Accessorius

Accessorius is the Latin adjective meaning “accessory” and is used in anatomical nomenclature to denote structures that assist or complement others. In human anatomy the term is most familiar in reference to the nervus accessorius, or the accessory nerve (cranial nerve XI).

The accessory nerve is commonly described as having two roots: a spinal root that arises from the ventral horns of the upper cervical spinal cord (typically C1–C5 or C6) and a cranial root that originates from the medulla oblongata. The spinal portion ascends into the skull and typically joins the cranial portion; together they exit the skull through the jugular foramen alongside the glossopharyngeal and vagus nerves. Some authorities consider the cranial component to be part of the vagus nerve rather than a separate nerve.

Functionally, the nervus accessorius is a motor nerve that innervates two main muscles: the sternocleidomastoid and the trapezius. Through these muscles, it enables head rotation and tilting (via the sternocleidomastoid) and shoulder elevation, scapular rotation, and stabilization (via the trapezius), which collectively facilitate arm abduction beyond about 90 degrees.

Clinical considerations include weakness or paralysis from nerve injury, which can result in difficulty turning the head to the opposite side, shoulder droop, and limited ability to raise the arm above the horizontal. Causes of injury include surgical trauma in the neck, blunt or penetrating trauma, and compression.

See also nervus accessorius; cranial nerves.