NEUS 609 - Audition

mechanotransduction

process of transforming mechanical energy into neural transmission

• used in audition (hearing) and vestibular (balance) mechanisms

outer ear

part of the ear made of the pinna/auricle, the external auditory meatus, and the auditory canal

pinna

aka auricle

external cartilage

external auditory meatus

hole leading to auditory canal

auditory canal

tunnel between meatus and tympanic membrane

tympanic membrane

limit between outer and middle ear

middle ear

part of ear made of the 3 ossicles and the tympanic cavity

ossciles

malleus + incus + stapes

3 bones vibrating on each other and amplifying sound using a lever-like system

oval window

small surface limit between middle and inner ear

• what the stapes vibrate/concentrate pressure waves onto

inner ear

made of bony labyrinth and membranous labyrinth, aka cochlea and cochlear duct

bony labyrinth

cochlea aka fluid-filled spiraling tunnel in temporal lobe

membranous labyrinth

cochlear duct aka scala media aka fluid-filled cavity in the MIDDLE of the cochlea

vestibuli, media, tympani

cochlea is made of 3 main cavities: scala __ on top, scala __ in the middle, and scala __ on the bottom

basilar

the roof of the scala media is the vesicular membrane, the floor of the scala media is the ___ membrane

organ of Corti

part of the basilar membrane containing the hair cell and overhung by tectorial membrane

tectorial membrane

___ is the membrane in the scala media that is over the organ of corti and against hair cells

transduction mechanism

1. pressure waves from stapes vibrations on oval window propagates in fluid-filled space of the cochlea and deforms basilar membrane

2. deformation of basilar membrane deflects hair cells in uniform direction against tectorial membrane

3. tip links between the stereocilia of hair cells get pulled on and open the potassium channels

4. cell depolarizes, opening voltage-gated calcium channels which create action potential and send nerve impulse down auditory nerve aka cranial nerve VIII

inner hair cells

• hair cells forming 1 row on organ of corti

• stereocilia form 3 straight rows on EACH hair cell

• specialized mechanosensory cells transducing sound waves into neural impulses

• afferent innervation from dendrites of auditory sensory neurons

• ~ 3500 total

outer hair cells

• form 3 rows on organ of corti

• stereocilia form 3 V-shaped rows on EACH hair cells

• efferent innervation from axons coming from the brain

• specialized contractile cells that help sharpen the frequency resolution aka tuning cells

• ~ 12 000

stereocilia

long, non-motile microvilli

• form 3 rows on EACH hair cell: 1 short, 1 medium height, and 1 tall

• straight in inner hair cells

• V-shaped in outer hair cells

divide

hair cells DO NOT ___

• reason why hearing loss is permanent

tip links

each row of stereocilia is connected to the row next to it by ___

• made of extracellular filaments

• lose when stereocilia are upright

• tight/pulled when stereocilia are deflected against tectorial membrane

• connect the plasma membrane of the taller one to the potassium (K+) channel of the shorter one

deflection

__ of stereocilia causes tip links to tighten and thus open the K+ channels and depolarize the cell, causing the opening of Ca2+ voltage gated channels and vesicular GLU release.

upright

when stereocilia are ___ then the tip links are loose and the K+ channels are close and the cell is hyperpolarized

compression

high pressure waves

rarefaction

low pressure waves

frequency

aka pitch

• measured in hertz (Hz)

• how close the pressure waves are to each other

• greater the ___ = higher pitch

intensity

aka loudness

• measured in decibels

• how high the wave peaks are/the peak amplitude

• greater ___ = the louder the sound

sound amplification mechanism

amplified 22 times

1. mechanical amplification via the lever system formed by the ossicles in the middle ear

2. concentration of pressure wave on the oval window using the difference in surface area between tympanic membrane (big) and oval window (small).

wider, thinner

the basilar membrane is ___ and ___ at the apex (aka farther from cochlea) which is better at detecting low frequencies

narrower, thicker

the basilar membrane is ___ and ___ at the base (aka closer to cochlea) which is better at detecting higher frequencies

specific location

we perceive pitch due to the fact that basilar membrane vibrates maximally in a ___ along its length thus we detect specific frequencies

stiffening

outer hair cells help tuning by contracting and __ the basilar membrane for better recognition of higher/maximal vibration location

• aka better frequency distinction