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4. BIOL257 > Special senses II: hearing > Flashcards

Special senses II: hearing Flashcards

1
Q

physics of sound:

A
  • compressional/ longitudinal wave caused by variation in air pressure
  • medium (air) where sound moves oscillates in direction of propagation of wave
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2
Q

sound wave consists of:

A
  • alternate compressions

- rarefractions

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3
Q

define compression:

A
  • where air molecules are pushed closer together
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4
Q

define rarefactions:

A
  • where air molecules are farther apart
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5
Q

define wavelength::

A

distance btw adjacent compressions/ rarefactions of sound wave

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6
Q

amplitude: features

A
  • ‘loudness’
  • proportional to difference in density of air within rarefaction vs air in compressions
  • greater the difference, louder the sound (greater amplitude)
  • dB
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7
Q

frequency: features

A
  • in Hz
  • reciprocal of time taken to complete one oscillation cycle
  • frequency of sound wave aka pitch
  • humans:: 20 - 22 000 Hz
  • best range: 2000 - 5000
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8
Q

define pinna:

A

aka auricle

- visible part of ear outside head

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9
Q

external ear features:

A
  • skin covered, cartilaginous
  • sound is collected by pinna (best when sound from front for humans, convolutions help localise sound)
  • pinna funnels sound waves into ear canal (meatus) and conducts them to tympanic membrane (eardrum)
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10
Q

middle ear: function

A

sound conduction and amplification

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11
Q

middle ear: features

A
  • sound waves displace (vibrate) tympanic membrane

- ossicles conduct and amplify vibrations from tympanic membrane to oval window

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12
Q

define oval window:

A

connection btw air-filled middle ear and fluid filled cochlea in inner ear

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13
Q

cochlea: features

A
  • compact spiral structure in inner ear

- 3 long thin fluid-filled compartments (scala vestibuli, scala media, scala tympani)

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14
Q

cochlea: compartments separated by

A
  • vestibular membrane

- basilar membrane

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15
Q

importance of basilar membrane:

A

carries Organ of corti

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16
Q

cochlea: sound transduction function

A
  • unlike gases, liquids can’t be compresses

- variations in air pressure from ossicles to oval window -> displacements of perilymph

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17
Q

cochlea: sound transduction - how does perilymph move

A
  • vestibular and basilar membranes joined together close to tip (helicotrema) so scala vestibuli + scala tympani are continuous
  • pressure on oval window pushes perilymph around cochlea -> pushes onto round window (flexible to let perilymph move back and forth)
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18
Q

movement of perilymph causes:

A
  • deflection of membranes in cochlea

- membrane cause bending of stereocilia of sensory hair cells in Organ of corti

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19
Q

organ of corti: types of hair cells

A
  • inner hair cells (IHC)
  • outer hair cells (OHC)
  • both have stereocilia on apical surface (facing fluid in scala media- endolymph)
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20
Q

organ of corti: inner hair cells

A
  • primarily responsible for sending auditory info to brain perceived as sound
  • 3000-3500/ cochlea
  • synapse w Type I spiral ganglion cells (bipolar afferent sensory nerves, 90-95%) CN VIII
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21
Q

organ of corti: outer hair cells

A
  • mostly control cochlear sensitivity and frequency response
  • 10 000- 12 000/ cochlea
  • synapse w Type II spiral ganglion cells (bipolar/pseudounipolar afferent nerves, 5-10%) CN VIII
  • also receive efferent input via CN VIII indicating top-down (central) influences on hearing sensitivity/ frequency response
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22
Q

organ of corti: features

A
  • covered by gelatinous membrane called tectorial membrane, firmly attached to cochlea on one edge, weakly to organ of corti
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23
Q

organ of corti: outer hair cells attachment

A
  • tips embedded in tectorial membrane

- displaced when hair cell/ basilar membrane moves relative to tectorial membrane

24
Q

organ of corti: inner hair cells attachment

A
  • stereocilia not attached to tectorial membrane

- displaced by movement of fluid (endolymph)in scala media

25
Q

hair cells: what type of receptor

A

mechanoreceptors

26
Q

hair cells: features

A
  • filamentous ‘tip links’ btw adjacent cilia are connected to mechanoelectrical transduction (MET) channels
  • deflection of stereocilia cause change in no. of MET channels open = change ionic permeability of hair cell membrane
27
Q

MET channels are permeable to:

A
  • cations

- K+, Na+, Ca2+

28
Q

mechanotransduction of hair cells: unique features

A
  • highly sensitive
  • v fast (GCPR would be too slow to resolve high frequency sounds)
  • unique ionic properties of cochlear fluids and anatomical properties of hair cells facilitate fast, sensitive transduction system
29
Q

ionic basis of hair cell mechanotransduction:

A
  • hair cells derived from epithelial
  • joined by tight junctions
  • prevent movement of fluid and solutes btw cells
  • apical and basal parts of cell have different functions
  • tissue maintain different ionic env. on either side of epithelium
30
Q

in cochlea parts of hair cell exposed:

A
  • apical: stereocilia exposed to endolymph

- basal: soma to perilymph

31
Q

compare ionic comp. of endo/perilymph

A

perilymph:

  • scala vestibuli, scala tympani
  • low K+
  • high Na+

endolymph:

  • scala media
  • v high K+
  • v low Na+
32
Q

big difference in electrical potential of fluid:

A
  • endocochlear potential (endolymph has voltage = +80 mV from high K)
  • resting Vm = -45 mV relative to perilymph (0 mV)
  • therefore electrical force of +125 mV operating on cations in endolymph (mostly K)
33
Q

explain neurochemical process: hair cell mechanotransduction

A
  • displacement of longest stereocilium (kinocilium) opens MET channels
  • K enters cell DOWN electrochemical gradient
  • influx of +ve charges from endolymph depolarises hair cell
34
Q

hair cell mechanotransduction: rapid depolarisation-

A
  • causes voltage gated Ca channels in basolateral membrane to open
  • increased [Ca]in triggers increase in exocytosis of NT (glutamate) at synapse onto dendrites of 1˚ sensory afferent neuron
35
Q

hair cell mechanotransduction: rapid repolarisation-

A
  • causes voltage gated K channels in basolateral membrane to open
  • increased [Ca]in causes Ca-gated K channels in basolateral membrane to open
  • increase permeability allows K to leave cell rapidly via basolateral membrane into perilymph
36
Q

hair cells produce what type of potentials:

A

graded potentials

37
Q

coding of sound intensity (loudness):

A
  • coded through changes in rate of NT release from hair cells = change firing rate of afferent neurons
38
Q

coding of sound intensity (loudness): flowchart

A

louder sound - larger displacement of stereocilia - larger depolarisation - greater NT release - larger depolarisation of postsynaptic membrane (1˚ afferent neuron) - increase firing rate of AP

39
Q

coding of sound intensity: pitch- basilar membrane

A
  • basiliar membrane: stiffer/narrower at base (near oval window), more flexible/wider near apex (helicotrema)
40
Q

coding of sound intensity: pitch- features

A
  • sound pressure wave on oval window -> displacement wave in basilar membrane
  • amplitude increases along cochlea til reach max = decrease rapidly
  • for particular sound frequency, specific part of basilar membrane will resonate most strongly, stimulating adjacent IHCs
41
Q

coding of sound intensity: pitch- cochlear has place code

A
  • high freq. greatest displacement of basilar membrane at base of cochlear
  • low freq. at helicotrema
42
Q

coding of sound intensity: pitch- brain

A
  • interprets sound freq. on basis which IHCs were stimulated
  • auditory afferents travel via CN VIII (vestibulocochlear n) to brainstem and onwards to forebrain
  • tonotopic map of sound freq. in thalamus and primary auditory cortex
  • 1˚ auditory cortex in temporal lobe on sup temporal gyrus (adjacent to lateral sulcus)
43
Q

cochlear amplifier:

A
  • additional active mechanism

- sound induced physical movements of OHCs (electromotility)

44
Q

cochlear amplifier: name 2 mechanisms

A
  • somatic motor

- hair bundle motor

45
Q

cochlear amplifier: somatic motor

A
  • voltage sensitive proteins (prestins) in OHC somatic membrane
  • contract when cell depolarises (response to deflection of stereocilia and opening of MET channels)
46
Q

cochlear amplifier: hair bundle motor

A
  • OHC stereocilia deflected in response to sound wave, but actively rebound (‘twitch;) back to og position
  • therefore influx of Ca through MET channels
47
Q

cochlear amplifier: electromotile behaviour

A
  • of OHCs responding to movement of stereocilia causes larger displacement in basilar membrane for given sound intensity
  • additional movement in basilar mem. cause large displacement of endolymph -> move stereocilia of IHC
  • greatly amplify acoustic signal
  • non-linear: amplifies quiet sounds more than loud, maximise dynamic range of hair cells
48
Q

primary (tonotopic) auditory pathway: features

A
  • slow acting

- considerable processing of auditory info

49
Q

primary (tonotopic) auditory pathway: cochlear nuclei

A
  • ventral

- decode duration, intensity and frequency of sound

50
Q

primary (tonotopic) auditory pathway: sup olive and inf colliculi

A
  • help localise sound (comparing ipsilateral/ contralateral inputs from both ears)
51
Q

primary (tonotopic) auditory pathway: thalamus

A
  • prepares brain for motor response (speech)
52
Q

primary (tonotopic) auditory pathway: cortex

A
  • recognises, remembers and integrates sound signals
53
Q

non-primary (reticular) auditory pathway: features

A
  • fast acting pathway

- eg. auditory reflexes (startle)

54
Q

non-primary (reticular) auditory pathway: cochlear nuclei

A
  • dorsal
55
Q

non-primary (reticular) auditory pathway: reticular formation

A
  • selects which sensory info to pay attention to (eg. person can read book when listening to music)
56
Q

non-primary (reticular) auditory pathway: thalamus

A
  • MGN
  • relays info to cortex
  • connections to limbic system (eg. hypothalamus) for autonomic responses to auditory info
57
Q

non-primary (reticular) auditory pathway: cortex

A
  • auditory association cortex

4. BIOL257 (18 decks)

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