Definition of Accessory Spinal Nerves

Accessory spinal nerves or spinal accessory nerves are older names for cranial nerve number XI, the accessory nerves. These nerves are unique among the 12 cranial nerves in that they have both cranial and spinal origins to provide muscle innervation of the neck and back. The accessory nerves combine with the vagus nerve lower in the spinal cord. This co-mingling of nerve fibers is where the name accessory nerve arises.

Origin of Accessory Nerves

• Each cranial nerve is paired and has a left and right branch for each side of the body. The accessory nerve has a cranial branch originating in the medulla oblongata and a spinal branch originating from the spinal cord in the C1 to C5 cervical region.

Cranial Branch Function

• The cranial branch of the accessory nerve originates directly from the brain and combines with fibers from another cranial nerve, the vagus nerve, to control or activate muscles of the voice box, throat and roof of the mouth. The cranial branch is the only part of the accessory nerve to mix with the vagus nerve.

Spinal Branch Function

• The spinal branch starts in the spinal cord and enters the skull through the largest opening in the cranium; the foramen magnum. It then temporarily joins with the cranial branch and leaves the skull through the same opening as the jugular vein; the jugular foramen. These branches then split into the different nerve branches. The spinal branch controls the sternocleidomastoid and trapezius muscles, large neck and back muscles.

Injury to Accessory Nerves

• The results of an injury to an accessory nerve depends on severity. If both nerves are damaged or if the nerve is completely severed the damage is total. If only partial damage is done then there is a chance for recovery.

Muscle Paralysis

• Since the trapezius and sternocleidomastoid are the largest muscles served by the accessory nerve, damage or paralysis is most obvious in these two muscles. However, the cranial branch innervates, which means activates or controls, muscles used for chewing, speaking and swallowing. So damage to the cranial branch may have a large impact on one or all of these activities.