius muscles, enabling movements such as head rotation and shoulder elevation. In clinical practice, damage to cranial nerve XI can lead to weakness in these muscles, resulting in difficulties with head movement and shoulder shrugging. Assessment of this nerve is often part of a neurological examination to evaluate motor function in the neck and upper back.