The accessorius nerve is a motor nerve known as the eleventh cranial nerve. It has two distinct branches and innervates the sternocleidomastoid and trapezius muscles for motor function. Damage to the nerve can result in head-turning or trapezius palsy.
What is the accessorius nerve?
In the human body, the nervous system consists of motor, sensory, and mixed nerves. Sensory nerves are responsible for transporting sensations in the form of excitation. Motor nerves are responsible for active responses to the environment in the form of reactive movements and voluntary movements. Mixed nerves are nerves with parts of sensory as well as motor fibers. The accessorius nerve or eleventh cranial nerve is a motor nerve consisting of two distinct branches in terms of rami. The ramus internus originates in the brainstem and the ramus externus originates in the spinal cord. Cranial nerves are all those nerves that originate directly from the specialized nerve cell assemblies or cranial nerve nuclei in the brain area. Like a portion of the accessorius nerve, most cranial nerves arise directly from the brainstem. Although a different portion of the accessorius nerve arises from the spinal cord, it is included among the cranial nerves. The eleventh cranial nerve was first described by Thomas Willis and, because of its two distinct origins, its anatomy includes a spinal cord root and a cranial root. The radix spinalis or spinal cord root of the accessorius nerve originates from the upper cervical segments on the spinal cord. The radix cranialis, or cranial root, takes its origin below the vagus nerve, where it emerges from a groove called the sulcus posterolateralis within the medulla oblongata.
Anatomy and structure
In the lateral region, the radix spinalis emerges from the spinal cord. The fibers of the root originate in a motor neuron cluster called the nucleus motorius nervi accessorii or nucleus principalis nervi accessorii. Along the spinal cord, the individual nerve fibers ascend in the subarachnoid space. They pass through the foramen magnum in the area of the posterior fossa. The cranial root receives branchio-motor fibers from the so-called nucleus ambiguus, whose fibers participate in several cranial nerves. The fibers of the ramus externus and ramus internus reach union within the skull and exit the skull through the foramen jugulare, where they separate again. Intracranially lateral to the medulla oblongata, the ramus internus passes and sends fibers to the ganglion jugulare. After separating outside the skull, the rami join the vagus nerve and branches to the pharynx and larynx. The ramus externus enters the funiculus lateralis of the spinal cord and extends cranially to leave the spinal cord at the sulcus lateralis posterior and enter and exit the foramen magnum as an independent nerve cord. The ramus externus, after exiting the skull, runs caudally downward and passes ventrally or dorsally along the internal jugular vein. Thus, the ramus reaches the sternocleidomastoid and trapezius muscles, where it receives fibers from the cervical plexus and forms a plexus.
Function and Tasks
The accessorius nerve is a motor nerve. As such, it is responsible for the motor connection of muscles to the central nervous system. Motor nerves transmit efferent commands from the central nervous system to muscles, causing them to contract or relax. The radix spinalis of the accessorius nerve, in the form of the ramus externus, supplies the sternocleidomastoid and trapezius muscles with motor fibers and is consequently involved in the contraction of these two muscles. The trapezius muscle lies on both sides of the upper spine and runs from the occiput to the lower thoracic vertebrae. Laterally, it extends to the scapula. The trapezius muscle is responsible for various movements. It is responsible in the lifting of the arms above the horizontal, is solely involved in the rotation of the scapula upwards and towards the center. The sternocleidomastoid muscle is a ventral neck muscle known as the great head turner. It causes lateral head tilt toward the shoulder and is involved in slight posterior head extension.Both muscles motor innervated by the accessorius nerve are presumably sensitively innervated via rami musculares of the cervical plexus.
Diseases
Clinically, the condition of the accessorius nerve is checked by having the patient turn his or her head against resistance. If the nerve is paralyzed, the affected shoulder hangs. This phenomenon corresponds to trapezius palsy, which prevents elevation of the arm above the horizontal. Proximal damage to the nerve is associated with tumors of the skull base. Proportional paralysis is often preceded by removal or biopsy of the cervical lymph nodes within the lateral triangle of the neck, as is done for suspected tuberculosis and other lymphomas. Less commonly, lesions of the accessory nerve are due to whiplash injuries. Equally rare are anomalies of the craniocervical junction or skull base fractures. In patients undergoing radiation therapy, lesions of the nerve may correspond to radiation damage. Distal nerve damage to the accessory nerve is usually preceded by surgical excision or other disease of the cervical lymph nodes. In addition, syringomyelia and poliomyelitis can damage the accessorius nerve in the anterior horn of the spinal cord, causing functional impairment of the muscles it innervates. Syringomyelia is usually associated with cerebrospinal fluid outflow disorders. Poliomyelitis is polio, which is due to a viral trigger.
