CN VIII: Vestibulocochlear nerve

CN VIII: Vestibulocochlear nerveAnatomy and function of the vestibulocochlear nerve
~ 7 min

In this note, we are going to discuss the anatomy and function of the vestibulocochlear nerve (n. vestibulocochlearis), which is the CN VIII.

Vestibulocochlear nerve (n. vestibulocochlearis)
Vestibulocochlear nerve (n. vestibulocochlearis)

It consists of two parts: the vestibular part and the cochlear parts, as it relates to the transmitting structures of the two corresponding systems. These two parts provide the functions of balance and hearing, respectively.

According to the generally accepted classification of functional components, it contains special somatic afferent fibers (SSA). That means that it is a sensory nerve.

Let’s learn more about each part of the nerve.

The vestibular part (pars vestibularis) of the vestibulocochlear nerve is formed by the axons of bipolar sensory neurons

Vestibular part (pars vestibularis) Vestibular nerve (nervus vestibularis)
Vestibular part (pars vestibularis) Vestibular nerve (nervus vestibularis)
Vestibular part (pars vestibularis) Vestibular nerve (nervus vestibularis)
Vestibular part (pars vestibularis) Vestibular nerve (nervus vestibularis)

located in the vestibular ganglion (ganglion vestibulare).

Vestibular ganglion (ganglion vestibulare)
Vestibular ganglion (ganglion vestibulare)
Vestibular ganglion (ganglion vestibulare)
Vestibular ganglion (ganglion vestibulare)

The ganglion itself is located in the internal acoustic meatus.

Internal auditory canal (meatus acusticus internus)
Internal auditory canal (meatus acusticus internus)

The dendrites of these cells are associated with the receptors of the vestibular system, namely the maculae of the utricle and macule of the saccule (maculae utriculi et sacculi)

Utricle (utriculus)
Utricle (utriculus)
Saccule (sacculus)
Saccule (sacculus)
Vestibule (vestibulum) location of the utricle and saccule
Vestibule (vestibulum) location of the utricle and saccule

as well as the ampullary crests (crista ampullaris) of semicircular canals of the membranous labyrinth.

All these structures are collectively referred to as the “vestibular system».

Posterior semicircular canal (canalis semicircularis posterior)
Posterior semicircular canal (canalis semicircularis posterior)
Lateral semicircular canal (canalis semicircularis lateralis)
Lateral semicircular canal (canalis semicircularis lateralis)
Anterior semicircular canal (canalis semicircularis anterior)
Anterior semicircular canal (canalis semicircularis anterior)

The hair cells of the utricle and saccule detect linear acceleration, or back-and-forth motion of the head, and the hair cells of the semicircular canals detect angular acceleration, or rotational movements of the head.

Axons of the vestibular ganglion form the vestibular nerve (nervus vestibularis),

Vestibular nerve (nervus vestibularis)
Vestibular nerve (nervus vestibularis)

which passes through the internal acoustic opening (porus acusticus internus)

Internal auditory opening (nervus cochlearis)
Internal auditory opening (nervus cochlearis)

along with the cochlear nerve (nervus cochlearis)

Cochlear nerve (porus acusticus internus)
Cochlear nerve (porus acusticus internus)
Vestibulocochlear nerve in the internal auditory canal
Vestibulocochlear nerve in the internal auditory canal
Vestibulocochlear nerve in the internal auditory canal
Vestibulocochlear nerve in the internal auditory canal

and enters the brain in the region of the cerebellopontine angle (angulus pontocerebellaris).

Vestibulocochlear nerve (n. vestibulocochlearis) enters / exits the brainstem
Vestibulocochlear nerve (n. vestibulocochlearis) enters / exits the brainstem

The vestibular part of the nerve ends at the neurons of the vestibular nuclei of the brainstem, in the vestibular area, in the lateral angle of the rhomboid fossa.

Vestibular area (area vestibularis)
Vestibular area (area vestibularis)
Vestibular nuclei (nuclei vestibulares)
Vestibular nuclei (nuclei vestibulares)
Vestibular nuclei (nuclei vestibulares)
Vestibular nuclei (nuclei vestibulares)

Two vestibular nuclei, inferior Roller’s nucleus (nucleus vestibularis inferior) and medial Schwalbe’s nucleus (nucleus vestibularis medialis), are located in the medulla oblongata,

Roller’s inferior vestibular nucleus (nucleus vestibularis inferior)
Roller’s inferior vestibular nucleus (nucleus vestibularis inferior)
Schwalbe`s medial vestibular nucleus (nucleus vestibularis medialis)
Schwalbe`s medial vestibular nucleus (nucleus vestibularis medialis)

and another two, superior Bechterew’s nucleus (nucleus vestibularis superior) and lateral Deiters’s nucleus (nucleus vestibularis lateralis), are located in the pons.

Bechterew`s superior vestibular nucleus (nucleus vestibularis superior)
Bechterew`s superior vestibular nucleus (nucleus vestibularis superior)
Deiters`s lateral vestibular nucleus (nucleus vestibularis lateralis)
Deiters`s lateral vestibular nucleus (nucleus vestibularis lateralis)

From the vestibular nuclei, the system of different tracts goes in the following directions:

1. Vestibulospinal tract (tr. vestibulospinalis), medial and lateral, is formed by axons of neurons of the lateral nucleus and ends at the motor nuclei of the anterior horns of the gray matter of the spinal cord. This tract is involved in maintaining balance.

Vestibulospinal tract (tr. vestibulospinalis)
Vestibulospinal tract (tr. vestibulospinalis)
Vestibulospinal tract (tr. vestibulospinalis)
Vestibulospinal tract (tr. vestibulospinalis)

2. Medial longitudinal fasciculus (fasciculus longitudinalis medialis), which connects the lateral vestibular nucleus with the nuclei of cranial nerves, related to eye movements. It ensures that the gaze direction is maintained when the head position changes.

Medial longitudinal fasciculus (fasciculus longitudinalis medialis)
Medial longitudinal fasciculus (fasciculus longitudinalis medialis)
Medial longitudinal fasciculus (fasciculus longitudinalis medialis)
Medial longitudinal fasciculus (fasciculus longitudinalis medialis)
Medial longitudinal fasciculus (fasciculus longitudinalis medialis)
Medial longitudinal fasciculus (fasciculus longitudinalis medialis)

3. Vestibulocerebellar tract (tr. vestibulocerebellaris) goes tot che cerebellum and coordinate our movements. For the most part, it is formed by axons of the inferior and medial nuclei. The tract passes through the inferior cerebellar peduncles to the fastigial nucleus and the vermis.

Vestibulocerebellar tract (tr. vestibulocerebellaris)
Vestibulocerebellar tract (tr. vestibulocerebellaris)

4. Vestibulohypothalamic tract (tr. vestibulohypothalamicus), which provides the autonomic reaction in response to stimulation of the vestibular system.

Hypothalamus (hypothalamus)
Hypothalamus (hypothalamus)

In addition, some fibers have connections with the area postrema of the medulla oblongata, which causes nausea and vomiting, for example, when motion sickness occurs on road.

5. The vestibulothalamic tract (tr. vestibulothalamicus), which is formed by the axons of the neurons of the vestibular nuclei of the opposite side that pass to the ventral basal complex of the thalamus.

Thalamus (thalamus)
Thalamus (thalamus)

From here, the received sensory information on the change in body position is transmitted to the cerebral cortex. The cortical end of the vestibular system is located in the temporal and parietal lobes. This tract allows conscious perception of the spatial body position.

Cochlear part

Let’s move on to the second component of the vestibulocochlear nerve.

The cochlear part (pars cochlearis) of the cranial nerve VIII is formed by axons of the sensory neurons of the spiral ganglion located in the spiral canal of the modiolus (canalis spiralis modioli).

Cochlear part (pars cochlearis) / Cochlear nerve (nervus cochlearis)
Cochlear part (pars cochlearis) / Cochlear nerve (nervus cochlearis)
Cochlear part (pars cochlearis) / Cochlear nerve (nervus cochlearis)
Cochlear part (pars cochlearis) / Cochlear nerve (nervus cochlearis)
Spiral ganglion (ganglion spirale cochleae)
Spiral ganglion (ganglion spirale cochleae)
Spiral canal of the modiolus (canalis spiralis modioli)
Spiral canal of the modiolus (canalis spiralis modioli)

Remember that in the cochlea there are several structures involved in the perception, conduction of sound waves, as well as their transformation into a nerve impulse.

Cochlea (cochlea)
Cochlea (cochlea)

These are:

Scala vestibuli (scala vestibuli)

Scala vestibuli (scala vestibuli)
Scala vestibuli (scala vestibuli)
Scala vestibuli (scala vestibuli)
Scala vestibuli (scala vestibuli)
Scala vestibuli (scala vestibuli)
Scala vestibuli (scala vestibuli)

Cochlear duct (ductus cochlearis)

Cochlear duct (ductus cochlearis)
Cochlear duct (ductus cochlearis)
Cochlear duct (ductus cochlearis)
Cochlear duct (ductus cochlearis)
Cochlear duct (ductus cochlearis)
Cochlear duct (ductus cochlearis)

Scala tympani (scala tympani)

Scala tympani (scala tympani)
Scala tympani (scala tympani)
Scala tympani (scala tympani)
Scala tympani (scala tympani)
Scala tympani (scala tympani)
Scala tympani (scala tympani)

The scala vestibuli communicates with the middle ear via the oval window (fenestra vestibuli), and contains perilymph.

Oval window (fenestra vestibuli)
Oval window (fenestra vestibuli)

The cochlear duct is filled with endolymph and contains a spiral organ, or the organ of Corti (organum spirale), which is an organ of hearing, it contains hair cells that respond to vibrations of sound waves.

Organ of Corti (organum spirale)
Organ of Corti (organum spirale)
Organ of Corti (organum spirale)
Organ of Corti (organum spirale)

The scala tympani is filled with perilymph and communicates with the middle ear via the round window (fenestra cochleae).

Round window (fenestra cochleae)
Round window (fenestra cochleae)
Round window (fenestra cochleae)
Round window (fenestra cochleae)

So, the sound wave first passes through the external ear and transmits vibration to the tympanic membrane.

External acoustic meatus (meatus acusticus externus)
External acoustic meatus (meatus acusticus externus)
Tympanic membrane (membrana tympani)
Tympanic membrane (membrana tympani)

In its turn, it redirects the vibration to the auditory ossicles (malleus, incus, and stapes), and through the oval window, it enters the inner ear, causing fluctuations of the perilymph in the scala vestibuli.

Malleus (malleus)
Malleus (malleus)
Malleus (incus)
Malleus (incus)
Stapes (stapes)
Stapes (stapes)

These fluctuations are transmitted to the hair cells of the organ of Corti, which converts them into a nerve impulse. This is where the transmitting part of the auditory system begins.

The signal from the dendrites enters the spiral ganglion.

Spiral ganglion (ganglion spirale cochleae)
Spiral ganglion (ganglion spirale cochleae)

The axons of the neurons of the spiral ganglion form the cochlear nerve (nervus cochlearis) that enters the brain along with the vestibular part of the CN VIII

Cochlear nerve (nervus cochlearis)
Cochlear nerve (nervus cochlearis)
Cochlear nerve (nervus cochlearis)
Cochlear nerve (nervus cochlearis)
Vestibulocochlear nerve (n. vestibulocochlearis) at the exit/entry site of the brain
Vestibulocochlear nerve (n. vestibulocochlearis) at the exit/entry site of the brain

and ends at the cells of the dorsal cochlear nucleus (nucleus cochlearis posterior) and ventral cochlear nucleus (nucleus cochlearis anterior) located within the lateral part of the pons.

Dorsal cochlear nucleus (nucleus cochlearis posterior)
Dorsal cochlear nucleus (nucleus cochlearis posterior)
Ventral cochlear nucleus (nucleus cochlearis anterior)
Ventral cochlear nucleus (nucleus cochlearis anterior)

The dorsal nucleus perceives mainly low-frequency sound waves, while the ventral nucleus perceives high-frequency sound waves.

Cochlear nuclei (nuclei cochleares)
Cochlear nuclei (nuclei cochleares)
Cochlear nuclei (nuclei cochleares)
Cochlear nuclei (nuclei cochleares)

The fibers of the auditory pathway begin from the cochlear nuclei, and they form three so-called cochlear (or acoustic) striae or stripes are formed.

От улитковых ядер начинаются волокна слухового пути, которые направляются к промежуточным слуховым центрам своей и противоположной стороны.

The posterior cochlear stripe (stria cochlearis posterior) and the intermediate cochlear stripe (stria cochlearis intermedia) extend from the dorsal nucleus. And the anterior cochlear stripe (stria cochlearis anterior) extends from the ventral nucleus.

All three striae pass through the reticular formation of the pons, some of them go to the contralateral side and at the level of the superior olivary nucleus (nucleus olivaris superior) gather into a thick fasciculus called the lateral lemniscus (lemniscus lateralis).

Superior olivary nucleus (nucleus olivaris superior)
Superior olivary nucleus (nucleus olivaris superior)
Lateral lemniscus (lemniscus lateralis)
Lateral lemniscus (lemniscus lateralis)
Lateral lemniscus (lemniscus lateralis)
Lateral lemniscus (lemniscus lateralis)

The other, non-decussated part of the fibers passes to the ipsilateral olivary nuclear complex.

Non-decussated part of the lateral lemniscus
Non-decussated part of the lateral lemniscus

Eventually, the lateral lemniscus reaches the inferior colliculi (colliculi inferiores) in the midbrain.

Inferior colliculi (colliculus inferior)
Inferior colliculi (colliculus inferior)
Inferior colliculi (colliculus inferior)
Inferior colliculi (colliculus inferior)

They are the centers that sense the pitch of the sound, its rhythm and the localization of the source.

Then the colliculi transmit the signal to the medial geniculate body of the thalamus (corpus geniculatum mediale) in the diencephalon.

Medial geniculate body (corpus geniculatum mediale)
Medial geniculate body (corpus geniculatum mediale)
Medial geniculate body (corpus geniculatum mediale)
Medial geniculate body (corpus geniculatum mediale)

Axons of neurons of the medial geniculate body pass in the posterior limb of the internal capsule and, forming the acoustic radiation (radiatio acustica),

Acoustic radiation (radiatio acustica)
Acoustic radiation (radiatio acustica)
Acoustic radiation (radiatio acustica)
Acoustic radiation (radiatio acustica)

pass to the ipsilateral auditory cortex, which is located in the superior temporal gyrus (gyrus temporalis superior).

Superior temporal gyrus (gyrus temporalis superior)
Superior temporal gyrus (gyrus temporalis superior)
Superior temporal gyrus (gyrus temporalis superior)
Superior temporal gyrus (gyrus temporalis superior)
Dictionary

CN VIII: Vestibulocochlear nerve

Vestibulocochlear nerve
n. vestibulocochlearis
Vestibular part
pars vestibularis
Vestibular nerve
nervus vestibularis
Vestibular ganglion
ganglion vestibulare
internal acoustic meatus
meatus acusticus internus
Macula of the utricle
macula utriculi
Utricle
utriculus
Macula of the saccule
macula sacculi
Saccule
sacculus
Vestible
vestibulum
Ampullary crest
crista ampullaris
Posterior semicircular canal
canalis semicircularis posterior
Lateral semicircular canal
canalis semicircularis lateralis
Anterior semicircular canal
canalis semicircularis anterior
Cochlear nerve
nervus cochlearis
Internal acoustic opening
porus acusticus internus
Crebellopontine angle
angulus pontocerebellaris
Vestibulocochlear nerve
n. vestibulocochlearis
Vestibular area
area vestibularis
Vestibular nuclei
nuclei vestibulares
Inferior vestibular nucleus
nucleus vestibularis inferior
Medial vestibular nucleus
nucleus vestibularis medialis
Superior vestibular nucleus
nucleus vestibularis superior
Lateral vestibular nucleus
nucleus vestibularis lateralis
Vestibulospinal tract
tr. vestibulospinalis
Medial longitudinal fasciculus
fasciculus longitudinalis medialis
Vestibulocerebellar tract
tr. vestibulocerebellaris
Vestibulohypothalamic tract
tr. vestibulohypothalamicus
Hypothalamus
hypothalamus
Vestibulothalamic tract
tr. vestibulothalamicus
Thalamus
thalamus
Cochlear part
pars cochlearis
Spiral ganglion
ganglion spirale cochleae
Spiral canal of the modiolus
canalis spiralis modioli
Cochlea
cochlea
Scala vestibuli
scala vestibuli
Cochlear duct
ductus cochlearis
Scala tympani
scala tympani
Oval window
fenestra vestibuli
Spiral organ (Organ of Corti)
organum spirale
Round window
fenestra cochlea
External acoustic meatus
meatus acusticus externus
Tympanic membrane
membrana tympani
Malleus
malleus
Incus
incus
Stapes
stapes
Dorsal cochlear nucleus
nucleus cochlearis posterior
Ventral cochlear nucleus
nucleus cochlearis anterior
Cochlear nuclei
nuclei cochleares
Posterior cochlear stria
stria cochlearis posterior
Intermediate cochlear stria
stria cochlearis intermedia
Anterior cochlear stria
stria cochlearis anterior
Superior olivary nucleus
nucleus olivaris superior
Lateral lemniscus
lemniscus lateralis
Inferior colliculus
colliculus inferior
Medial geniculate body
corpus geniculatum mediale
Acoustic radiation
radiatio acustica
Superior temporal gyrus
gyrus temporalis superior
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