Accessory nerve (Latin)

The radix spinalis of the n. accessorius originates from the cell bodies of motor neurons located in the upper 5-6 cervical segments of the spinal cord which collectively form the ncl. nervi accessorii. Multiple rootlets exit the spinal cord between the radices anteriores/posteriores of the nn. spinales and combine to form the nerve trunk, which ascends the spinal cord and passes through the foramen magnum to enter the cranium. The radix cranialis arises from motor neurons in the ncl. ambiguus of the medulla oblongata. Rootlets from this nucleus exit from the dorsolateral medulla oblongata and temporarily join with the fibers of the radix spinalis. Together, they then exit the skull via the foramen jugulare. The nerve then divides into two again; the r. internus (radix cranialis) joins with the n. vagus to innervate the mm. palati mollis/mm. interni laryngis while the r. externus (radix spinalis) descends into the lateral cervical region of the neck, passing superficially through its trigonum posterius. Here the n. accessorius passes deep to the m. sternocleidomastoideus, which it innervates via a r. muscularis. It continues to descend in a posterolateral direction before terminating as the r. trapezius which spans out across the m. trapezius, supplying it with motor function.Supplementary innervation of the m. sternocleidomastoideus/trapezius is also delivered via homonymous branches of the plexus cervicalis. These branches contain afferent/sensory neurons, whose cells bodies are located in the ganglia spinalia of nn. spinales C2-C4. Some motor fibers are also believed to be delivered to the muscles via this pathway.