slideshow Q's Hearing science exam 2

OE: what is the structure of the ear canal?

outer 1/3 cartilaginous, inner 2/3 bony

OE: what is the tool that is used to perform an assessment of the outer ear and Tympanic Membrane?

Otoscope

OE: what is the roll off of a filter determined by?

octaves of the cutoff frequency

OE: what are acoustic functions of the outer ear?

sound transmission and localization

localization 

where in space a sound is coming from

sound transmission

gathering, funneling, and amplifying sounds

Interaural timing difference (ITD)

difference in time of arrival of a sound between your two ears 

• sound reaches one ear slightly earlier than the other 

• the brain uses the delay to figure out the direction of the sound 

OE: at which frequencies are ITD (Interaural time difference) most efficient?

low frequencies (<850 Hz) 

OE: which sound localization cue does the Head Shadow Effect relate to? 

Interaural Intensity (Level) Differences (IID) 

Interaural Intensity (Level) Differences (IID) 

difference in sound levels (loudness) between your two ears for the same sound

• the sound is louder in the closer ear and softer in the further ear

Head Related Transfer Function (HRTF)

describes how your head, ears, and body change incoming sound before it reaches your eardrum 

• monaural localization cue 

what is the resonant frequency of the ear canal

~2500 Hz

why should you not use q tips? 

• can rupture your tympanic membrane 

• disrupts the outward migration of cerumen; hair cells only in the outer 1/3 portion of EAC, so it pushes it into osseous portion where it can get stuck.

ME: what frequencies do stiffness work against? 

low frequency transmission 

what are the functions of the middle ear?

1. transduce acoustic to mechanical energy

2. overcome the impedance mismatch 

how does the middle ear act as an impedance matcher

1. areal ratio

2. lever action

3. curved membrane buckling principle

ME: areal ratio

the TM is larger than the stapes footplate, therefore pressure is 17 times greater at the stapes footplate than the TM. moving sound from a large to small area increases pressure and adds gain 

ME: Lever action 

the length of the malleus is 1.3 times larger than the length of the incus. the incudomalleolar joint acts as a fulcrum. if we apply effort to the longer side, we can move more on the other end 

ME: curved membrane buckling principle

there is unequal distribution of force along the TM, with more displacement laterally. the umbo is fixed and moves the least, doubling the amount of force. 

ME: how much sound is attenuated due to the abrupt change in impedance from air to cochlea?

~30 dB SPL

ME: how much gain does each aspect of the impedance matching function of the middle ear add?

areal ratio: 24.6 dB SPL

lever action of the ossicles: 2.3 dB SPL 

curved membrane buckling principle: 6 dB SPL

impedance

the overall opposition to the flow of energy 

ME: what protection does the middle ear provide?

1. pressure equalization

2. acoustic reflex 

ME: how does the eustachian tube equalize pressure in the middle ear space?

when you yawn, sneeze, or swallow, the eustachian tube opens to allow air through the middle ear space, which equalizes middle ear pressure to atmospheric pressure

ME: Boyle’s law

as pressure increases, volume in cavity decreases

ME: how does the middle ear equalize pressure when a plane takes off?

pressure decreases → volume inside the middle ear space increases → TM bulges → eustachian tube opens to release build up of air

ME: why are children more likely to get otitis media?

children’s eustachian tubes are more horizontal so fluid cannot drain as easily

ME: otitis media with effusion 

ear infection with fluid buildup 

ME: how does the middle ear provide protection?

1. pressure equalization

2. acoustic reflex

ME: how does the acoustic reflex work?

• Sounds with inputs of 70-90 dB SL elicit this reflex

• The stapedius muscle constricts, increasing the stiffness of stapes and ME structures 

  • This also increases the impedance of the ME system 

  • Increased impedance = reduction of acoustic energy 

ME: how does the eustachian tube equalize pressure in the middle ear space?

when you yawn, sneeze, or swallow, the eustachian tube opens to allow air through into the middle ear space, which equalizes middle ear pressure to atmospheric pressure

ME: how much attenuation does the acoustic reflex provide? what frequencies does it affect the most?

~ 14-20 dB attenuation of loud inputs

• affects low frequencies the most (because of increased stiffness) 

ME: what cranial nerve is involved in the acoustic reflex? 

CN VII (cranial nerve 7) 

ME: when input levels are too loud, the middle ear will attenuate sounds. what type of system is this and why?

nonlinear system; the amount of gain is changed across input levels

IE: which ions are perilymph? which ones are endolymph?

Perilymph: sodium (Na+)

Endolymph: Potassium (K+)

IE: what is the function of the vestibular system?

detect angular acceleration/deceleration of our head movements in space: rotations, front to back, up and down, side to side 

IE: which labyrinths are filled with perilymph? which are filled with endolymph?

• perilymph: bony labyrinth → scala vestibuli and scala tympani

• endolymph- membranous labyrinth → scala media

IE: Outer Hair cell characteristics

• cylindrical shape

• arranged in rows of 3

• 12,000 total

• 100-150 stereocilia on each hair cell

• V shaped stereocilia bundles

• provides gain

IE: what direction does a traveling wave move?

displacement of the basilar membrane away from the base and towards the helicotrema and apex

IE: helicotrema 

connects the scala vestibuli to the scala tympani

tonotopic organization of the basilar membrane

• due to the mass-stiffness gradient

  • base is thin/narrow and stiff: higher freqs

  • apex is wide and floppy: lower freqs

what is the basilar membrane composed of?

overlapping bandpass filters

what is the characteristic frequency (CF)?

the lowest dB SPL required to maintain a constant displacement on the basilar membrane; the resonant frequency at that spot

vibrations of the _______ at the _________ displace ________ in the ___________, resulting in deflections of the ___________

stapes footplate, oval window, perilymph, scala vestibuli, basilar membrane

IE: X and Y axes of a tuning curve

x-axis: dB SPL/impedance

y-axis: frequency 

IE: what would a tuning curve look like for healthy OHCs vs. damaged OHCs look like?

Healthy: sharp tuning, active system, freq-specific, narrower bandwidth

damaged: broad tuning, passive system, less freq specific, wider bandwidth

IE: what does increased intensity result in for the mechanical properties of the basilar membrane?

non-linear displacement at the CF, changes in basilar membrane velocity, and frequency selectivity 

IE: what is the shearing action?

sideways movement of the tectorial membrane against stereocilia bundles as they move upward; allows for the sensation of hearing

what happens during a rarefaction during mechano-electrical transduction?

depolarization → tip links fully open, K+ flows in → voltage becomes more positive → basilar membrane moves up → stereocilia are pushed away from the modiolus and excited

what happens during a rest during mechano-electrical transduction?

• intracellular voltage at rest (70mV)

• endolymphatic potential (+80 mV) 

IE: what happens during a repolarization during mechano-electrical transduction?

• compression → tip links fully close, K+ flow stops → voltage becomes more negative 

IE: how does electromotility work? which hair cells does it apply to?

• outer hair cells

• elongate during depolarization and contract during hyperpolarization 

• this adds gain

IE: what neurotransmitter is released at the base of the hair cells once they are depolarized?

Glutamate

IE: which hair cells generate electrical events that are passed to the brain?

Inner hair cells

IE: what is the mass-stiffness gradient along the basilar membrane? what does it contribute to?

base: stiffness increases

apex: stiffness decreases

contributes to tonotopic organization 

IE: the absence of what electrical event signifies that the OHC stereocilia are damaged?

cochlear microphonic

• an electrical response generated by OHC’s of the cochlea that mimics the waveform of an acoustic stimulus, as if the cochlea were acting as a microphone

IE: the basilar membrane tunign curve has a _______ high freq roll off and a ______ low freq roll off

sharp; gradual 

IE: where is pressure released in the inner ear?

the round window

IE: in what bone is the inner ear housed?

temporal bone

what type of neurons do OHC’s have? what type do IHCs have?

OHC’s: efferent neurons (top-down)

IHC’s: afferent neurons (bottom-up)

what type of energy best describes the movement of inner ear fluid?

Hydrodynamic energy 

what is the purpose of cerumen?

• acts as a waterproof barrier preventing debris from entering the ear canal and damaging TM

• moisture regulation