Anatomists secrete twelve pairs of nerves that have specific functions and are located within the head and neck. One of them is the vestibulo-cochlear nerve. He is responsible for special sensitivity: hearing and a sense of balance. Violation of its function or anatomy can lead to a deep disability of a person.
Structure
What is the vestibulo-cochlear nerve? Its anatomy is quite complicated, since, based on the name, it includes two separate roots having different functions. The first is the vestibular, is responsible for balance and innervates the semicircular canals of the inner ear. The second - auditory, conducts impulses from the labyrinth of the cochlea to its root.
The nerve originates on the lower surface of the hemispheres, emerging from the gray matter at the olive kernels in the medulla oblongata and located below the facial nerve. The auditory branch starts from the nodule of the cochlea, and its peripheral processes end in the spiral organ, and the central branch extends through the top of the pyramid of the auditory bone into the brain and reaches the cochlear nuclei.
The second, vestibular, branch also begins with a nodule, which is located in the inner ear. The dendrites of these neurons go to the semicircular canals, spherical and elliptical sacs. And the axon as part of the vestibular root goes into the rhomboid fossa and ends there, on the vestibular nuclei.
Hearing support
The system of perception of sound in humans is quite complicated. There is an outer, middle and inner ear, but the vestibulo-cochlear nerve innervates exclusively the inner part. At first, the sound wave is perceived by the eardrum. Its vibrations are transferred to the hammer, anvil and stapes, interconnected. A swirling wave touches an oval window located in anticipation of the maze. Fluctuations cause movements of the perilymph and endolymph within the maze. Together with the fluid, the sections of the secondary tympanic membrane, or basilar plate, also oscillate. On it are sound-picking hairs that generate a nerve impulse. It is transmitted to a spiral node located in the inner ear. The processes from the nerve cells that make up the node exit through the hole in the ear canal and, connecting with the vestibular nerve, go to the bridge, where they end in the substance of the cochlear nuclei in the rhomboid fossa.
Axons of cochlear neurons intersect and form a lateral loop. Then the fibers are separated. A small part of them ends on the lower mounds of the quadruple plate (midbrain). The rest go to the medial cranked bodies in the diencephalon or to the median nuclei of the thalamus.
Equilibrium function
For the balance of the body in space during movement and at rest, the vestibulo-cochlear nerve is also responsible. The scheme of its innervation in the uninitiated can cause a flutter, since to ensure this function, the synchronous work of many parts of the nervous system is necessary.
The main function of the vestibular apparatus is to analyze the position of the head in space at each moment of time and adjust the position of the body and muscle tone. The organ responsible for balance is located next to the labyrinth in the middle ear and consists of three intersecting oval-shaped channels that end in elliptical and spherical sacs. Inside these structures are hairs that are sensitive to changes in the position of the head, angular and linear acceleration, as well as changes in gravity.
From sensitive hairs, the peripheral processes of neurons are directed to the vestibular node located at the bottom of the temporal bone. Entering the substance of the brain, the nerve goes into the rhomboid fossa to the vestibular nuclei. From the bridge, the processes of neurons diverge into the spinal cord (to the nuclei of the anterior horns), the cerebellum (the cortex of the worm), the thalamus (vestibular nuclei) and the reticular formation (nuclei of the cranial nerves). All of these structures provide friendly body responses to vestibular receptor irritation. All information from the subcortical structures enters the region of the middle and lower temporal gyrus, where the center of motor functions, the center of general sensitivity and the center of the body circuit are located.
Hearing tests
What should be done to check whether the vestibulo-cochlear nerve performs its functions well? Its two branches are examined separately. Hearing tests are performed by ENT doctors, neuropathologists, and even psychiatrists; therefore, tests uniform for all specialties have been developed.
It all starts with a simple hearing test. Normally, a person should hear whispering addressed to him from a distance of five meters. Hearing loss or its absence can cause not only damage to the outer or middle ear, but also the inner one. Therefore, it is so important to understand the causes of the disease.
- The Schwabach test is based on measuring the duration of bone conduction. The tuning fork is turned on and mounted on the mastoid process behind the ear. If the patient does not hear the sound, then the problem is in the inner ear, if the sound is heard longer than necessary, then the pathology is in the middle section of the analyzer.
- The Rinne test determines the difference between air and bone conduction. The included tuning fork is set on the mastoid process, and the patient is asked to say when he will no longer hear the sound. After that, the instrument is transferred to the auricle. In the case when the patient is healthy, the sound will still be heard.
- Weber test. The newly turned on tuning fork is placed on the parietal region of a person, and the doctor asks which side the sound is heard better. If the patient indicates a sick side, then this speaks in favor of damage to the middle ear, and if it is healthy, then the problems are in the inner ear.
Balance assessment
The vestibulo-cochlear nerve is also responsible for balance, so neuropathologists often resort to various tests during a comprehensive examination to check the patient for stability:
- Romberg pose is one of the most common options. The patient is asked to stand up so that the feet are on the same line, and the heel of one leg rests on the toe of the second. Hands should be spread apart or straightened in front of you. Then the doctor asks you to take a few steps forward, first with your eyes open, and then with your eyes closed. Unsteadiness of gait in the second case indicates damage to the inner ear.
- Mittelnaer test. The patient steps in place with his eyes closed. If there is a lesion of the vestibular apparatus, then gradually it will turn towards the focus.
The defeat of the cochlear branch
The defeat of the vestibulo-cochlear nerve in the area responsible for the processing of auditory impulses has specific clinical manifestations. There are two options for reducing:
- violation of sound conduction, or conductive hearing loss (damage to the middle ear);
- sensorineural hearing loss with damage to the inner ear.
In the first case, the causes of the condition may be inflammatory processes, tissue sclerosis, or neoplastic diseases. The second variant of the disease can also be caused by inflammatory phenomena, neuroma, as well as damage to brain matter in the areas of the nuclei of the eighth pair of cranial nerves.
Clinically, this is manifested by complaints of ear noise, headache, and general hearing loss. If the pathological process is located in the thickness of the brain, then there may be a loss of functions and neighboring nerves, such as the vestibular, trigeminal and facial. This community of symptoms is called "alternating syndrome."
The defeat of the vestibular part
Pathology of the vestibulo-cochlear nerve in the area of the vestibular branch will primarily manifest itself as dizziness, nausea (sometimes with vomiting) and nystagmus. This nerve is partly responsible for the position of the eyeballs when the position of the head changes, so when it is damaged, a change in the movement of the eyes can be observed. Namely, small horizontal or vertical twitches.
In addition, the patient has an unstable gait, and he needs to widely spread his legs (like on a ship during pitching) in order to maintain balance, as well as constantly monitor his legs. Therefore, in such people, the doctor can guess the diagnosis at the moment when they enter his office.
Neurinoma of the vestibulo-cochlear nerve
The innervation of the vestibulo-cochlear nerve suggests that its fibers are covered by a membrane of Schwanian cells. This is a kind of isolation, so that the nerve impulse does not pass to other fibers. But in rare cases (one in one hundred thousand people), a benign tumor can grow from the cells of the membrane.
It manifests itself slowly and, as a rule, when the neoplasm has already reached considerable size. Patients complain of hearing loss on one side, dizziness, pain in half of the face, as well as the presence of a combined pathology of the facial and abducent nerve. This is manifested by impaired speech, difficulty in eating. The tumor compresses the nerve endings, which causes the corresponding clinic.
If neurinoma has arisen on both sides, then such a patient is recommended to undergo a genetic examination for the presence of neurofibromatosis (hereditary connective tissue disease). Treatment is usually surgical.
Meniere's Syndrome
The vestibulo-cochlear nerve can be indirectly damaged in Meniere's disease. The pathology itself is associated with a violation of the production and outflow of fluid in the inner ear. Its excess exerts pressure on sensitive hairs, which is manifested in imbalance.
The disease manifests itself with bouts of dizziness, which are accompanied by tinnitus and a feeling of fullness on the affected side. In addition, patients complain of progressive hearing loss. As the disease progresses , the symptoms intensify, and can go so far that the person cannot get out of bed or turn his head during an attack.
Treatment comes down to stopping unpleasant sensations during an attack and taking sedative medications at bright intervals. If conservative therapy does not help, they resort to a radical remedy and destroy the labyrinth or cross the vestibular branch of the vestibulo-cochlear nerve.