The accessory nerve (eleventh cranial nerve, CN XI, also known as the spinal accessory nerve, latin: nervus accessorius) is a motor nerve supplying the sternocleidomastoid and trapezius muscles.
Traditionally, the accessory nerve is described as arising from both medulla oblongata and the spinal cord, consisting of a cranial and a spinal root. However, the existence of a cranial contribution to the accessory nerve is controversial. (See the discussion below.)
The accessory nerve originates from the spinal accessory nucleus located in the posterolateral aspect of the anterior horns of spinal cord at the levels C2 - C5 (or C6). The fibers form delicate rootlets emerging from the lateral surface of the spinal cord and collect to form the accessory nerve. The nerve ascends, enters the cranial cavity through the foramen magnum, and continues on the lateral aspect of the medulla oblongata. The accessory nerve joins with the aberrant vagal fibers as they emerge from the medulla as a distinct rootlet. These two groups of nerve fibers travel alongside for a short distance through the jugular foramen, and then separate to go in different directions. The aberrant vagal fibers (previously called the “cranial root” of the accessory nerve) join the vagus nerve to ultimately reach the intrinsic laryngeal muscles.
The accessory nerve exits the skull via the jugular foramen, and then travels downward behind the digastric and stylohyoid muscles to reach the upper part of sternocleidomastoid muscle. It enters the deep surface of the muscle and provides motor branches to innervate it, and forming anastomosis with fibers from C2 and/or C3. After piercing the sternocleidomastoid it descends further, crosses over the posterior triangle of the neck deep to the cervical plexus, reaches the deep surface of the upper part of the trapezius muscle to supply it with motor innervation.
The fibers carried by the accessory nerve are mostly general somatic efferents.
The reports on the cranial contribution to the accessory nerve have been inconsistent, and there has been a controversy regarding the anatomy of the eleventh cranial nerve. There are three main views about the cranial root of the accessory nerve.
The long-established view is that the accessory nerve is composed of a cranial root emerging from the medulla oblongata and a spinal root - from the spinal cord. The cranial root of the accessory nerve joins and distributes through the internal branch of the vagus nerve (CN X), so it has been described to functionally be a part of the vagus nerve. It has been suggested that the cranial root of the accessory nerve carries motor fibers that eventually innervate muscles of larynx, pharynx, and soft palate.
The second view is that there is no cranial root of the accessory nerve, as some reports show. (Lachman et al., 2002) In this case, the “cranial root” of the accessory nerve can be viewed as aberrant vagal fibers arising from the nucleus ambiguus in the medulla oblongata. These fibers emerge as a distinct rootlet, that travels along the accessory nerve for a short distance, and joins the vagus nerve. The accessory nerve, on the other hand, originates only from a nucleus located in the spinal cord. Many neuroanatomists no longer consider the accessory nerve as a true cranial nerve, rather as a unique spinal nerve.
Other studies suggest that there is a morphologically distinct cranial root of the accessory nerve as recent reports show, but it exists variably (Wiles et al., 2007), while some reports show that the cranial root exists in the majority (Tubbs et al., 2014), and in some cases in all the specimens. (Liu et al., 2014)
Although the “cranial root” of the accessory nerve appears to be distinct from the vagus nerve in the cranial cavity, this rootlet and the vagus nerve can be regarded functionally as the same nerve based on their distribution in the laryngeal muscles. (Liu et al., 2015)
Lachman N, Acland RD, Rosse C. 2002. Anatomical evidence for the absence of a morphologically distinct cranial root of the accessory nerve in man. Clin Anat 15:4-10.
Liu, HF, Won, HS, Chung, IH, Oh, CS, and Kim, IB. (2014). Variable composition of the internal and external branches of the accessory nerve. Clin. Anat., 27: 97-101.
Liu HF, Won HS, Chung IH, Kim IB, Han SH. (2015). Distribution of the internal branch of the human accessory nerve. Anat Sci Int., 90(3):180-186.
Wiles, CCR, Wrigley, B. and Greene, JRT. (2007). Re-examination of the medullary rootlets of the accessory and vagus nerves. Clin. Anat., 20: 19-22.
Tubbs, RS, Benninger, B, Loukas, M, and Cohen-Gadol, AA. (2014). Cranial roots of the accessory nerve exist in the majority of adult humans. Clin. Anat, 27: 102-107.