Nolte Chapter 14 – Hearing and Balance: The Eighth Cranial Nerve
The membranous labyrinth is suspended within the bony labyrinth, a cavity in the temporal bone.
The cochlea extends anteriorly from an enlargement called the vestibule, to which three semicircular
canals are attached.
The vestibule contains two enlargements of the membranous labyrinth: the utricle(where we find the
semicircular ducts) and the saccule(which is connected to the cochlear duct)
The bony labyrinth is filled with perilymph which is similar in composition to the CSF. The perilymphatic
space is continuous with the subarachnoid space through the cochlear aqueduct.
The membraneous labyrinth is filled with endolymph, which is high in Pottassium concentration.
Swelling of this fluid in this membranous labyrinth cases tinnitus.
Hair Cells
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one end of the cell goes into the endolymphatic interior of the membranous labyrinth. The
other end of each hair cell synapses on peripheral processes of the eigth nerve fibers
steriocilia are lined up in graduated rowas so that the tallest are toward one side of the hair cell.
Adjacent to the tallest stereocilia is “the single true cilium”, the kinocilium.
tips of the kinocilium and the tallest stereocilia are usually embedded in a specialized mass of
gelatinous material and movement of this mass relative to the hair cells causes deflections and
action potentions.
the real barrier between endolymph and perilymph is a series of tight junctions near the top of
the hair cells.
stereocilia are packed of cross-lined actin which allow them to not bend and instead pivot at
their bases.
various linking molecules interconnect neighboring steriocilia so that the whole hair bundle
moves as a unit in response to mechanical stimuli
o tip links extend from the tip of each steriocilium to the next tallest neighbor.
A mechanically gated cation channel is located at one or both ends of each tip link and is
normally open part of the time. Deflecting the hair bundle towards the tallest steriocilia
stretches the tip links and opens the channel.
o this allows Pottassium ions to flow down the electrical gradient into the negative
interior of the hair cells and depolarize the cells, open voltage gated potassium channels
and release neurotrainmitters onto the eighth nerve.
o deflecting in the other direction closes the channels more than their regular opening
and is similar to inhibition.
 perpendicular direction has no effect.
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The cochlea
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the outer ear is a funnel consisting of the aurcle and conducts sound to the tympanic
membrane.
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sound-induced vibrations are then transferred along the ossicles (chain of three small bones)
that traverse the middle ear cavity
o the handle is the malleus and it is attached the medial surface of the tympanic
membrane
o the incus pivots on that to the stapes, which hits the oval window
 on the other side of that is the perilymph filled vestibule of the bony labyrinth
o this middle ear apparatus acts as a transformer and amplifies the current.
o two tiny muscles attached to the middle ear bones modulate the transmission of
vibrations
 the tensor tympani(innervated by the trigeminal motor) is attached to the
malleus and when it contracts it increases the tension on the tympanic
membrane and decreases the transmission of vibrations.
 the stapedius (innervated by the facial) is attached to the neck of the stapes and
decreases transmission as well.
the cochlea has a core of spongy bone called the modiolus, from which the osseous spiral lamina
projects like threads of a screw.
o a winging cavity within houses the spiral ganglion which contains the cell bodies of the
auditory primary afferent fibers. The central processes of these cells collect at the base
of the cochlea to form the cochlear division of the eighth nerve.
the cochlear duct is triangular so that there is a partition between two perilymphatic spaces
o except at the tip (helicotrema) where perilymph can flow from both sides.
o the top of this duct is the scala vesibuli
o the bottom is the scala tympani
 this ends at the round window so that the oval window puts pressure on the
top, it flows around the helicotrema and releases pressure through the round
window.
o the space enclosed by the duct is filled with endolymph and is called the scala media.
 the basilar membrane spans the gap between the edge of the osseuous spiral
lamina and the spiral ligament
The intensity of a stimulus is coded by the rate of action potential firing in population of nerve
fibers and by the number of nerves responding.
Frequency is encoded by which part of the organ of corti (a strip of hair cells that rest on the
basilar membrane) is active.
o inner hair cells and outer hair cells.
 these are separated by a perilymphatic space called the tunnel of corti through
which the peripheral processes of the 9th nerve must pass on their way to the
outer hair cells.
 inner are the sensory cells and outer are the amplifiers
 a single inner hair cell typically synapses on about 10 different auditory
afferents.
 the sensitivity of the outer hair cells can account for the amazing
preffered frequencies of each section that cannot be accounted for
solely by the basilar membrane.
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outer hair cells can actually change length and amplify vibrations of the
basilar membrane thereby making the inner hair fire stronger.
o the steriocilia of the outer hair cells are inserted into the gelatinous tectorial membrane
so that vibrations of the basilar membrane cause oscillations of the hairs and therefore
oscillation of the membrane potential of the hair cells.
 the inner hair cells get stimulated by the endolmph movement inside the duct
A pressure pulse delivered to scala vestibula by movement of the staped on the oval window
causes a travelling wave in this upper section and reaches a peak amplitude at a location that
depends on the frequency of the stimulus.
o the entire basilar membrane responds to intense low-frequency sounds, but closer to
threshold it is driven most effectively by sounds of pressively higher frequencies as one
moves from the apex to the base of the cochlea.
 (higher is closer to the oval window)
o this mechanical tuning of the basilar membrane is known as tonotopic organization
where frequencies are mapped in an orderly fashion.
Auditory Transmission
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auditory afferent, whose cells bodies are located in the spiral ganglion of the modiolus enter the
brainstem at the pontomedullary junction.
o one branch goes to the dorsal cochlear nucleu and another goes to the ventral cochlear
nucleus.
some fibers loop over the top of the inferior cerebellar peduncle, cross the midline and join the
lateral lemniscus (the major ascending auditory pathway of the brainstem)
vritually all fibers of the lateral lemniscus terminate in the inferior colliculus which then projects
through the brachium of the inferior colliculus to the MGN.
these then project tonotopically to A1, which is located in the transverse remporal gyri(Hescl) on
the superior surface of the temporal love
other fibers (from the ventral cochlear nucleus) pass beneath the cerebellar peduncle and head
for the superior olivary nucleus where there is a comparison of timing of arrival and intensity of
the two auditory inputs. This allows for sound localization.
crossing from one cochlear nucleus to the contralateral superior olvary nucleus occurs in the
trapezoid body, a large collection of second-order fibers that pass through and ventral to the
medial lemnisci.
the superior olive then projects through each of the lateral lemniscus to the ipsilateral inferior
colliculus.
loud sounds can trigger the acoustic reflex where the olive projects to facial and then contracts
the stapedius
o this can also be used to help with attention to one particular stimulus.
o the tensor tympani plays a role in this but only for extremely loud noises.
the olive can also project back to the cochlea to the outer hair and inner hair cells to help
suppress the contractility of outer hair cells in the presence of high-frequency noise
o this helps the inner hairs concentrate on things within its range, like human speech.
inputs to the organ of corti from the inferior colliculus can change with attention.
The Vestibular System
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Inside the bony labyrinth there are three semicircular canals(horizontal, anterior, posterior) and
two or more otolithic organs(utricle and saccule)
o each semicircular duct communicates at both ends with the utricle.
At one end of each duct is a dilation called an ampulla that contains a crista, which is a
transversely oriented ridge of tissue covered by supporting cells and sensory hair cells.
the geltanious mass here is called a cupula and the hair cells sit below it in the same way as the
ear.
all the hair cells of a given crista are aligned with their kinocilia facing in the same direction, so
deflection of the cupula in one direction causes the afferents that innervate that crista to
increase their firing rate and deflection in the opposite direction to decrease their firing rate.
the most straightforward way to deflect a cupula is to rotate its semicircular duct about an axis
perpendicular to it.
o at first the endolymph will lage behind because of inertia.
 this motion of duct and endolymph will deflect the cupula and sitmulate the hair
cells.
o at the end of the rotations, the endolymph, due to intertia, will continue to move for a
short time and the cupula will bulge in the opposite direction.
The utricle and saccule have a patch of supporting cells and hair cells called a macula
o uculla , in an upright standing human, point towards the sky.
 sensitive to linear acceleration forward to backward
o the saccular macula has its steriocilia pointing laterally.
 sensitive to linear acceleration in the sagittal plane.
o they are also embedded in a gelatinous membrane. that contains minute crystals of
calcium carbonate called otoconia (which is why it is called the otolithic membrane)
 these otoconia make the membrane denser than endolymph so that the
membrane flops around and stays flopped with positions in head change.
Vestibular Transmission
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Primary afferents have their cell bodies in scarpa’s ganglion
they enter the brainstem at the pontomedullary junction
some fibers go straight to the cerebellum through the juxtarestiform body on the medial edge of
the inferior cerebellar peduncle to synapse on the nodulus.
most primary affernts end in the vestibular nuclei of the rostral medulla and caudal pons.
o the cerebellum and spinal cord will also project here.
The vestibular complex primarily goes to the cerebellum (vermis and flocculonodular via the
juxtarestiform body), the thalamus, the spinal cord in the lateral/medial vestibulospinal tracts,
motor nuclei of extraocular muscles, and the vestibular apparatus.
Lateral vestibulospinal tract
o travels through the ventral part of the lateral funiculus and project to motor neurons for
antigravity and postural change.
o the medial version of this helps to stabilize head position.
During head rotations that are too alrge to be compensated for by the VOR, the reflex is periodically
interrupted by very rapid eye movements in the opposite direction. The resulting back-and-forth eye
movements, with a slow phase in one direction and a fast phase in the other is known as nystagmus.
nystagmus is named for its “fast” phase.
nystagmus helps to ensure that there is a stable image on the retina and is partially coordinated by the
cupula
Warm water instilled into the left ear causes the endolymph in the left horizontal canal to warm and
rise, causing a convection current of endolymph in a clockwise direction, which mimics an individual
rotating to the left. This causes nystagmus to the left. This process is known as caloric nystagmus. COWS
= cold opposite, warm same.
If you are on a train and tracking visual stimuli as they pass, you follow until out of view and then cycle
back to the front. This nystagmus is called optokinetic nystagmus.
The vestibular nystagmus system involves the MLF
As blood alchol levels rise, alcohol initially leaves the capillaries and infiltrates the cupulae, making them
less dense than the surrounding endolymph. if you hold the head in a position that lets the cupula float,
then you get nystagmus. After blood alcohol levels drop, alcohol leaves the cupulae first and makes
them more dense then the endolymph and causes positionally dependent nystagmus in the opposite
direction.
Efferents can control the sensitivity of vestibular hair cells from the reticular formation. They can help to
compensate for self-generated head movements. Since the hair cells receive the same input no matter
what, they should be modulated if things are intentional vs. not.
The visual system coordinates with vestibular signals, since vestibular alone provide no information
about the position of the body.
Mechanoreceptors in the neck detect the orientation of the head relative to the body and help with
orientation.
These can all combine in the vestibular nuclei.
Romberg’s sign is a greatly increased swaying when one closes their eyes.