The auditory nerve is one of the most important nerves, as it is responsible for transmitting acoustic information to the brain. If its function is disturbed – this can happen, for example, due to infections of the inner ear, strong noise or circulatory disorders – the hearing ability of the affected person is reduced. In the most severe cases, deafness may even result. Impairments in sound transmission or misinterpretation of signals should definitely be taken seriously, otherwise the patient will experience a severe impairment of his or her quality of life.
What is the auditory nerve?
The auditory nerve (vestibucochlear nerve) is the eighth of a total of 12 cranial nerves that lead from the balance organ of the inner ear to the brain. It is not a single nerve cord, as is the case with other nerves, but a twin nerve. It consists of the cochlear nerve – formerly called the acustic nerve – and the vestibular nerve (older name: static nerve). The cochlear nerve is the actual auditory nerve, while the vestibular nerve is the vestibular nerve. Both nerve cords run parallel until they unite in the brainstem to form the vestibucochlear nerve. The auditory nerve and vestibular nerve are located outside the inner ear.
Anatomy and structure
The cochlea and organ of balance, located in the inner ear, are filled with lymph fluid containing tiny hair cells. The auditory nerve originates from the center of the convolutions of the cochlea and terminates in the two auditory nuclei, ventral cochlear nerve and dorsal cochlear nerve, in the myelencephalon. From these auditory nuclei, nerve pathways lead to the auditory cortex (Heschl’s transverse convolutions) in the cerebrum. The vestibular nerve runs along the floor of the internal bony auditory canal. Six of its nerve cords are connected to the receptors of the vestibular organ in the inner ear.
Function and tasks
The hair cells of the cochlea are stimulated by movements of the lymph – triggered by the pressure oscillations of incoming sound waves – to emit nerve impulses, which the auditory nerve receives and, after exiting the cochlea, transmits in the form of bio-electrical signals via afferent pathways to the brain, where they are analyzed and evaluated in the auditory cortex (cerebral cortex). After stimulus processing, the acoustic signals are sent from there via efferent pathways to the inner ear, which then fine-tunes the sense of hearing. The hair cells located in the vestibular organ detect changes in movement and direction and also send out impulses. They are then sent via the vestibular nerve along afferent pathways to the brain (brainstem) for information processing and evaluation. From there, they reach the cerebellum. There, as a result of a comparison of the incoming information with other sensory impressions from the muscle and eye sensors, the body’s sense of position is created. It is necessary for humans to perform coordinated movements.
Diseases
If the transmission of acoustic stimuli is disturbed or even temporarily or permanently damaged, diseases of the auditory nerve occur. It can be impaired in its function by acoustic neuroma, by tinnitus, hearing loss, deafness, inflammation and other diseases. Acoustic neuroma is a slow-growing benign tumor, but if its spread is not stopped, it can become life-threatening for the patient. It usually forms only on one side of the vestibular nerve or proliferates between the auditory and vestibular nerves. The factors involved in its development are still unclear. As it grows, the acoustic neuroma presses on the auditory nerve and therefore interferes with the transmission of information to the cerebral cortex. The bio-electrical signals no longer arrive in their correct form because of the growth: The patient hears something being said, but does not understand what was said. The main symptoms of the disease are unilateral hearing loss and hearing noises (tinnitus). In a later stage of the disease, the facial nerve may also be affected. If the tumor is so large that it causes serious health problems, the patient must undergo surgery in a specialized ENT clinic. Hearing loss is a sudden loss of hearing or hearing loss, usually occurring on one side. The affected person first has the feeling of hearing everything as if through absorbent cotton, then tinnitus usually develops.Due to the damage to the hair cells, the sound vibrations arriving in the ear can no longer be properly absorbed. The exact causes of the disease are not known. Doctors assume that it is triggered by circulatory disorders in the inner ear. Diabetes mellitus, strokes and infections can also cause it. Damage to the auditory nerve can also lead to sensorineural hearing loss and deafness. Sound is no longer transmitted properly through the eardrum and ossicles. The cause is usually a pathological change in the auditory nerve due to infection or injury. Tinnitus refers to hearing certain tones and sounds that have no external sound source. The patient hears ringing, buzzing, etc. Tinnitus patients usually have a hearing loss before the first onset of symptoms. In mild cases, perception and performance of the affected person are not affected. Chronic tinnitus with frequent misperceptions can even lead to incapacity to work. Causes of tinnitus include degenerative diseases of the inner ear, obstruction of the auditory canal, or stress. Other diseases that lead to damage of the auditory nerve are herpes zoster oticus. In this case, herpes viruses not treated with virustatics attack the tissues of the auditory nerve. In addition, meningitis pathogens can infect it. Auditory neuropathy occurs when the hair cells are not properly connected to the auditory nerve. Impaired auditory nerve also occurs in multiple sclerosis.
Typical and common ear disorders
- Ear flow (otorrhea)
- Otitis media
- Ear canal inflammation
- Mastoiditis
- Ear furuncle
