Download Cell Bio 14- Auditory Pathways All 3 parts necessary to hear

Cell Bio 14- Auditory Pathways
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All 3 parts necessary to hear airborne sounds
Only inner necessary to hear sounds conducted through bone
CN VIII
2 sensory divisions:
• Cochlear (sound)
• Vestibular (position/movement of the head)
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Sound Transmission
Sound waves enter external auditory canal
Causes tympanic membrane (eardrum) at end of canal to vibrate
Outer and Middle Ears
 Pinna, a skin-covered flap of cartilage, collects and focuses sound waves into the external auditory canal
 The external auditory canal ends at the eardrum (tympanic membrane)
 The middle ear is an air-filled chamber that is bounded by the tympanic membrane on one side and the oval
window on the other
 The primary function of the middle ear is to transfer the vibrations of the tympanic membrane to the oval
window
 It accomplishes this task with a chain of three bones called ossicles: the malleus, incus, and stapes
 Footplate of stapes occupies a hole in the temporal bone: oval window
o Other side of oval window = cochlea, organ of Corti
o Vibration of tympanic membrane ultimately  movement of inner ear fluids
o handle of malleus is attached to tympanic membrane, so sound-induced vibrations are transferred
directly to malleus, then incus, then stapes
Cell Bio 14- Auditory Pathways
Amplification from Outer to Inner Ear
Within the 4 cm or so occupied by the outer and middle ears, three distinct physical principles operate to magnify weak
vibrations in air so that they can establish pressure waves in a liquid:
1) The organ pipe resonance of the ear canal may increase the air pressure ~x10
2) The mechanical advantages of the bone lever system may nearly triple the air pressure
3) The pin-pointing arrangement of the eardrum and the oval window may provide another 30-fold increase
The result of these three mechanisms is an amplification of a sound wave by more than 800 times before it
sets the liquid of the inner ear in motion.
Hearing Threshold
• Humans are most sensitive to ~3000 Hz
• Frequency range in healthy young adults is ~20-20,000Hz
• Damage at 90dB or greater for sustained periods of time
Perilymph and Endolymph
• Bony labyrinth is filled with perilymph
• Similar to CSF (low K+ concentration, high Na+ concentration)
• Continuous with subarachnoid space through the cochlear duct
• Membranous labyrinth is filled with endolymph
• More like intracellular fluid (high K+ concentration, low Na+ concentration – positive electrical potential
inside)
• Tight junctions – barrier between endolymph & perilymph
Meniere’s Disease
• Transient attacks of vertigo, nausea, & hearing loss accompanied by ringing in the ears (tinnitus)
• Swelling of the membranous labyrinth usually due to an endolymphatic blockage
• Disruption of fluid secretion, damaging hair cells
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Hair Cells
Microvilli (stereocilia) – project as a bundle from the cell into the endolymphatic interior of the membranous
labyrinth
Other end synapses on CN VIII nerve fibers that send auditory/vestibular info to CNS
Stereocilia and apical surfaces of hair cells are bathed in endolymph while other hair cell surfaces & the
associated nerve fibers are in perilymph
The hair bundle consists of one large kinocilium, and 50 to 150 stereocilia
Little; 0.2-0.8 μm in diameter, 4-10 μm high
Within the bundle, stereocilia are connected to each other, but they can slide with respect to each other as the
bundle is deflected side to side
Hair cells are “bent” (pivot at the base – site of attachment to hair cell) to transduce a signal
Bending the hair bundle toward the kinocilium excites the cell and causes depolarization
Bending the hair bundle away from the kinocilium hyperpolarizes the cell
Mechanically-gated cation channel
Deflection towards kinocilium stretches links, opens channels, and causes K+ to flow from endolymph into hair
cells  depolarization
Ultimately causes release of neurotransmitter onto CN VIII nerve endings (mainly glutamate,  increased firing)
Deflection in opposite direction decreases tension in links, channels close  hyperpolarization
Hair cells are contained within a membranous labyrinth
The auditory portion of the labyrinth is the spiraling cochlea, which detects rapid vibrations (sound) transmitted
to it from the surrounding air
The vestibular portion contains two structures: the otolith organs, which detect gravity and head tilt, and the
semicircular canals, which are sensitive to head rotation
Cell Bio 14- Auditory Pathways
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How the Cochlea Functions
The pressure waves in the cochlea exert energy along a route that begins at the oval window and ends abruptly
at the membrane-covered round window, where pressure is dissipated
The pressure applied to the oval window at the stapes is transmitted to all parts of the cochlea
One of the two walls of the cochlear duct is the vibrating basilar membrane which functions to separate sounds
by frequency.
The membrane is narrow and taut at the end near the stapes and is wider and more pliant at the other
Hydraulic pressure waves in the cochlea induce a wave-like ripple in the basilar membrane which travels from
the taut to loose end
High tones produce greatest crests where the membrane is tight, low tones where the wall is slack
The position of this crest is important because it determines which nerve fibers will send signals to the brain
 High frequency tones cause the crest to occur at the base of the cochlea and the lower frequencies
towards the apex
Apart from airborne sounds, the basilar membrane also picks up vibrations in the skull from such sources as
teeth clicking
Cell Bio 14- Auditory Pathways
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Auditory and Vestibular Transduction
Sensation in both the auditory and vestibular systems begins with the inner ear, and both use a highly
specialized kind of receptor called the hair cell
Hair cells are mechanoreceptors that are specialized to detect minuscule movement along one particular axis
Inner Ear
Two membranes divide the cochlea into three fluid-filled compartments
Scala vestibuli
Reissner’s membrane
Scala media (where hair cells are)
Basilar Membrane
Scala tympani
Cochlea
• The cochlear duct is the auditory portion of the membranous labyrinth and is anchored to the bony labyrinth
• Space enclosed by cochlear duct = scala media
• Scala vestibuli above: directly continuous with the vestibule
• Scala tympani below: ends at the round window (secondary tympanic) membrane
• Vibrations reaching stapes are transferred to perilymph of the vestibule
• Round window membrane is elastic, allowing vibrations to enter the labyrinth
• Vibrations are passed from scala vestibuli to scala tympani, deforming the cochlear duct
• Cochlear duct contains auditory receptors, and this deformation stimulates some of them
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Encoding Sounds
Auditory system must encode intensity, frequency, and location of a stimulus
Intensity of sound encoded by frequency of APs
Frequency (perceived as pitch) encoded by the part of the organ of Corti/basilar membrane that is most active
Location assessed in the CNS by comparing sounds reaching the 2 ears
Organ of Corti
• Strip of hair cells and supporting cells that rests on the basilar membrane
• 2 groups: ~3500 inner hair cells, ~15000 outer hair cells
– Separated by perilymphatic space, the tunnel of Corti, through which peripheral processes of CN VIII
pass en route to outer hair cells
• Outer hair cells are inserted into a gelatinous tectorial membrane so that vibration of basilar membrane causes
oscillations of the hair cells
• Inner hair cells are stimulated directly by endolymph movement
Cell Bio 14- Auditory Pathways
Innervation of Cochlear Hair cells by sensory endings
• Although outer hair cells outnumber inner hair cells, ~90% of cochlear nerve fibers receive their entire input
from single inner hair cells
• Inner hair cells are sensory cells; each synapses on ~10 auditory afferents
• Outer hair cells serve as signal amplifiers
Otoacoustic Emissions
• Motion of OHCs can enhance basilar membrane vibration, which can travel backward through perilymph and
middle ear ossicles to vibrate the tympanic membrane
• Vibrating tympanic membrane behaves like a tiny loudspeaker, producing sound – can be recorded as
otoacoustic emissions
• For several ms after arrival of a brief sound at the ear, a sensitive microphone in the external auditory canal can
detect faint sounds emitted by the ear itself
• Requires normal functioning of middle ear & inner ear up to/including OHCs – basis for audiological tests,
particularly to screen for hearing problems in newborns
Basilar Membrane
• Vibration from stapes to scala vestibuli causes a traveling wave of deformation to move along the basilar
membrane
• Mechanical properties of basilar membrane vary along its length
• Traveling wave reaches its peak amplitude at a location that depends on the sound frequency
• Close to threshold, driven most effectively by higher frequencies at base, lower at apex
• Organ of Corti contains auditory receptor cells rests on basilar membrane – mechanical tuning of the basilar
membrane is the beginning of tonotopic organization (orderly mapping of frequencies)
Cochlear Implants
• Some common forms of deafness result from damage to or abnormal development of cochlear hair cells
– CN VIII nerve endings intact
• Substantial auditory function can be regained by implanting an array of electrodes through the round window
membrane and into the scala tympani
• Small microphone and associated electronics can be used to break sound frequencies down and stimulate CN
VIII nerve endings at tonotopically appropriate levels
• This direct stimulation of cochlear nerve fibers can make nearly normal speech perception possible
Cell Bio 14- Auditory Pathways
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Auditory Pathway
Auditory afferents (cell bodies in the spiral ganglion) enter brainstem between the pons and medulla
Each fiber bifurcates and sends one branch to the dorsal cochlear nucleus and one branch to the ventral
cochlear nucleus (both flank the ICP)
Some fibers (mainly from DCN) cross midline and join lateral lemniscus, the major ascending auditory pathway
of the brainstem
Other fibers project to ipsilateral lateral lemniscus; thus each LL has info from both ears
Fibers then project to inferior colliculus
From IC, fibers project bilaterally through the brachium of the IC to the medial geniculate nucleus (MGN)
MGN projects tonotopically to primary auditory cortex (transverse temporal gyri of Heschl)
Localizing Sound
 Auditory afferents (cell bodies in the spiral ganglion) enter brainstem between the pons and medulla
 Each fiber bifurcates and sends one branch to the dorsal cochlear nucleus and one branch to the ventral
cochlear nucleus (both flank the ICP)
 Some fibers (mainly from DCN) cross midline and join lateral lemniscus, the major ascending auditory pathway
of the brainstem
 Other fibers project to ipsilateral lateral lemniscus; thus each LL has info from both ears
 Fibers then project to inferior colliculus
 From IC, fibers project bilaterally through the brachium of the IC to the medial geniculate nucleus (MGN)
 MGN projects tonotopically to primary auditory cortex (transverse temporal gyri of Heschl)
Neural Pathways for Hearing
 Arrangement of neurons along auditory pathway is tonotopic
 Sound information from each ear is transmitted bilaterally through the thalamus to each auditory cortex
 Receptive fields of neurons in auditory cortex are biaural
 Important for sound localization
 Damage to cortex of one hemisphere does not cause hearing loss but does cause problems with sound
localization
 Two ears are necessary for the detection of sound in a horizontal plane
 Sounds must first be processed by the cochlea in each ear and then compared by neurons within the CNS to
estimate horizontal direction
Cell Bio 14- Auditory Pathways
Auditory Lesions
• Unilateral lesions of cochlea, cochlear nerve or cochlear nuclei: Profound ipsilateral hearing loss
• Unilateral lesions above cochlear nuclei: No significant hearing loss
• Bilateral central lesions: Profound hearing loss
• SON lesions: Difficulty localizing sounds in space; Not deaf
Definitions
• Presbycusis: Hearing loss associated with age
• Gradual, bilateral
• Most common cause of hearing loss
• Hypacusis: Reduction in hearing
• Anacusis: Absent hearing
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Middle Ear Muscles Filtering Sounds
Middle ear muscles contract in response to loud sounds – contraction of stapedius stiffens the ossicular chain
and alters transmission of vibrations
– When a loud sound enters one ear, both stapedius muscles contract: acoustic reflex (tested by
measuring changes in the amount of a test sound reflected back from an eardrum after a loud sound in
each ear)
– Person with damaged CN VII may complain that sound is too loud in ipsilateral ear (hyperacusis)
– Pathway from on VCN to both SON, then to both facial motor nuclei
Stapedial contractions selectively impede transmission of low frequencies – reduce noise
Tensor tympani contraction may help to filter out self-generated noise
Hearing Issues
Hearing can be tested by measuring an individual’s thresholds for hearing a series of pure tones at different
frequencies
– Air conduction: tones presented through earphones
• Hearing normally by air conduction requires normal function of outer, middle, and inner ears
• Most hearing aids amplify sound through this route
– Bone conduction: tongs applied to the mastoid process or forehead
• Hearing by bone conduction: direct transmission of vibrations from skull to fluids of the inner
ear, bypassing outer and middle
Audiogram plots threshold sound levels for each frequency
Conductive hearing loss: sound is prevented from reaching the labyrinth
– Someone who hears normally by bone conduction but not by air conduction (B; bilateral)
– Typical causes: accumulation of fluid in the middle ear due to infection, or bony growths that impede
vibration of the middle ear ossicles (otosclerosis)
– Can be helped by hearing aid
Sensorineural hearing loss: damage to hair cells, cochlear nerve fibers, or cochlear nuclei
– Hearing loss measured by bone conduction indicates a sensorineural problem (C; unilateral)
– Typical causes: hair cell damage, cochlear nerve damage (e.g., from an acoustic neuroma, Ménière’s
disease, damage from prolonged loud noises)
Cell Bio 14- Auditory Pathways
Weber Test
• Will localize hearing loss
• 256 Hz tuning fork on vertex of skull
• Ask patient to localize sound
• Sound in both ears if patient is normal
• Louder in ear with conduction deafness
• Louder in normal ear in patient with sensorineural deafness
Rinne Test
• Compares air vs. bone conduction
• Tuning fork on mastoid process, then next to ear
• Air > bone conduction in normal patient
• Bone > air in conduction deafness
• Air > bone in sensorineural deafness
Recap
Pinna reflects sound & affects ability to locate sounds
Outer & middle ear turns air vibration into bone vibration
Inner ear turns bone vibration into fluid vibration
Cochlea turns fluid vibration into tissue movement
Different parts of the cochlea resonate to different frequencies of sound
The basilar membrane of the cochlea contains ciliated, mechanosensitive receptor cells (which also resonate)
The receptor cells release neurotransmitter on 1° auditory neurons
1° auditory neurons of the spiral ganglion project in the 8th cranial nerve to the medulla
The place coding is preserved throughout the CNS (tonotopy)
High & low frequency sounds are localized in the horizontal plane by interaural intensity & timing differences,
respectively
11) Sound is localized in the vertical plane, and back/front ambiguity is resolved by spectral differences
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Key Concepts
Depolarization/hyperpolarization of hair cells; endolymph vs. perilymph K+/Na+ concentrations
Tonotopy of the cochlea; auditory lesions (slide 38 – don’t worry about knowing the entire pathway)
How intensity, frequency, and location of sounds are encoded
– interaural time difference, interaural intensity difference
Cochlear implants
Conduction deafness
Sensorineural deafness
Air vs. bone conduction; audiograms; Weber vs. Rinne tests