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neuro - yr 2 > hearing - from sound to synapses > Flashcards

hearing - from sound to synapses Flashcards

1
Q

what are the main causes of hearing loss *

A

loud traumatic sounds eg military, industrial, clubs

200 genetic conditions that can cause hearing problems

infections like meningitis or congenital ones eg rubella/syphilis

drugs that are used for severe heart infections and chemo

aging

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2
Q

what is the frequency range heard by humans *

A

20Hz to 20KHz

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3
Q

what is frequency *

A

cycles/second

percieved as pitch

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4
Q

define pitch *

A

the perception of frequency

most sensitive at 1-3kHz

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5
Q

what is timbre

A

it is what distinguishes 2 sounds at the same frequency and intensity - ie what distinguishes between 2 instruments playing the same note

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6
Q

how sensitive is the ear

A

the most sensitive sense

the internal ear can detect movements of size a fration of a nm

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7
Q

what is intensity *

A

the amount of energy given per second - how many Joules per second pass through 1 square m

percieved as volume

amplitude

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8
Q

what is the range of energy that we can hear

A

we can hear 1 watt of energy over size 3x UK - lowest intensity

highest intenstity - 12 orders of magntitude higher (this is pain)

ie range 10(power -12)W/sq m -> 1w/sq m

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9
Q

why do we use a decibel scale *

A

to make the numbers managable

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10
Q

what is teh decibel scale *

A

instead of measuring the intensity with respect to faintest percievable intesity of sound Io - compare logs - this is the bel scale

multiply everything by 10 = decibel scale

10Log(I) - 10log(Io) = 10log(I/Io) = dB

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11
Q

summarise the path of sound waves from the air to the hair cells *

A

ear detects sound waves in the air - stimulate the tympanic membrane

vibration go through the 3 small bones to the cochlear

here is the basilar membrane where there are 4 rows of hair cells - the sensory receptor of the internal ear

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12
Q

describe the hair cells in the cochlear *

A

they take their name from the hair bundle

the hair bundle is a cluster of modified microvilli called stereocilia

there are 3 rows of outer hair cells - 20000 in 3 rows, 1 axon innervates several cells

one row of inner hair cells - 3500 in 1 row - innervated by 10 sensory neurons per cell

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13
Q

describe how the ossicles are involved in hearing *

A

they transmit the vibration of the tympanic membrane to the cochlear

they match the impedance to reduce loss in energy as the vibration goes from the air to the cochlear

when sound moves from 1 medium to another - eg from air to fluid some sound is transmitted and some is reflected

the impedence measures the reluctance of a system in recieving energy from a source

if the impedence is the same between 2 mediums - all of the sound will be transmitted - the impedence depends on the mechanical properties locally

the position of incus and malleeus can be adjusted by the tensor tympanic muscle and stapedius muscle to control the tension of the tympanic membrane - providing an inbetween inpedance to air and liquid = increased transmission

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14
Q

what is the resonant frequency *

A

the frequency at which the impedence of the system is minimal - ie transmission of sound is max

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15
Q

what is the structure of the cochlear *

A

a snail shaped organ

filled with liquid

it is divided longitudinally by the basilar and vestibular membranes into 3 compartments

sound wavees casue these membranes to vibrate

the air cells are on the basilar membrane

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16
Q

what is conductive hearing loss *

A

when the ear is not capable of transmitting the vibration of sound waves onto the cochlear

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17
Q

what are the causes of conductive hearing loss *

A

earwax (cerumen)

infections eg otitis

tumours

in children fluid accumulation in the inner ear

perforated tympanic membrane

abnormal growth of bone (otosclerosis) can obstruct ear canal

barotrauma (temporary) - because of increased air or water pressure eg on a plane

congenital malformations

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18
Q

How is the cochlear involved in hearing *

A

the motion of the stapes generates a pressure difference between the 2 fluid filled chambers of the cochlear - this causes the vibration of the basilar membrane

the relative movement of the tectorial and basilar membrane stimulate the hair cells

19
Q

what makes up the organ of Corti *

A

the basilar membrane

tectoral membrane - gelatinous - doesnt vibrate with sound

hair cells

supporting cells - surround the hair cells

spiral ganglion is embedded in modiolus and innervates hair cells

stria vascularis secretes endolymph - high in K low in Na

20
Q

what is the function of the basilar membrane *

A

it is an elastic structure - it has heterrogenous mechanical properties at different positions along its length in response to different frequencies - ie vibrates at different positions depending on the frequencies

it breaks complex sounds down - distribute the energy of each component freq along length

sensory receptors (hair cells) are along the length of the membrane to detect frequencies

this is a tonotopic map - higher frequencies vibrate the membrane nearer the base, lower frequencies nearer the apex

21
Q

describe how the hair cells act as sensory receptors *

A

the motion of the basilar membrane deflects the hair bundles of the hair cells

the bending of the stereocilia towards the largest stereocillium changes the internal voltage of the cell -> electric signal that travels towards the brain - this is mechano-transduction

22
Q

how do stereocilia change the voltage in a cell *

A

they are connected by tip-links - filamentous linkages that act as springs

tip links share their location with ion channels

response currents in cells are because of the opening of these channels

the opening of channels relaxes the tip links -> measured stiffness of the hairbundles becomes -ve = bundle move actively when triggered by stimulus

23
Q

describe the 4 aspects of the active process with the hair cells *

A

amplification - living basilar membrane vibrates more than a dead membrane (passive)

frequency tuning - dead mem produces broadd response - not tuned for specific frequency, living selectively amplifies a single frequencies

competitive non-linearity - at low intensities, tehe basialr membrane moves more with increasing intesity - this ability decreases as intensity increases

spontanaeous otoacoustic emission - work produced as hair cells in normal conditions of low levels of sound to counteract the drag in the cochlear

24
Q

what are the differences between outer and inner hair cells *

A

more outer than inner

95% afferent projections are from inner - ie sensory

most efferent are to outer - they are involved in the active process and do the work

25
Q

what is electromotility *

A

the cell bodies of the outer hair cells shorten when their internal voltage is changed

due to the reorientation of the protein prestin

may be the orrigin of the cochlear amplificationand otoacoustic emissions

26
Q

describe the connection of the hair cells to cochlear ganglion *

A

there are synapses

there are many channels - in case one breaks

the ganglion cells in a particualr area of the spinal ganglion respond best top the resonant frequency of teh basilar membrane in the same area - this is the tonotopic map

27
Q

what is sensorineural hearing loss *

A

when the problem is rooted in the sensory apparatus of the inner ear or in the vestibulocochlear nerve

28
Q

what is the cause of sensineural hearing loss *

A

load noises can cause temp/permenant damage

many genetics affect the organ of Corti

aminoglycoside AB - toxic to hair cells

congenital disease- rubella/toxoplasmosis

acoustic neuroma (tumour on the cochlear nerve)

meniere’s disease

hereditory disorders

viral infection

injury to central auditory pathway

aging (presbycusis)

malfunctioning of auditory pathways - demyelination and blast injuries can affect the balance between inhibition and excitation in the superior olivary complex

29
Q

how can you treat hearing loss

A

bypass the hair cells and stimulate the nerve cells directly - ie detect cells, break them into the constituent frequencies and send signal to auditory nerves by antennas

elongated coil inserted into cochlear - pairs of electrodes corresponding to single frequencies

30
Q

describe the organisation of the ventral cochlear nucleus *

A

nerve fibres arranged tonotopically

low frequencies ventrally, high frequencies dorsally

31
Q

describe the role of the dorsal cochlear nucleus

A

it locates sound in a vertical plane

high frequency sounds produce interference with the body ie when the wavelength is the size of the head and external ear

the ears detect and react to these differently depending on their spectral cues ie when sounds come from different directions and have assymetrical shape

32
Q

what is the role of the superior olivary complex *

A

compares the bilateral activity of the cochlear nuclei

localise sounds in the horizontal plane

medial superior olive - computes the interneural time difference - sounds at near ear detected first, a map of interneural delay can be formed due to delay lines - neuron fires when stimulated by both ears at the same time

lateral superior olive - detects differences in intensity between the 2 ears >2kHz due to head size - this is interneural level difference because of head shadow. excitation that arrives ipsilaterally must arrive at same time as inhibition contralaterally - inhib signal carried by large axons with large synapses (the large calyxes of Held) axons for excitation are smaller and conduct more slowly

33
Q

what is the connection between the lateral superior olive and the cochlear *

A

superior olivary complex neurons send feedback to hair cells - to afferent fibres and to outer hair cells

this modulates the amplification done by the cochlear - protect the ears

34
Q

describe the inferior collicus *

A

made of the central nucleus (tonotopically mapped), dorsal cortex, external cortex

carries information about sound location

all ascending pathways converge here - sensitive to complex features

involved in precedence effect - brain filters out sound not necessary to localise it ie sounds of loer intesnity withing 30sec window

involved in reflexes - cause head to turn

35
Q

describe the superior collicus *

A

all ascending pathways converge here too

auditory and visual maps merge

neurons are tuned to respond to stimuli with specific sound directions

auditory map here is fundamental for reflexes - head turn in direction of sound

36
Q

describe the auditory cortex *

A

neurons respond to complex sounds

  • primary auditory cortex located in superior bank of the temporal lobe - central area of the auditory cortex - tonotopically mapped - loudness rate and freq modulation are mapped here - reflexes for complex tasks related to gaze control - can be trained to help dyslexia etc which have impaired auditory function
37
Q

describe the anatomy of the ear

A

outer - auricle and external acoustic meatus

middle - Air-filled chamber in bone, lying between tympanic membrane laterally, and oval and round windows medially

inner - cochlear and organs of balance, hair cells

38
Q

describe how the middle ear can be protective *

A

Reflex contraction of tensor tympani and stapedius muscles reduces amplitude of vibrations passing through ossicles

Protects against natural sounds but maybe not against man-made sounds

Auditory tube allows equilibration of air pressure on either side of tympanic membrane

39
Q

summarise how the middle ear is involved in amplication *

A

achieved by the lever system of the ossicles

also the ratio of the tympanic membrane to the oval window

40
Q

what is sound

A

pressure wave in air

41
Q

function of the outer ear

A

collects and conducts sound waves to tympanic membrane

42
Q

describe the transduction mechanism in the cochlear *

A

basilar membrane vibrates in response to sound

upward movement displaces cilia away from mediolus - K channels open - K enters from endolymph - hair cells depolarise

downward movement displace cilia to modiolus - K ion channels close - hair cell hyperpolarises

this requires the stria vascularis to maintain the endolymph at +80mV by stria vascularis

depolarisation opens Ca channels in body of hair cell

glutamate released from base depolarises axon of spinal ganglion cell -> AP

43
Q

summarise the auditory pathways *

A

complex bilateral pathways through the brain

colateral pathways go to reticular formation and cerebellum

lateral inhibition in ascending pathways enhances resolution of similar frequencies

descending pathways provide feedback at all levels

primary auditory cortex is divided according to frequency response

cells respond to specific features of sound - on/off, duration, repitition, intensity, rising and falling frequencies, animal vocalisations

secondary cortex - neurons respond to more compelx sound patterns

44
Q

describe the mechanism of amplification *

A

conduction through middle ear amplifies sound by 30db

lever system of articulated ossicles and ratio of area of tympanic membrane to oval window

neuro - yr 2 (24 decks)

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