auditory and vestibular Flashcards
Review: laminar organization of the retina
inside out organization whereby light can pass through the cells, reflect off the back of the eye, and excite the rods and cones. R and C synapses onto bipolar cells, and to ganglion cells (with horizontal and amacrine cell input)
Production of sound
these are variations in air pressure
alternating compressed and spaced out sound waves propagated by elastic medium (the hair) at ~340 m/s
ears must capture this mechanical energy, transmit to receptor organ, and transduce to electrical siganls for neural analysis
comparison of auditory and visual pathways
auditory receptors in cochlea -> brain stem neurons -> mgn -> auditory cortex
photoreceptors in eye -> other retinal neurons -> LGN -> visual cortex
gross anat. of ear
pinna, auditory canal (meatus), tympanic membrane, ossicles, and cochlea
pinna
outer part of ear that’s right above the earlobe
helps collect sound from wide area
auditory canal (meatus)
entrance to internal ear; conducts sound
tympanic membrane
“eardrum”
separates the outer from inner ear
ossicles
tiny bones transfer tympanic membrane reverberations to oval window
cochlea
contains fluid that physically transduces the oval window reverberation into neuronal signals
What is the pathway of sound waves through the ear?
sound wave moves the tympanic membrane
tympanic membrane moves ossicles
ossicles move membrane at oval window
motion at oval window moves fluid into cochlea
fluid into cochlea causes response in sensory neurons
middle ear
malleus, incus, stapes, and eustachian tube
malleus (the hammer)
attached to tymp. membrane, forms rigid connection to incus
incus (anvil)
forms flexible connection to stapes
stapes (the stirrups)
connects to oval window like piston
eustachian tube
air inside is continuous with nasal cavity, although closed by a valve; pressure changes distend (inward or outward by the tymp membrane, causing pain)
sound amplification
goes through tympanic membrane and middle ear because:
cochlea filled with fluid, not air
ossicles transmit sound waves v concentratedly (because oval window is small compared to tmp mem)
sound at oval window thus is ~20x more intense than at tmp membrane
attenuatoin reflex
contains tensor tympani muscle and stapedius muscle
contraction increases rigidity of ossicles
may be to adapt the ear to continuous high intensity sound
may protect the inner ear, small delay though
activated when we speak so we don’t hear our own voices so loudly
tensor tympani muscle
originates at the bone in the skull and inserts into the malleus
stapedius muscle
originates at bone in skull and inserts onto stapes
cochlear anatomy
roughly size of pea
three fluid-filled membranes: scala vestibuli, scala media, and scala tympani
reissner’s membrane separates vestibuli from media
organ of corti surmounts basilar membrane and contains hair cells covered by tectorial membrane
basilar membrane of the cochlea
cochlea narrows from base to apex – however the basilar membrane widens from base to apex (by ~5x)
stiffness of basilar membrane decreases from base to apex (by ~100x)
flexible and bends in response to sound
traveling wave in basilar membrane
when sound pushes footplate of stapes at oval window, perilymph is displaced in scala vestibuli AND endolymph is displaced at scala media
stapes can also pull at oval window and displace perilymph in opposing fashion
basilar membrane of cochlea
similar to rope/extension cord/hose – high frequency waves dissipate quickly but low frequency waves propagate very far
response of bm establishes place code in which different locations of membrane are maximally deforced
organ of corti
contains hair cells, rods of corti, and various supporting cells
hair cells - contain ~100 stereocilia that bend in response to sound and initiate the transduction of sound into neural signal
rods of corti span the reticular lamina and basilar membrane and provide structural support
stereocilia of outer hair cells extend through reticular lamina, into endolymph, and ends in tectorial membrane
stereocilia of inner hair cells extend through reticular lamina, into endolymph, but do not reach tectorial membrane
compression
downward deflection
causes stereocilia to bend in and hyperpolarize
rarefaction
upward deflection
stereocilia bend outwards and depolarize
stereocilia bend
regulated by TRP (transient receptor potential) and TRPA1 channels (mediates mechanoelectrical transduction of noise
outer hair cells
one spiral ganglion synapses into multiple
inner hair cells
one spiral ganglion synapses onto one
amplification by outer hair cells
amplifies the movement of basilar membrane
mechanism of sound localization
horizontal localization = needs comparison of sounds reaching BOTH ears
vertical localization = one ear is sufficient
interaural time delay
interaural time delay
sound from the right/.left will reach right/left ear first and there will be a delay to the other ear
doesn’t work well with continuous high frequency tone
interaural intensity difference
due to head effectively casting sound shadow over opposite ear
what are delay lines and neuronal sensitivity to interaural delay
monaural neurons and binaural neurons
monaural neurons
respond to sound from one ear (cochlear nuclei)
binaural neurons
respond to sound from both ears (superior olive nuclei and “above”)
sound localization based on reflection from pinna
localization of sound in vertical plane heavily relies on pinna
tonotopic organization
axons leaving MGN project to primary auditory cortex via internal capsule in array called acoustic radiation
semicircular canals
part of vestibular system
used for detecting head roation
otoliths
detects force of gravity, tils of head, and linear acceleration based on the calcium carbonate crystals inside it
macular orientation
when the head is upright:
vertically orientated with saccule, horizontally orientated with utricle
mirror image orientation of saccule and utricle on both sides of head
with movement, hair cells on one side are excited and on opposite side are inhibited – allowing CNS to unambiguously interpret all possible linear movements
ampula and semicircular canal
cilia of hair cells extend into gelatinous cupula
cupula is bathed in endolymph
when canal rotates, endolymph lags behind and applies force to cupula in opposite direction
push-pull activation of semicircular canals
head rotation causes excitation of hair cells in one horizontal semicircular canal and inhibition of hair cells in other
figure here shows that long lasting head rotation leads to adaptation of firing in vestibular axons
when rotation is stopped, vestibular axons from each side begins firing again, but with opposite patterns of excitation and inhibition
presbyacusis
hearing loss due to physiologic process of aging
can be due to degeneration of cochlear hairs + supporting cells; thickening of basilar membrane; degen of neurons; changes in auditory cortex/processing
