Final answer:
The accessory nerve, also known as the spinal accessory nerve, controls the sternocleidomastoid and trapezius muscles, essential for head and neck movements and shoulder elevation. Dysfunction in this nerve affects muscle strength and can be clinically evaluated through resistance tests.
Step-by-step explanation:
The accessory nerve, also known as the spinal accessory nerve or the eleventh cranial nerve, is a motor nerve that controls the motion of the neck and shoulder muscles. Specifically, it innervates the sternocleidomastoid and trapezius muscles.
These muscles play a critical role in various head and neck movements. When both sternocleidomastoid muscles contract, the outcome is forward flexion of the neck. When contracted individually, they facilitate rotation to the opposite side. The trapezius muscle acts in opposition, generating extension and hyperextension of the neck. Lateral flexion of the neck toward the shoulder engages both muscles as synergists.
These muscles not only assist in head motion but also in lifting the shoulders—evident when a person shrugs. Injury or dysfunction of the accessory nerve can present as weakness or asymmetry in shoulder elevation or head orientation. Thus, this nerve is pivotal for both motor functions and routine clinical evaluation of neck muscle strength.