Cranial nerve VIII is the vestibulocochlear nerve. It supplies the special senses of hearing and balance, with integration to thalamic and cerebellar structures causing postural changes. In this article, we will divide CN VIII into its two components, the vestibular and cochlear nerves, and discuss their structure and clinically relevant sensory functions.
The three superior nuclei project fibres superiorly to the MLF, connecting to cranial nerves III, IV and VI. The three inferior nuclei project fibres inferiorly to the vestibulospinal tract for balance control.
The cochlear nuclei
There are two cochlear nuclei:
Ventral cochlear nucleus – ventrolateral to the inferior cerebellar peduncle – inferior colliculus
Dorsal cochlear nucleus – dorsolateral to the inferior cerebellar peduncle – inferior colliculus and auditory cortex
The vestibulocochlear nerve
Fibres entering both the vestibular and cochlear nuclei converge in the dorsolateral pons, emerging from the cerebellopontine angle as the vestibulocochlear nerve. The vestibulocochlear nerve then enters the internal auditory meatus with the facial nerve (CN VII).
In the distal segment of the internal auditory meatus, the vestibulocochlear nerve diverges into the separate vestibular and cochlear nerves.
The vestibular nerve travels to the vestibular system of the inner ear and detects changes in head motion to represent balance.
The cochlear nerve travels to the cochlea of the inner ear and forms the spiral ganglia, which detect sound waves transmitted as electrical impulses from the inner ear structures.
The vestibular division of CN VIII detects changes in the position of the head relative to gravity – our sense of balance and equilibrium.
It does this through the use of two sets of vestibular hair cells:
The first are located in the otolith organs of the utricle and saccule. These vestibular hair cells respond to linear head movement.
The second are located in the otolith organs of the semicircular canals. These vestibular hair cells respond to rotational head movement.
The vibration of these hair cells creates an action potential that travels to the cell bodies of the vestibular nerve located in the vestibular ganglion, which is found in the distal internal auditory meatus.
These balance signals converge at the four vestibular nuclei for integration with two important pathways:
Medial longitudinal fasciculus – control of conjugate movements of the eye by integrating the frontal eye field (frontal lobe) and head movement information (CN VIII) with the extraocular muscle-controlling nerves, CN III, IV and VI.
Vestibulospinal tract – detecting acute changes in head movement and adjusting anti-gravity/postural muscles to maintain the correct (usually erect) stance.
Both of these pathways are integrated to produce the vestibulo-ocular (VOR) reflex. The VOR allows images detected on the retina to be stabilised as the head is turning, by moving the eyes in the opposite direction (such that turning the head to the left will cause both eyes to move right for fixation on the same object).
The pathway of hearing will be explained from the time that sound waves oscillate the tympanic membrane to the impulses sent in the cochlear nerve, but first, a note on the structure of the cochlea.
The cochlea is a spiral-shaped structure that contains three major chambers:
Scala vestibuli – containing perilymph
Scala tympani – containing perilymph
Scala media – containing endolymph
Scala vestibuli is the chamber that abuts the oval window. Thus, it is the chamber that is affected by the vibration of the stapes as sound is transmitted.
Scala tympani is the chamber that abuts the round window. As the amount of perilymph should be fixed and remain unchanged, the changes in pressure exerted by the stapes on the oval window must be mitigated. One of the functions of the round window is to bulge to accommodate this pressure change and prevent damage to the cochlear structures.
The scala media contains the organ of Corti and hair cells of the cochlea. Lying between the scala vestibuli and the scala media is Reissner’s membrane. Reissner’s membrane is a flexible membrane that moves as sound is transmitted into the scala vestibuli from the oval window. As Reissner’s membrane vibrates, the tectorial membrane vibrates, causing electrical discharging of the organ of Corti hair cells. This produces electrochemical impulses representing sound.
These impulses are transmitted through the cochlear nerve and reach the cochlear nuclei for integration in two important pathways:
Inferior colliculus – integration of sound with head and neck movements to allow reflexive turning of the head in response to ‘danger’ or ‘alert’ sounds.
Primary auditory cortex – the transverse gyri of Heschl in the deep temporal lobe receive fibres bilaterally from the left and right cochlear nuclei complex.
Clinical relevance – assessing the vestibulocochlear nerve
Knowing the functions of the vestibulocochlear nerve can help make the cranial nerve examination easier. The specifics of each step are explained here.
Considering the vestibular division of CN VIII, we know that we must test anti-gravity muscle balance and the MLF:
Anti-gravity muscle balance – gait and Romberg’s test
MLF – object tracking for nystagmus
Considering the cochlear division of CN VIII, we know that we must test sound conduction and hearing:
External and middle ear – otoscopic examination
Crude hearing – crude hearing test (whisper or finger rustle)
BPPV is a relatively common condition that is caused by dislodging of otolith (calcium) crystals within the semicircular canals. It causes patients to experience vertigo, nausea and vomiting, often exacerbated by specific head positions.
It is often treated with simple exercises that re-position the otolith organs within the semicircular canals. Another manoeuvre to attempt is the Epley manoeuvre, which uses gravity to move the otolith crystal causing the vertigo.
CN VIII is the vestibulocochlear nerve
It originates in the pontomedullary region
It provides special somatic afferent fibres for hearing and balance
The cochlea transmits sound waves to mechanical ossicle movements to electrochemical action potentials
The vestibular apparatus detects changes in head motion
It passes through the internal auditory meatus and does not leave the skull
Sinnatamby, C. S. (2011). Last’s Anatomy, International Edition: Regional and Applied. Elsevier Health Sciences.
Moore, K. L., Dalley, A. F., & Agur, A. M. (2013). Clinically oriented anatomy. Lippincott Williams & Wilkins.
Nolte, J. (2002). The human brain: an introduction to its functional anatomy.
Snell, R. S. (2010). Clinical neuroanatomy. Lippincott Williams & Wilkins.
Patrick J. Lynch, medical illustrator [CC BY 2.5 (https://creativecommons.org/licenses/by/2.5)]. Modified by Dr Lewis Potter.