Accessory nerve (CN XI)

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The accessory nerve (CN XI) is primarily a motor nerve meaning that it supplies efferent motor function to muscles. Traditionally, it has been considered to have two parts, a cranial root and a spinal root. The classification of the cranial root is somewhat controversial however, with many anatomists nowadays considering these fibers to be part of the vagus nerve. Therefore, when clinicians and textbooks refer to the accessory nerve, they usually are specifically referring to the spinal part only (making the accessory nerve the only cranial nerve not to originate within the cranium). Consequently, many no longer technically consider the accessory nerve to be a cranial nerve but continue to include it as one, so as not to defy traditional convention.

When considering the spinal part only, the function of the accessory nerve is to supply motor innervation to two muscles, the sternocleidomastoid and trapezius. The type of innervation carried by the accessory nerve to these muscles is disputed however. Some references describe it as carrying special visceral efferent (SVE, branchiomotor) fibers (due to a close relationship between the nucleus of accessory nerve with the nucleus ambiguous), while others describe it as carrying general somatic efferent (GVE) fibers (due to the unknown/disputed embryological origins of the sternocleidomastoid and trapezius muscles).

Find out more about the pathway, branches and function of the accessory nerve by watching the video below!

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Summary