The Anatomy of the Cochlear Nerve

Also called the acoustic or auditory nerve

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The cochlear nerve, also known as the acoustic or auditory nerve, is the cranial nerve responsible for hearing. It travels from the inner ear to the brainstem and out through a bone located on the side of the skull called the temporal bone.

Pathology of the cochlear nerve may result from inflammation, infection, or injury. Rarely, people are born without a cochlear nerve or with a shortened one.

Cochlear Nerve's Role in Hearing: A Complex Phenomenon


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Anatomy

The cochlear nerve is purely a sensory nerve (it has no motor or movement function) and is one of two parts of the vestibulocochlear nerve, also known as cranial nerve eight (VIII). The cochlear nerve is responsible for hearing. The vestibular nerve is responsible for balance, motion, and position.

Structure

To understand the structure of the cochlear nerve, it's best to start with the anatomy of the ear.

Your ear has three main parts:

  • Outer ear: Contains the pinna (the fleshy, visible part of your ear) and ear canal
  • Middle ear: Contains three ear bones (called ossicles), the eardrum (also called the tympanic membrane), and the eustachian tube
  • Inner ear: Contains the cochlea, cochlear nerve, and vestibular organ

Your inner ear sits in a hollowed-out part of the temporal bone (located on the side and base of your skull). The cochlea, which is a spiral-shaped organ that contains fluid (perilymph and endolymph), is found in the inner ear.

The cochlea houses the cell bodies of the cochlear nerve within a region called the spiral ganglion. Nerve cells (neurons) in the spiral ganglion project sound signals to tiny hair cells also located within the cochlea. These hair cells convert the sound signals into nerve impulses that are carried by the cochlear nerve trunk to the brainstem and eventually to the brain, for interpretation.

Overall, the cochlear nerve trunk contains over 30,000 sensory nerve fibers and is approximately 1 inch in length.

Location

The cochlear nerve trunk travels from the base of the cochlea to the brainstem via the internal auditory canal. It's within the internal auditory canal that the cochlear nerve trunk joins the vestibular nerve to form cranial nerve VIII.

When the cochlear nerve enters the brainstem, it innervates or supplies nerve signals to the cochlear nuclei located at the pontomedullary junction (where the pons and medulla of the brainstem meet). The cochlear nerve exits the skull at the temporal bone while information from the cochlear nuclei is carried to the primary auditory (hearing) cortex of the brain for sound analysis.

Function

The cochlear nerve is a sensory nerve that allows you to hear. This complex, precise job starts and end with the following steps:

  1. The pinna of your ear collects sound waves and funnels them through your ear canal to your eardrum. The waves cause your eardrum to vibrate.
  2. The vibration from your eardrum sets your ear bones (malleus, incus, stapes) into motion.
  3. This motion stimulates the cochlear nerve cells (within the spiral ganglion) to form synaptic connections with the hair cells (also located within the cochlea).
  4. The hair cells then convert the sound vibrations into electrochemical (nerve) signals.
  5. The nerve signals are then relayed back through the cochlear nerve to the brainstem.
  6. From the brainstem, the signals are carried to the auditory cortex located in the brain where they are interpreted and "heard."

Associated Conditions

The structure and function of the cochlear nerve may be affected by inflammation from an autoimmune disease, trauma, a congenital malformation, a tumor, an infection, or a blood vessel injury.

Depending on the specific condition, the following symptoms may occur:

Some conditions that may affect the cochlear nerve include:

Vestibular Labyrinthitis

Vestibular labyrinthitis involves the swelling of the vestibulocochlear nerve (both the vestibular and cochlear nerve).

Symptoms include sudden and severe vertigo, hearing loss, tinnitus, and balance problems. The cause of this condition is linked to a viral infection, such as the herpes virus, flu, measles, mumps, and hepatitis.

Multiple Sclerosis

Around 3 to 6% of people with the autoimmune disease multiple sclerosis (MS) experience hearing loss as a result of an MS lesion (site of inflammation) on the cochlear nerve or at another site within the auditory pathway.

With MS, a person's own immune system misguidedly attacks and inflames the insulating covering (myelin) of nerve fibers in their brain, spinal cord, and/or eyes. If the cochlear nerve is affected, a patient may experience sudden hearing loss and vertigo.

Acoustic Neuroma

A slow-growing tumor of the Schwann cells that insulates the vestibulocochlear nerve may result in progressive hearing loss, tinnitus, and vertigo.

This noncancerous tumor (called a vestibular schwannoma or acoustic neuroma) typically occurs on one cochlear nerve. If the tumor develops bilaterally, it may be a sign of a genetic condition called, neurofibromatosis type 2.

Anterior Inferior Artery Cerebellar Stroke

A cerebellar stroke in the territory of the anterior inferior cerebellar artery (AICA) usually leads to infarction of the vestibulocochlear nerve (when the nerve's blood supply is cut off) resulting in sudden, one-sided hearing loss and vertigo.

Other symptoms like loss of coordination, walking difficulties, and one-sided facial weakness and numbness may also occur, depending on the extent of the stroke.

Trauma

A labyrinthine concussion or other trauma to the temporal bone (e.g., from a blow to the side of the head) may damage the cochlear nerve within the inner ear and result in hearing loss.

Congenital Malformation

A congenital malformation of the cochlear nerve—either aplasia (no nerve) or hypoplasia (small nerve)—is a rare cause of sensorineural hearing loss.

Treatment

Treatment depends on the specific pathology that is affecting the cochlear nerve.

The treatment of vestibular labyrinthitis often entails taking medications to control symptoms, such as Antivert (meclizine) and Valium (diazepam) to reduce dizziness, and Zofran (ondansetron) to reduce nausea.

Steroids may be given for hearing loss, and an antiviral medication like Zovirax (acyclovir) may be given if the herpes virus is the suspected cause. Lastly, a balance rehabilitation program may be recommended if the patient's vertigo and balance problems persist for longer than a few weeks.

MS-related inflammation of the cochlear nerve often requires that a patient take corticosteroids. Long-term treatment of MS involves taking a disease-modifying therapy, such as an injectable interferon drug or an infused drug, like Ocrevus (ocrelizumab).

For an acoustic neuroma, there are three treatment options: watchful waiting, radiation therapy, or surgery. The purpose of radiation therapy is to stunt the growth of the tumor while surgery completely removes the tumor.

An anterior inferior cerebellar artery stroke warrants immediate thrombolysis with a tissue-type plasminogen activator. Unfortunately, some patients do not receive this therapy because of the potential difficulties and subtleties in recognizing this type of stroke.

Regardless, all patients with a cerebellar stroke require close monitoring for potential brain swelling. Patients will also undergo a thorough workup to sort out the etiology of the stroke and take an anticoagulant, if the stroke was caused by a blood clot (called an ischemic stroke).

For patients with severe cochlear nerve trauma or cochlear nerve aplasia or hypoplasia, cochlear implants may restore hearing by carrying sound signals from the patient's inner ear to their brain (although, the outcomes are variable).

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