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The accessory nerve is the eleventh of the 12 cranial nerves. It possesses both cranial and spinal root components with the latter originating from the lateral gray matter of the C1 to C5 parts of the spinal cord [rx–rx]. The cervical spine contribution enters the skull through the foramen magnum and coalesces with the cranial roots to form a trunk which exits the skull through the jugular foramen [rx–rx]. After exiting the skull, the accessory nerve descends into the neck and branches into the sternocleidomastoid (SCM) and trapezius muscles [rx–rx].
The fibres of the spinal accessory nerve originate solely in neurons situated in the upper spinal cord, from where the spinal cord begins at the junction with the medulla oblongata, to the level of about C6.[rx][rx] These fibres join together to form rootlets, roots, and finally the spinal accessory nerve itself. The formed nerve enters the skull through the foramen magnum, the large opening at the skull’s base.[rx] The nerve travels along the inner wall of the skull towards the jugular foramen.[rx] Leaving the skull, the nerve travels through the jugular foramen with the glossopharyngeal and vagus nerves.[rx] The spinal accessory nerve is notable for being the only cranial nerve to both enter and exit the skull. This is due to it being unique among the cranial nerves in having neurons in the spinal cord.[rx]
After leaving the skull, the cranial component detaches from the spinal component. The spinal accessory nerve continues alone and heads backward and downwards. In the neck, the accessory nerve crosses the internal jugular vein around the level of the posterior belly of the digastric muscle. As it courses downwards, the nerve pierces through the sternocleidomastoid muscle while sending it motor branches, then continues down until it reaches the trapezius muscle to provide motor innervation to its upper part.[rx]
The fibres that form the spinal accessory nerve are formed by lower motor neurons located in the upper segments of the spinal cord. This cluster of neurons, called the spinal accessory nucleus, is located in the lateral aspect of the anterior horn of the spinal cord, and stretches from where the spinal cord begins (at the junction with the medulla) through to the level of about C6. The lateral horn of high cervical segments appears to be continuous with the nucleus ambiguus of the medulla oblongata, from which the cranial component of the accessory nerve is derived.[rx]
The spinal accessory nerve innervates two muscles – the sternocleidomastoid and trapezius.
- Attachments – Runs from the mastoid process of the temporal bone to the manubrium (sternal head) and the medial third of the clavicle (clavicular head).
- Actions – Lateral flexion and rotation of the neck when acting unilaterally, and extension of the neck at the atlanto-occipital joints when acting bilaterally.
- Attachments – Runs from the base of the skull and the spinous processes of the C7-T12 vertebrae to the lateral third of the clavicle and the acromion of the scapula.
- Actions – It is made up of upper, middle, and lower fibers. The upper fibres of the trapezius elevate the scapula and rotate it during abduction of the arm. The middle fibres retract the scapula and the lower fibres pull the scapula inferiorly.