Cranial nerve XI is the accessory nerve, supplying somatic motor fibres to two large muscles: the sternocleidomastoid and trapezius. In this article, we look in more detail at the structure, function and clinical relevance of the accessory nerve.
The accessory nerve is traditionally described as arising from two nuclei (cranial and spinal):
Cranial nucleus: joins the vagus nerve; based on function, this is not a true part of the accessory nerve, and will not be discussed further.
Spinal nucleus: lower motor neurons clustered in the lateral aspect of the ventral horn of the superior cervical cord (C1-C6). This lateral aspect of the ventral horn appears continuous with the nucleus ambiguus of the medulla oblongata.
Fibres exiting this nucleus from these spinal levels exit the superior cervical cord and lower medulla at the post-olivary sulcus. As some roots begin arising below the skull, CN XI is the only cranial nerve that enters the skull passing through the foramen magnum.
It then courses inferolaterally with CNs IX and X to reach the jugular foramen. Here, it transiently interacts with the cranial portion of the accessory nuclear complex. It then passes through the jugular foramen.
Extracranial accessory nerve
Upon passing through the jugular foramen, the spinal accessory nerve travels inferiorly with the internal carotid artery to reach and supply the sternocleidomastoid muscle. It continues travelling through the posterior triangle of the neck to supply motor fibres to the trapezius.
Clinical relevance: assessing the accessory nerve
Examination of the accessory nerve involves assessing two muscles: the trapezius and sternocleidomastoid.
When assessing the trapezius, ask the patient to shrug their shoulders, then apply resistance. Accessory nerve weakness will present with weak or no resistance to your downward force.
When assessing the sternocleidomastoid, ask the patient to turn their head as far to the left as they can. With your left hand bracing their left shoulder, pull their chin forward. Weakness or no resistance to your pull suggests accessory nerve weakness.
The accessory nerve travels between the prevertebral and investing fascial layers of the neck. This superficial location predisposes it to injury.
Clinical relevance: tension headaches
The motor innervation of the trapezius and sternocleidomastoid has been discussed in some detail, but what about the sensory innervation?
The sternocleidomastoid derives sensory supply from the dorsal rami of spinal nerves C2 and C3. The trapezius derives sensory supply from the dorsal rami of C3 and C4.
Chronic contraction in these muscles can lead to increased tension and local inflammatory mediators such as substance P and prostaglandins. These inflammatory mediators may stimulate the C2-C4 nerve roots and cause a referred pain in the distribution of C2 to C4 – the head and neck.
Stretching and massage of the sternocleidomastoid and trapezius muscles may help to alleviate this pain.
CN XI is the accessory nerve
It originates in the medulla oblongata and superior cervical cord region
It provides general somatic efferent fibres to the trapezius and sternocleidomastoid
It exits the spinal cord in the neck and enters the skull through the foramen magnum
It then leaves the skull by the jugular foramen with CNs IX and X
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Patrick J. Lynch. License: [CC BY]. Modified by Dr Lewis Potter.