The spinal accessory nerve arises from the ventral rami of the cervical spinal cord between C2-4. The rootlets arise from the posterior half of the spinal cord and ascend up into the skull through the foramen magnum. The spinal accessory briefly joins the cranial accessory nerve before they separate again.
The spinal accessory nerve continues out through the jugular foramen anterior to the internal jugular vein in the intermediate compartment . It passes deep to the tip of the styloid process and then deep to the posterior belly of digastric and the occipital artery. It supplies sternocleidomastoid before piercing the muscle and entering the posterior triangle of the neck. The nerve terminates as it innervates trapezius.
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