Final answer:
Injury to the spinal accessory nerve in the posterior triangle of the neck can result in difficulty with neck movements and shoulder elevation, due to the nerve's role in innervating the sternocleidomastoid and trapezius muscles.
Step-by-step explanation:
The spinal accessory nerve (CN XI) is vulnerable to injury in the posterior triangle of the neck, such as during surgical procedures like cervical lymph node dissection. Injury to this nerve can lead to weakness or paralysis of the muscles it innervates, specifically the sternocleidomastoid and trapezius muscles. As a result of such an injury, a person might experience difficulty in neck movements, such as turning the head or shrugging the shoulders, and may display asymmetry when performing these actions due to the impaired function of the affected muscles.
The sternocleidomastoid muscles, when contracting together, flex the head forward and rotate it to the opposite side when contracting individually. The trapezius muscles, on the other hand, can cause extension and hyperextension of the neck. These muscles are essential for changing the position of the head and for elevating the scapula and clavicle. An injury to the spinal accessory nerve can thus significantly affect movements of the trunk or shoulders, as well as head orientation.