Nucleus of accessory nerve

The nucleus of accessory nerve (a.k.a. spinal accessory nucleus or accessory nucleus proper) is part of the central group of motor nuclei in the anterior gray horns of the cervical spinal cord. Motor rootlets originating from this nucleus form the spinal rootles of the accessory nerve, which ascend through the foramen magnum to enter the cranial cavity. Within the cranial cavity, the spinal rootlets coalesce with the cranial accessory nerve, arising from the nucleus ambiguous in the medulla oblongata. The combined accessory nerve exits the skull via the jugular foramen, after which efferent fibers from the spinal accessory nucleus separate and proceed to innervate the trapezius and sternocleidomastoid muscles.

The nucleus of accessory nerve, along with the phrenic nucleus, is positioned between the medial and lateral cell groups of the cervical anterior gray horn.