What is the anatomy of the spinal accessory nerve (SAN)?

Updated: Feb 09, 2018
  • Author: Rohan R Walvekar, MD; Chief Editor: Arlen D Meyers, MD, MBA  more...
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Answer

The spinal accessory nerve (SAN) is formed by 2 parts. The spinal or motor portion includes fibers that originate in the ventral horn of the upper 4 or more cervical segments of the spinal cord. The fibers may originate from as low as the fifth and rarely the seventh cervical segment. These fibers ascend lateral and parallel to the spinal cord, entering the skull through the foramen magnum. These fibers join the second or accessory component of the SAN that originates in the nucleus accumbens brain stem nucleus of the medulla oblongata in the posterior fossa. The 2 parts of the SAN, now joined, leave the skull through the jugular foramen in the same dural compartment as the vagus nerve. The nerve passes through the jugular foramen and then divides variably into the 2 original components.

The superior branch, also known as the accessory or internal branch, joins the vagus either directly or through the ganglion nodosum and then contributes to the pharyngeal, laryngeal, and cardiac sympathetic fibers.

The inferior branch, also known as the spinal or lateral branch, is essentially a pure motor nerve and innervates the sternocleidomastoid (SCM) and trapezius muscles. [27] It passes beneath the posterior belly of the digastric and the upper end of the SCM muscle along the internal jugular vein. It may travel either anterior or posterior to the occipital artery, and it communicates with the second cervical nerve before it enters the SCM. [28]

The nerve emerges from the posterior border of the SCM and obliquely crosses the posterior cervical triangle downward before entering the trapezius (see the image below). The posterior or lateral cervical triangle is bordered ventrally by the SCM, dorsally by the trapezius muscle, and caudally by the clavicle. [6] The length of the SAN can vary from 4-5 cm when it is lax (chin pointing forward) to 9-10 cm when it is extended (chin pointing to the opposite shoulder). [2, 29]

Course of the spinal accessory nerve (SAN) in the Course of the spinal accessory nerve (SAN) in the posterior cervical triangle. DG = posterior belly of digastric muscle; T = trapezius; LS = levator scapulae; IJV = internal jugular vein; black arrow = SAN.

The coiled and redundant nature of the nerve as it traverses the posterior triangle allows for this variability in length. A recent cadaveric study compared this distal portion of the spinal accessory nerve with the less redundant proximal segment between the skull base and sternocleidomastoid muscle. The authors concluded that this coiled, redundant appearance was purely a functional characteristic of the nerve, as the 2 segments were identical under electron microscopy. [30]


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