In this note, we are going to discuss the anatomy and function of the vestibulocochlear nerve (n. vestibulocochlearis), which is the CN VIII.
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
located in the vestibular ganglion (ganglion vestibulare).
The ganglion itself is located in the internal acoustic meatus.
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)
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».
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),
which passes through the internal acoustic opening (porus acusticus internus)
along with the cochlear nerve (nervus cochlearis)
and enters the brain in the region of the cerebellopontine angle (angulus pontocerebellaris).
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.
Two vestibular nuclei, inferior Roller’s nucleus (nucleus vestibularis inferior) and medial Schwalbe’s nucleus (nucleus vestibularis medialis), are located in the medulla oblongata,
and another two, superior Bechterew’s nucleus (nucleus vestibularis superior) and lateral Deiters’s nucleus (nucleus vestibularis lateralis), are located in the pons.
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.
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.
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.
4. Vestibulohypothalamic tract (tr. vestibulohypothalamicus), which provides the autonomic reaction in response to stimulation of the vestibular system.
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.
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.
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).
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.
Scala vestibuli (scala vestibuli)
Cochlear duct (ductus cochlearis)
Scala tympani (scala tympani)
The scala vestibuli communicates with the middle ear via the oval window (fenestra vestibuli), and contains perilymph.
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.
The scala tympani is filled with perilymph and communicates with the middle ear via the round window (fenestra cochleae).
So, the sound wave first passes through the external ear and transmits vibration to the tympanic membrane.
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.
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.
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
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.
The dorsal nucleus perceives mainly low-frequency sound waves, while the ventral nucleus perceives high-frequency sound waves.
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).
The other, non-decussated part of the fibers passes to the ipsilateral olivary nuclear complex.
Eventually, the lateral lemniscus reaches the inferior colliculi (colliculi inferiores) in the midbrain.
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.
Axons of neurons of the medial geniculate body pass in the posterior limb of the internal capsule and, forming the acoustic radiation (radiatio acustica),
pass to the ipsilateral auditory cortex, which is located in the superior temporal gyrus (gyrus temporalis superior).
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
- Macula of the saccule
- macula sacculi
- 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
- Vestibulothalamic tract
- tr. vestibulothalamicus
- Cochlear part
- pars cochlearis
- Spiral ganglion
- ganglion spirale cochleae
- Spiral canal of the modiolus
- canalis spiralis modioli
- 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
- 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