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Accessory nerve
Medical expert of the article
Last reviewed: 07.07.2025
The accessory nerve (n. accessories), or nerve of Willis, is formed by the processes of the motor nuclei located in the tegmentum of the medulla oblongata and in the spinal cord.
The cranial roots (radices craniales) of the accessory nerve emerge from the posterior lateral groove of the medulla oblongata, behind the olive.
The spinal roots (radices spinales) emerge from the posterolateral groove of the spinal cord, ascend through the foramen magnum into the cranial cavity and join the cranial roots behind the lobule of the cerebellar hemisphere (cerebellar tonsils). At the exit from the jugular foramen, the accessory nerve gives off an internal and external branch. The internal branch (r. internus), which is thinner, is part of the vagus nerve above its inferior ganglion. The external branch (r. externus) of the accessory nerve goes behind the styloid process of the temporal bone and the muscles originating from it, passes behind the posterior belly of the digastric muscle and is directed toward the sternocleidomastoid muscle. Some of the fibers of the external branch pierce the sternocleidomastoid muscle and enter the anterior edge of the trapezius muscle, which it innervates.
The accessory nerve gives off communicating branches to the anterior branches of the III and IV cervical spinal nerves and to the hypoglossal nerve.
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