Spinal accessory nerve


Cranial nerve XI.

Originates from neuronal cell bodies located in the cervical spinal cord and caudal medulla.

Cranial Nerve XI, the accessory nerve signals the contraction or relaxation of the the sternocleidomastoid.


The Cranial Nerve XI, the accessory nerve’s nucleus is in the anterior horn of the spinal cord around C1-C3, where lower motor neuron fibers mark its origin. 


Most neuronal cell bodies are located in the spinal cord and ascend through the foramen magnum and exit the cranium through the jugular foramen.

It innervates the sternocleidomastoid and trapezius muscles in the neck and back.

The cranial root of the accessory nerve originates from cells located in the caudal medulla, and they are found in the nucleus ambiguus and leave the brainstem with the fibers of the vagus nerve joining the spinal root to exit the jugular foramen.

The fibers from the accessory nerve nucleus travel upward to enter the cranium via the foramen magnum. 
After a signal reaches the accessory nerve nucleus in the anterior horn of the spinal cord, the signal is conveyed to motor endplates on the muscle fibers located at the clavicle. 
Acetylcholine (ACH) is released from vesicles and is sent over the synaptic cleft to receptors on the postsynaptic bulb. 
The ACH causes the resting potential to increase above -55mV, thus initiating an action potential which travels along the muscle fiber. 
The nerve rejoins the vagus nerve and distribute to the same targets as the vagus and is considered to be functionally part of the vagus nerve.

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