The vestibulocochlear nerve or auditory vestibular nerve, also known as the eighth cranial nerve, cranial nerve VIII, or simply CN VIII, is a

cranial nerve Cranial nerves are the nerves that emerge directly from the brain (including the brainstem), of which there are conventionally considered twelve pairs. Cranial nerves relay information between the brain and parts of the body, primarily to and ...

that transmits sound and equilibrium (balance) information from the

inner ear The inner ear (internal ear, auris interna) is the innermost part of the vertebrate ear. In vertebrates, the inner ear is mainly responsible for sound detection and balance. In mammals, it consists of the bony labyrinth, a hollow cavity in the ...

to the brain. Through olivocochlear fibers, it also transmits motor and modulatory information from the superior olivary complex in the brainstem to the cochlea.

Structure

The vestibulocochlear nerve consists mostly of bipolar neurons and splits into two large divisions: the cochlear nerve and the vestibular nerve. Cranial nerve 8, the vestibulocochlear nerve, goes to the middle portion of the

brainstem The brainstem (or brain stem) is the posterior stalk-like part of the brain that connects the cerebrum with the spinal cord. In the human brain the brainstem is composed of the midbrain, the pons, and the medulla oblongata. The midbrain is cont ...

called the pons (which then is largely composed of fibers going to the

cerebellum The cerebellum (Latin for "little brain") is a major feature of the hindbrain of all vertebrates. Although usually smaller than the cerebrum, in some animals such as the mormyrid fishes it may be as large as or even larger. In humans, the cerebel ...

). The 8th cranial nerve runs between the base of the pons and

medulla oblongata The medulla oblongata or simply medulla is a long stem-like structure which makes up the lower part of the brainstem. It is anterior and partially inferior to the cerebellum. It is a cone-shaped neuronal mass responsible for autonomic (involun ...

(the lower portion of the brainstem). This junction between the pons, medulla, and cerebellum that contains the 8th nerve is called the cerebellopontine angle. The vestibulocochlear nerve is accompanied by the labyrinthine artery, which usually branches off from the anterior inferior cerebellar artery at the cerebellopontine angle, and then goes with the 7th nerve through the internal acoustic meatus to the internal ear. The cochlear nerve travels away from the cochlea of the

where it starts as the spiral ganglia. Processes from the organ of Corti conduct afferent transmission to the spiral ganglia. It is the inner hair cells of the organ of Corti that are responsible for activation of afferent receptors in response to pressure waves reaching the basilar membrane through the transduction of sound. The exact mechanism by which sound is transmitted by the

neurons A neuron, neurone, or nerve cell is an electrically excitable cell that communicates with other cells via specialized connections called synapses. The neuron is the main component of nervous tissue in all animals except sponges and placozoa. N ...

of the cochlear nerve is uncertain; the two competing theories are place theory and temporal theory. The vestibular nerve travels from the

vestibular system The vestibular system, in vertebrates, is a sensory system that creates the sense of balance and spatial orientation for the purpose of coordinating movement with balance. Together with the cochlea, a part of the auditory system, it constitutes ...

of the inner ear. The vestibular ganglion houses the cell bodies of the bipolar neurons and extends processes to five sensory organs. Three of these are the cristae located in the ampullae of the semicircular canals. Hair cells of the cristae activate afferent receptors in response to rotational acceleration. The other two sensory organs supplied by the vestibular neurons are the maculae of the saccule and utricle. Hair cells of the maculae in the utricle activate afferent receptors in response to linear acceleration, while hair cells of the maculae in the saccule respond to vertically directed linear force.

Development

The vestibulocochlear nerve is derived from the

embryo An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male spe ...

nic otic placode.

Function

This is the

nerve A nerve is an enclosed, cable-like bundle of nerve fibers (called axons) in the peripheral nervous system. A nerve transmits electrical impulses. It is the basic unit of the peripheral nervous system. A nerve provides a common pathway for the e ...

along which the sensory cells (the hair cells) of the inner ear transmit information to the brain. It consists of the cochlear nerve, carrying details about hearing, and the vestibular nerve, carrying information about balance. It emerges from the pontomedullary junction and exits the inner skull via the internal acoustic meatus in the temporal bone. The vestibulocochlear nerve carries axons of type special somatic afferent.

Clinical significance

Symptoms of damage

Damage to the vestibulocochlear nerve may cause the following symptoms: * hearing loss * vertigo *false sense of motion *loss of equilibrium (in dark places) * nystagmus * motion sickness *gaze-evoked tinnitus.

Examination

Examinations that can be done include the Rinne and Weber tests. Rinne's test involves Rinne's Right and Left Test, since auditory acuity is equal in both ears. If bone conduction (BC) is more than air conduction (AC) (BC>AC) indicates Rinne Test is negative or abnormal. If AC>BC Rinne test is normal or positive. If BC>AC and Weber's test lateralizes to abnormal side then it is Conductive hearing loss. If AC>BC and Weber's test lateralizes to normal side then it concludes Sensorineural hearing loss. After pure-tone testing, if the AC and BC responses at all frequencies 500–8000 Hz are better than 25 dB HL, meaning 0-24 dB HL, the results are considered normal hearing sensitivity. If the AC and BC are worse than 25 dB HL at any one or more frequency between 500 and 8000 Hz, meaning 25+, and there is a no bigger difference between AC and BC beyond 10 dB at any frequency, there is a sensori-neural hearing loss present. If the BC responses are normal, 0-24 dB HL, and the AC are worse than 25 dB HL, as well as a 10 dB gap between the air and bone responses, a conductive hearing loss is present. The modified Hughson–Westlake method is used by many audiologists during testing. A battery of (1) otoscopy, to view the ear canal and tympanic membrane, (2) tympanometry, to assess the immittance of the tympanic membrane and how well it moves, (3) otoacoustic emissions, to measure the response of the outer hair cells located in the cochlea, (4) audiobooth pure-tone testing, to obtain thresholds to determine the type, severity, and pathology of the hearing loss present, and (5) speech tests, to measure the patients recognition and ability to repeat the speech heard, is all taken into consideration when diagnosing the pathology of the patient.

History

Etymology

Some older texts call the nerve the ''acoustic'' or ''auditory nerve'', but these terms have fallen out of widespread use because they fail to recognize the nerve's role in the vestibular system. ''Vestibulocochlear nerve'' is therefore preferred by most.

References

Additional images

File:Cranial endobasis of a 19-20 weeks foetus.JPG, Vestibulocochlear nerve File:Gray689.png, Superficial dissection of brain-stem. Ventral view. File:Gray705.png, Dissection showing the projection fibers of the cerebellum. File:Gray719.png, Hind- and mid-brains; postero-lateral view. File:Gray792.png, Upper part of medulla spinalis and hind- and mid-brains; posterior aspect, exposed in situ. File:Gray922.png, Position of the right bony labyrinth of the ear in the skull, viewed from above. File:ThreeNeuronArc.png , Vestibulo-ocular reflex File:Slide5nnn.JPG, Vestibulocochlear nerve

External links

* * () {{Authority control Cranial nerves