The Accessory Nerve
The accessory nerve is the eleventh of the Twelve Cranial Nerves. The fibers consist of efferent motoric and afferent proprioceptive fibers. The word afferent means toward the centre, as in from a peripheral area of a limb to the central nervous system. The word efferent is the opposite of afferent, meaning away from the centre and toward the periphery; when the stimulus is carried back to the brain from a peripheral area.
The pathway for the efferent motoric fibers of the accessory nerve is the following (from the initiation in the brain to the termination in the periphery):
- The efferent motoric fibers emerge from the nucleus ambiguous which is situated in the tuberculum gracile in the brainstem.
- Fibers also continue to emerge down the fasciculus gracilis.
- The fibers from the nucleus ambiguous run in the cranial root of the accessory nerve, while the fibers from the fasciculus gracilis run in the spinal radix of the accessory nerve.
- Together these two branches exit the skull via the foramen magnum in the base of the skull.
- After exiting the skull, the nerve bundles momentarily bind together as the accessory nerve, without mixing their fibers.
- They then diverge and the fibers from the nucleus ambiguous synapse on the superior jugular ganglion of the vagus nerve (CN X). From then on the accessory nerve fibers run within the vagus and end up in the recurrent laryngeal nerve which supplies the muscles of the larynx, excluding the cricothyroid muscle.
- Meanwhile the fibers from the fasciculus gracilis continue as the main branch of the accessory nerve and innervate the sternocleidomastoid muscle and the trapezius muscle.
- As well as emerging from the fasciculus gracilis and merging into the spinal radix of the accessory nerve, fibers also run directly from the fasciculus gracilis into the cervical spinal nerves C1-C4 and merge into the main branch of the accessory nerve as it descends from the foramen magnum towards the musculature which it innervate
The pathway for the afferent proprioceptive fibers of the accessory nerve is the following (from the initiation in the periphery to the termination in the brain):
- Proprioceptive fibers stem from the muscular fibers of the trapezius muscle and the sternocleidomastoid muscle.
- The sternocleidomastoid muscle fibers and the trapezius muscle fibers run into the cervical spinal nerves C3 and C4 respectively.
- The fibers synapse upon spinal ganglia lodged between the cervical vertebrae before finally synapsing in the spinal cord within the fasciculus gracilis and entering a spinal tract.