Vestibulocochlear Nerve: What It Is And What Functions It Has
The vestibulocochlear nerve is the eighth cranial nerve of nerves and its function is essential for our survival, since it is thanks to it that we can hear and maintain our balance.
The way it sends information to the brain is somewhat complex, involving multiple pathways and activating several regions and specific types of neurons. Let’s look in greater depth at the importance of this nerve.
The vestibulocochlear nerve (scientific name: nervus vestibulocochlearis) is the eighth cranial nerve (CN VIII), which It is divided into two parts, the vestibular and the cochlear, both divisions being in charge of sensory function. This nerve carries somatic afferent fibers from structures of the inner ear. While the cochlear part of the nerve is responsible for the sense of hearing, the vestibular part is responsible for aspects related to balance.
The vestibulocochlear nerve is responsible for sending information from the cochlea and vestibule to the brain stimuli which will be interpreted in the form of sound and balance.
When a sound reaches the ear, the sound waves impact the internal structures of the ear, making them vibrate. The cochlea transforms these vibrations into electrical impulses, which travel through a series of structures that flow into the auditory cortex of the brain.
Regarding balance, When we move our head the vestibule detects these movements and sends signals to the brain to indicate what position we are in or if we have lost our balance momentarily. Within this structure we have a liquid that, when moving, activates cells, called cilia or hair cells of the ear, which act as sensors. These cells send the signal to the brain, an organ that will activate the muscles necessary to correct position and maintain balance.
Parts of this nerve
Below we will look at both sections in greater depth:
1. Cochlear nerve
The cochlear nerve (scientific name: nervus cochlearis) is one of the two divisions of the vestibulocochlear nerve, responsible for hearing.
At the beginning of this section it is located in the sensory receptors of the organ of Corti traveling through the inner ear until it reaches the brain, where the auditory stimulus is processed.
Auditory information first passes through the thalamus and subsequently reaches the auditory cortex of the temporal lobe.
The cells responsible for receiving the auditory stimulus are hair cells found in the organ of Corti, which is located in the cochlea.
The information is sent to pseudounipolar neurons located in the spiral ganglion, located in the center of the cochlea. The axons of these pseudounipolar neurons are what form the cochlear nerve itself.
After leaving the cochlea, the nerve enters the internal meatus where it joins the vestibular nerve, forming the vestibulocochlear nerve itself.
Both sections of the complete nerve travel to the posterior cranial fossa, entering the brain through the cerebellopontine angle along with the facial nerve (CN VII).
In the brainstem pons, cochlear nerve fibers synapse with the posterior and anterior cochlear nuclei. The axons of the anterior nucleus form the trapezoid body.
Many of these fibers decussate and end up in the superior olive complex The axons of the neurons that reach here, together with those of the posterior cochlear nucleus, form the lateral lemniscus, which travels until it reaches the inferior colliculus and the medial geniculate bodies.
Axons from the medial geniculate nucleus form the acoustic radiation of the brain, which passes through the internal capsule and ends in the superior temporal gyrus and the transverse temporal gyrus (Brodmann areas 41 and 42). Here they synapse with cortical neurons.
2. Vestibular nerve
The vestibular nerve (nervus vestibularis) is the other division of the vestibulocochlear nerve. It receives the stimulation captured by the sensory receptors located in the membrane of the auditory labyrinth.
The vestibular nerve is responsible for the sense of balance spatial orientation and motor skills.
Most of the fibers of this nerve end up in the brain, in the vestibular nuclei, but some of them go directly to the reticular nuclei without having to make synapses along the way, and they also end up in the cerebellar nuclei.
The vestibular nerve arises from the receptors of the macules of the inner ear, specifically the utricle and the saccule in addition to the receptors of the semicircular canals of the membranous labyrinth.
The receptors receive primary stimuli, and the neurons of the vestibular ganglion transmit the information from the receptors through their dendrites.
Axons arising from neurons in the vestibular ganglion form the vestibular nerve, which joins its partner, the cochlear nerve in the internal meatus of the ear, forming the vestibulocochlear nerve.
Fibers from the vestibular nerve reach the vestibular area in the brain, where they synapse with the vestibular nuclei. The axons of the neurons in these nuclei travel in several directions:
Injuries to the vestibulocochlear nerve
Damage to this nerve can affect the sense of hearing and balance, which manifest themselves mainly in the form of hearing loss, vertigo, dizziness, false sensation of movement and loss of balance. When this nerve is affected, it is usually due to tumors, such as acoustic neuromas, which hinder its functioning.
To evaluate damage to this nerve, fingers are placed in both ears and clicked asking the patient if he hears the sounds bilaterally and if they are equal in intensity.
It should be said that it is not always easy to detect diseases that can affect the vestibulocochlear nerve, although symptoms such as those mentioned above will appear, especially those that involve loss of hearing and balance ability. Hearing loss is usually a symptom associated with age, although being exposed to high-intensity noise or having consumed drugs whose side effect can be deafness are also potential causes of nerve damage.
If the fibers that make up the cochlear nerve are destroyed, the person begins to have difficulty understanding what they hear This difficulty increases when you are in noisy environments, in conversations in which there are more than two people speaking at the same time and if there is background noise.
Another symptom that indicates that the vestibular nerve is affected is the appearance of tinnitus, which is the subjective perception of sounds that do not really exist. It is believed that the appearance of this phenomenon is due to the fact that the nerve is damaged and sends involuntary signals to the brain, an organ which interprets them as sounds that are actually invented.
Although the intensity of tinnitus varies from person to person, it can greatly affect the quality of life of those who suffer from it, especially if this phenomenon appears in the company of hearing loss. As a result, people with tinnitus may become depressed, irritable, and have trouble falling asleep.
If tinnitus is due to injuries to the auditory nerve, it is very difficult to completely eliminate it, since it is necessary to repair damaged cells in the nervous tract and this involves a very delicate surgical intervention. One of the best options to deal with them, in addition to the surgical route, is to teach the patient to live with them.
It is for this reason that, taking all this into account, it is necessary to highlight the importance of prevention and good hearing hygiene.
To avoid having such annoying phenomena as tinnitus or different degrees of acquired deafness, it is advisable to avoid environments with high intensity sounds, in addition to taking preventive measures when going to places with concerts and discos, such as not getting too close to the speakers. If working in a noisy environment, such as a construction site where there are drills, protective headphones should be used.
