Otolaryngology: Applied Basic Sciences for YO-IFOS
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8: The accessory nerve and anatomy of the neck
Function
Voluntary efferent motor control of the trapezius and sternocleidomastoid muscles (SCM).
There is some debate as to whether the accessory nerve is a branchial or somatic motor nerve. Somatic motor nerves innervate muscles derived from somites (like the tongue or the extraocular muscles). Branchial motor nerves innervate muscles derived from the branchial arches (like the muscles of facial expression, mastication muscles, and the laryngeal muscles. Both somatic and branchial motor nerves are efferent motor nerves that innervate muscles involved in voluntary motor pathways. For the purposes of our knowledge a distinction is unnecessary.
Anatomy of the accessory nerve
The accessory nerves are paired nerves that innervate the ipsilateral SCM and contralateral trapezius muscles.
Information from the premotor association cortex and other cortical areas feeds to the motor cortex via association fibres.
Axons of the cortical neurons descend in the corticospinal tract through the posterior limb of the internal capsule.
Neurons destined to:
Supply the SCM muscle
Descend to the ipsilateral accessory (spinal) nucleus in the lateral part of the anterior grey column of the upper 5-6 segments of the cervical spinal cord (approximately in line with the nucleus ambiguous).
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Supply the trapezius muscle
Axons cross the midline in the pyramidal decussation to synapse in the contralateral accessory nucleus.
From the accessory nucleus, postsynaptic fibres emerge from the lateral white matter of the spinal cord as a series of rootlets to form the accessory nerve.
The rootlets emerge posterior to the ligamentum denticulatum (denticulate ligament), but anterior to the dorsal roots of the spinal cord.
The rootlets form a nerve trunk that ascends rostrally in the subarachnoid space and parallel to the spinal cord as far as the foramen magnum.
At the foramen magnum, the nerve passes posterior to the vertebral artery, to enter the posterior cranial fossa.
The fibres join caudal fibres from cranial nerve X and then separate from them within the jugular foramen.
As the accessory nerve emerges from the jugular foramen, it passes posteriorly, medial to the styloid process, descends obliquely, and enters the upper portion of the sternomastoid muscle on its deep surface. Some of the fibres terminate in this muscle, and the remaining fibres pass through the muscle to emerge at the midpoint of its posterior border.
These fibres then cross the posterior triangle of the neck, superficial to the levator scapulae where they are closely related to the superficial cervical lymph nodes. Five centimetres above the clavicle, the nerve passes deep to the anterior border of the trapezius to supply it.
Anatomy of the neck
There are various classifications of the neck areas which serve different purposes.
Triangles of the neck
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