The accessory nerve (in English ” accessory nerve “) is the eleventh nerve belonging to the group of twelve cranial nerves (XI cranial nerve) and is a motor nerve that has thepeculiarity of being equipped with both spinal root (which originates from the spinal cord) and a cranial root (which originates from the medulla oblongata). Its main function is the motor function relative to the shoulder-trapezium-neck area.
Spinal portion of the accessory nerve
The spinal part of the XI cranial nerve originates from the lower portion of the bulb, up to the anterior horns of the cervical medullary portion (C1-C5) and represents the spinal accessory nerve. Its core is composed of a series of motor neurons arranged in the anterior horns of the spinal cord. The radicles emerge from the lateral face of the spinal cord and along their ascent merge with each other into a single trunk that runs through the foramen. Here it joins the fibers of the cranial portion and together they will cross the posterior lacerated hole behind the vagus nerve. It continues downwards and laterally, crosses the internal jugular vein posteriorly and then reaches the SCOM (sternocleidomastoid). From here it proceeds downwards and is brought medially passing over the shoulder blade and then reaching thetrapezius muscle . The trapezius is not, however, innervated only by the spinal accessory nerve but, in many individuals, it is also disputed by the innervation of the upper portion of the cervical plexus. Along its course, the spinal accessory nerve also releases proprioceptive sensitive fibers to the aforementioned muscles. Lesions of this nerve lead to the establishment of an ipsilateral weakness in the trapezius muscle .
Cranial portion of the accessory nerve
The cranial portion of the accessory nerve is composed of somatic motor neurons deriving from the caudal part of the ambiguous nucleus, the axons of these neurons travel within the vagus nerve taking the name of the accessory vagus nerve . This portion is not considered as a stand-alone nerve, but as part of the vagus nerve. Its fibers emerge just below the emergence of the fibers of the vagus nerve. Their function is to innervate part of the musculature of the soft palate, the adductor muscles of the vocal cords. Some of its fibers participate, albeit in a not very defined way, in the visceral innervation provided by the vagus nerve. Its injury causes difficulty in head rotation and elevation of the scapula .