A sensory nerve conducts two special senses, hearing and balance.
The bipolar sensory neurons in the organ of Corti pass impulses from the hair cells of the cochlea through the peripheral axonal process to the spiral ganglia (this is the collection of the cellular bodies of the cochlear nerve and is located inside the cochlea).
The impulse then travels through the central axonal process towards the cochlear nucleus in the brainstem from these ganglia.
The fibers are projected from the cochlear nucleus to the inferior colliculi, the medial geniculate bodies, and then through the internal capsule to the auditory cortex located in the temporal lobe.
The vestibular apparatus is the organ that detects changes in the position of the head to gravity. The movement causes a fluid vibration that displaces the hair cells and activates the vestibular part of the eighth nerve.
Bipolar sensory neurons transmit impulses from vestibular hair cells in the utricle, saccule, and semicircular canals to the vestibular ganglion. The vestibular ganglion is located inside the internal auditory meatus.
The peripheral parts of the eighth nerve travel a short distance from the bodies of the nerve cells at the base of the corresponding sense organs. The central part of the nerve travels through the internal auditory meatus with the facial nerve from these peripheral sensory nerve cells.
The eighth nerve enters the brainstem at the junction of the pons and the lateral medulla to the facial nerve. The auditory component of the eighth nerve ends in a sensory nucleus called the cochlear nucleus at the junction of the pons and the medulla.
The vestibular part of the eight nerves ends in the vestibular nuclear complex located on the floor of the fourth ventricle.
The vestibulocochlear nerve is the eighth cranial nerve. It is made up of two parts: vestibular fibers and cochlear fibers. Both have a purely sensory function.
The vestibular and cochlear portions of the vestibulocochlear nerve are functionally discrete, so they originate from different nuclei in the brain:
- Vestibular component: arises from the vestibular nucleus complex in the swell and the medulla.
- Cochlear part: derives from the ventral and dorsal cochlear nuclei, located in the inferior cerebellar peduncle.
Both sets of fibers combine in the swell to form the vestibulocochlear nerve. The nerve emerges from the brain at the cerebellopontine angle and exits the skull through the internal acoustic meatus of the temporal bone.
Within the distal aspect of the internal acoustic meatus, the vestibulocochlear nerve is divided to form the vestibular nerve and the cochlear nerve.
The vestibular nerve innervates the inner ear’s vestibular system, responsible for detecting balance. The cochlear nerve travels to the cochlea of the inner ear, forming the spiral ganglia that serve the sense of hearing.
Basilar skull fracture
A basilar skull fracture is a fracture of the base of the skull, usually due to increased trauma. The vestibulocochlear nerve can be damaged inside the internal acoustic meatus, producing vestibular and cochlear nerve damage symptoms.
Patients may also exhibit signs related to the other cranial nerves, bleeding from the ears and nose, cerebrospinal fluid leaking from the ears (CSF otorrhea) and nose (rhinorrhea of the CSF).
Special sensory functions
The vestibulocochlear nerve is unusual in that it consists mainly of bipolar neurons. It is responsible for the special senses of hearing (through the cochlear nerve) and balance (through the vestibular nerve).
The cochlea detects the magnitude and frequency of sound waves. The internal hair cells of the organ of Corti activate the ion channels in response to the vibrations of the basilar membrane. The action potentials travel from the spiral ganglia, which house the cell bodies of the neurons of the cochlear nerve.
The magnitude of the sound determines how much the membrane vibrates and how often the action potentials are activated. More robust sounds make the basilar membrane vibrate more, which causes action potentials to be transmitted from the spiral ganglia more frequently and vice versa.
The sound frequency is encoded by the position of activated inner hair cells along the basilar membrane.
The vestibular apparatus detects changes in the position of the head about severity. The vestibular hair cells are located in the otolithic organs (the uterus and the saccule), where they detect linear movements of the head, and in the three semicircular canals, where they see rotational movements of the head.
The cellular bodies of the vestibular nerve are found in the vestibular ganglion located on the outer part of the internal acoustic meatus.
Information about the position of the head is used to coordinate the balance and the vestibular-ocular reflex. The vestibule-ocular reflex (also called the oculocephalic reflex) allows the images in the retina to stabilize when the head rotates, moving the eyes in the opposite direction.
It can be demonstrated by keeping a finger still at a comfortable distance in front of you and turning your head from side to side while staying focused on the finger.
Vestibular neuritis refers to inflammation of the vestibular branch of the vestibulocochlear nerve. The etiology of this condition is not fully understood, but it is believed that some cases are due to the reactivation of the herpes simplex virus.
Symptoms of vestibular nerve damage
- Vertigo is a false sensation that one or the environment is spinning or moving.
- Nystagmus: a repetitive and involuntary oscillation of the eyes back and forth.
- Loss of balance (especially in low light conditions).
- Nausea and vomiting.
The condition is usually automatic resolution. The treatment is symptomatic, usually in antiemetics or vestibular suppressors.
Labyrinthitis refers to the inflammation of the membranous labyrinth, which damages the vestibular and cochlear branches of the vestibulocochlear nerve.
The symptoms are similar to vestibular neuritis, but they also include indicators of cochlear nerve damage:
- Sensorineural hearing loss.
- Tinnitus: a false sound of buzzing.