Final answer:
Intramedullary lesions affect fibers of the spinal accessory nerve only if they occur at the cervical level of the spinal cord where there might be an intersection of nerve fibers. Such lesions can cause motor deficits in neck muscles and asymmetry during shoulder movement. Lesions at other levels of the spinal cord would typically not affect the spinal accessory nerve.
Step-by-step explanation:
The question asks whether intramedullary lesions affect the fibers of the spinal accessory nerve, which is responsible for the contraction of neck muscles like the sternocleidomastoid and trapezius. Intramedullary lesions occur within the spinal cord itself, whereas the spinal accessory nerve, being the eleventh cranial nerve, primarily exits from the brainstem and extends down into the neck. The nerve innervates the sternocleidomastoid and trapezius muscles, facilitating movements such as flexion, extension, and rotation of the head as well as shrugging of the shoulders. It is important to note that the spinal accessory nerve primarily interfaces with the muscles in the neck, and while it may receive some input from cervical spinal nerves, intramedullary lesions in the spinal cord could affect motor functions in these muscles only if the lesions are located at the cervical level where the nerve fibers might intersect.
Spinal accessory nerve involvement may also result in asymmetry during shoulder shrugging, as well as affect lateral flexion of the neck towards the shoulder. Furthermore, any deficits associated with the spinal accessory nerve can impact orientation of the head. It is also crucial to note that intramedullary lesions can lead to other motor deficits and sensory disruptions depending on their location within the spinal cord, with injuries potentially causing paralysis below the affected level.