The vestibulocochlear nerve is a sensory nerve cord composed of the cochlear nerve, the auditory nerve, and the vestibular nerve, the vestibular nerve. The nerve cord is also referred to as the 8th cranial nerve. The afferent sensory nerves transmit auditory and vestibular messages to the corresponding brain nuclei. The auditory nerve in particular also contains efferent fibers that make adjustments possible via “instructions” from the corresponding brain nuclei to the auditory organ.
What is the vestibulocochlear nerve?
In the inner ear, the organs for balance feedback and for hearing are practically located together because they also form a single unit in evolutionary terms. The afferent derivatives of the auditory organ, together with their efferent feeders, are referred to as the cochlear nerve because the translation of received sound waves into nerve impulses takes place in the cochlea, the auditory cochlea. The afferent sensory fibers of the vestibular organs are called the vestibular nerve. The two nerve cords together form the 8th cranial nerve called the vestibulocochlear nerve. The vestibular nerve is composed of afferent fibers from each of the vestibular organs (3 arcuate and 2 otolith organs each). The nerve cords of the auditory nerve and the vestibular nerve join to form the vestibulocochlear nerve, which is surrounded by a common connective tissue sheath and extends into the brainstem. Just before reaching the cranial nerve nucleus or the cochlear and vestibular ganglia, the two nerve cords separate again. The cochlear and vestibular ganglia are each composed of several nerve nuclei, including one nucleus of the vestibular apparatus consisting of a collection of Purkinje cells of the cerebellum with a widely ramified system of dendrites.
Anatomy and structure
The vestibulocochlear nerve consists essentially of afferent sensory nerve fibers that report nerve impulses from the cochlea and vestibular organs to their ganglia or nuclei. These are axons surrounded by a common myelin sheath after union of the cochlear and vestibular nerves. The corresponding cranial nerve nuclei are responsible for further processing and distribution of the impulses. The nuclei vestibulares, for example, provide further wiring of information from the vestibular apparatus. Efferents travel to the thalamus, into the cerebellum, and to the nuclei of the eye muscles and the spinal cord. Thus, the vestibulo-ocular eye reflex can be activated almost without distortion because the eye muscles are directly induced to make appropriate contractions via the nuclei. The cochlear nerve, which participates in the vestibulocochlear nerve, unites about 30,000 fibers into one nerve cord, each for the left and right ear. The fibers also consist mostly of somatosensory afferent fibers, but also contain efferents. The so-called auditory pathway has a complex branching structure with a number of nerve nuclei in different brain regions and diverges strongly in the brainstem into parallel processing pathways.
Function and tasks
The main function of the somatosensory afferent fibers of the vestibulocochlear nerve is to transmit nerve impulses generated in the cochlea or vestibular organs by mechanoreceptors to the corresponding nerve nuclei, which perform initial processing of the signals. The signals coming in the opposite direction via the efferent fibers from specific brain regions or from the nuclei are transmitted to the vestibular organs or to the auditory organs, where they are converted. The interconnections of the cochlear as well as the vestibular afferents in different nuclei and brain regions are very complex, because the somatosensory impulses are partly information that is made available to various organs “in copy” in order to be able to trigger certain reflexes such as the vestibulo-ocular reflex without time delay and because it is a subset of multisensory information that is not always compatible with each other, so that in the case of incompatibility the brain must decide which of the information is “correct” or which is “wrong”. which is “wrong”. If visual impressions also play a role at the same time, these are always dominant and incompatible vestibular messages are suppressed. The same applies to cochlear somatosensory impulses.The impulses sent via the afferent fibers of the vestibulocochlear nerve enter our consciousness in a manipulated form only after appropriate processing by the responsible brain areas.
Diseases
Disease symptoms and dysfunctions related to the vestibulocochlear nerve are similar to symptoms that would result from a dysfunction of the cochlea or the vestibular organs themselves, because somatosensory afferent signals from the organs that are not relayed or are relayed incorrectly have a similar effect. Dysfunction of the vestibulocochlear nerve can result from traumatic brain injury (SHT), brain tumor, or other lesion of the nerve. Similarly, nerve inflammation, in this case vestibulocochlear neuritis, is often the cause of symptoms. Such nerve inflammation can be caused by a viral infection or by circulatory problems of various kinds. Lesions of the vestibulocochlear nerve caused by a SHT may manifest as mild to severe balance disorders, dizziness, and malaise, as well as limitations in hearing and even unilateral deafness. Unilateral balance disorders may also present with nystagmus of the eyes, an unconscious eye movement with a repetitive frequency similar to that seen with rotational accelerations and stopping of a rotational acceleration. Another symptom may be failure of the vestibulo-ocular eye reflex. In this case, when walking and running, the risk of stumbling and falling is great, because the eyes are not stabilized and the eyes follow only the much slower voluntary correction. If no organic disease is apparent in the organs themselves or in the vestibulocochlear nerve, symptoms of vertigo, tinnitus, and decreased hearing may also be due to prolonged stress.
