L25- Hearing and the ear

why is hearing important as an early warning system

• 360 degree coverage

• works at a distance

• no line of site needed

why is hearing important as a means for information

• environment sounds

• sophisticated

• speech

• music

define sound

longitudinal pressure wave travelling through air or other medium

how is sound defined by frequency

• pitch (tone) of sound

• measured in Hertz (Hz)= cycles per second

how is sound defined by amplitude (intensity)

• loudness

• measured in dB (deciBels)

• log scale- to better match perception

what is the equation for frequency

1/t Hertz (s-1)

• t= period (s)

define a pressure wave

alternate compression and rarefraction of molecules

what is the threshold for hearing damage

90dB

describe the pinna in the outer ear

• not (significantly) mobile in humans

• in other species, sound is gathered by orientation towards a source

describe externalisation of sounds in the outer ear

• listen without pinna

• sound perceived inside the head

describe the localisation of sounds in the vertical plane in the outer ear

sound waves interfere on the surface of the pinna

describe acoustic gain (due to ear canal resonance) in the outer ear

10 to 15 dB amplification for 1.5 kHz to 7kHz

why is equal pressure needed either side of the tympanic membrane

hear is impaired if not

• due to air travel, diving, blocked Eustachian tube

why do we need a middle ear

without it there is poor transfer of energy from air to cochlear fluid

what is in the inner ear

fluid

what transformer is the middle ear

impedance transformer

describe what happens if the area of tympanic membrane is greater than the footplate of the stapes

• force on the tympanic membrane acts over smaller area of stapes footplate

• increases the pressure = force/surface area

• ossicles act as levers

which is longer, the malleus or the incus

malleus

what is the increase in pressure between the tympanum and stapes at a lever ratio of ~1.3x and an area ratio of ~18x

x25

what are the measurements of uncoiled cochlea

• 2mm diameter

• 35mm long

what does the scala vestibuli do

sound in

what does the scala tympani do

sound out

what is endolymph

high K+ in scala media

how many rows of inner hair cells do human ears have

1 row (3500)

how many rows of outer hair cells do human ears have

three rows (12000)

where are stereocilia found

at the tips of hair cells

describe stereocilia

• bend in response to pressure changes

• stereocilia of the tallest row of outer hair cells are embedded in the tectorial membrane

where is high frequency in the cochlea

base (stiff)

where is low frequency in the cochlea

apex (floppy)

what does the position of maximum displacement of a travelling wave on the basilar membrane depend on

frequency

describe frequency analysis by the cochlea

sound enters

• basilar membrane vibrates

• waves travel up basilar membrane from base to apex

• position of maximal displacement depends on frequency of sound

• displacement of basilar membrane excites hair cells

where do low frequencies produce maximal displacement

apex of the cochlea

where do high frequencies produce maximal displacement

base of the cochlea

what is movement of the basilar membrane relative to

tectorial membrane causing bending of hair cell stereocilia

what does excitation of inner hair cells lead to

transmitter release and excitation of auditory nerve fibres

how does movement of the basilar membrane and interaction with the tectorial membrane activate hair cells

causes bending of stereocilia of inner and outer hair cells

how does bending towards the tallest stereocilium activate hair cells

increased inflow of potassium ions- depolarisation

how does bending towards the shortest stereocilium activate hair cells

reduces inflow of potassium ions- hyperpolarisation

how are (only) inner hair cells activated

• release neurotransmitter

• activate cochlear nerve fibres associated with the hair cells

what are the 2 ways the cochlea filters by frequency

• each neuron responds best to one frequency

• each location along the cochlea amplifies one frequency

how do outer hair cells act as cellular motors

• change length as stereocilia bend

• shorten and lengthen in response to stimulation

• inject and amplify basilar membrane movement

what motor protein drives outer hair cells in the OHC membrane

Prestin

what is the reason hearing is so sensitive

detect BM movements in nm range

outer hair cells are damaged by which ototoxic drugs

• some antibiotics (aminoglycosides)

• other drugs (cisplatin, furosemide, salicylate)

what affect of the basilar membrane does elongation and shortening of outer hair cells have

increases displacement

what affect does increasing displacement have on the basilar membrane

enhances hearing sensitivity and frequency selectivity