The accessory nerve is the 11th Cranial nerve and consists of a cranial and a spinal component.
Anatomy
Course
The cranial root arises from the Brainstem dorsal to the olive, as a caudal continuation of the root fibers of the Vagus nerve, and contains nerve fibers from the nucleus ambiguus[^1]. The spinal root contains nerve fibers from the nucleus of the accessory nerve and arises from the cervical Spinal cord between the dorsal and ventral roots, ascending through the Foramen magnum. Shortly before exiting the nervous part of the Foramen jugulare, the cranial and spinal roots unite to form a common nerve. After exiting the skull, the nerve divides again into its two components and gives off an internal branch to the vagus nerve. The internal branch supplies motor fibers to the pharyngeal and laryngeal musculature. The spinal component crosses the Internal jugular vein dorsally or ventrally and travels to the Sternocleidomastoid muscle and trapezius muscle to innervate them.
Clinical examination
In accessory nerve palsy, elevating the arm above the horizontal plane is difficult. If the branch to the sternocleidomastoid muscle is affected, the chin deviates toward the paralyzed side when the head is tilted, and turning the head toward the unaffected side against resistance is difficult[^1].