Special senses II: hearing Flashcards
physics of sound:
- compressional/ longitudinal wave caused by variation in air pressure
- medium (air) where sound moves oscillates in direction of propagation of wave
sound wave consists of:
- alternate compressions
- rarefractions
define compression:
- where air molecules are pushed closer together
define rarefactions:
- where air molecules are farther apart
define wavelength::
distance btw adjacent compressions/ rarefactions of sound wave
amplitude: features
- ‘loudness’
- proportional to difference in density of air within rarefaction vs air in compressions
- greater the difference, louder the sound (greater amplitude)
- dB
frequency: features
- 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
define pinna:
aka auricle
- visible part of ear outside head
external ear features:
- 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)
middle ear: function
sound conduction and amplification
middle ear: features
- sound waves displace (vibrate) tympanic membrane
- ossicles conduct and amplify vibrations from tympanic membrane to oval window
define oval window:
connection btw air-filled middle ear and fluid filled cochlea in inner ear
cochlea: features
- compact spiral structure in inner ear
- 3 long thin fluid-filled compartments (scala vestibuli, scala media, scala tympani)
cochlea: compartments separated by
- vestibular membrane
- basilar membrane
importance of basilar membrane:
carries Organ of corti
cochlea: sound transduction function
- unlike gases, liquids can’t be compresses
- variations in air pressure from ossicles to oval window -> displacements of perilymph
cochlea: sound transduction - how does perilymph move
- 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)
movement of perilymph causes:
- deflection of membranes in cochlea
- membrane cause bending of stereocilia of sensory hair cells in Organ of corti
organ of corti: types of hair cells
- inner hair cells (IHC)
- outer hair cells (OHC)
- both have stereocilia on apical surface (facing fluid in scala media- endolymph)
organ of corti: inner hair cells
- 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
organ of corti: outer hair cells
- 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
organ of corti: features
- covered by gelatinous membrane called tectorial membrane, firmly attached to cochlea on one edge, weakly to organ of corti
organ of corti: outer hair cells attachment
- tips embedded in tectorial membrane
- displaced when hair cell/ basilar membrane moves relative to tectorial membrane
organ of corti: inner hair cells attachment
- stereocilia not attached to tectorial membrane
- displaced by movement of fluid (endolymph)in scala media
hair cells: what type of receptor
mechanoreceptors
hair cells: features
- 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
MET channels are permeable to:
- cations
- K+, Na+, Ca2+
mechanotransduction of hair cells: unique features
- 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
ionic basis of hair cell mechanotransduction:
- 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
in cochlea parts of hair cell exposed:
- apical: stereocilia exposed to endolymph
- basal: soma to perilymph
compare ionic comp. of endo/perilymph
perilymph:
- scala vestibuli, scala tympani
- low K+
- high Na+
endolymph:
- scala media
- v high K+
- v low Na+
big difference in electrical potential of fluid:
- 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)
explain neurochemical process: hair cell mechanotransduction
- displacement of longest stereocilium (kinocilium) opens MET channels
- K enters cell DOWN electrochemical gradient
- influx of +ve charges from endolymph depolarises hair cell
hair cell mechanotransduction: rapid depolarisation-
- 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
hair cell mechanotransduction: rapid repolarisation-
- 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
hair cells produce what type of potentials:
graded potentials
coding of sound intensity (loudness):
- coded through changes in rate of NT release from hair cells = change firing rate of afferent neurons
coding of sound intensity (loudness): flowchart
louder sound - larger displacement of stereocilia - larger depolarisation - greater NT release - larger depolarisation of postsynaptic membrane (1˚ afferent neuron) - increase firing rate of AP
coding of sound intensity: pitch- basilar membrane
- basiliar membrane: stiffer/narrower at base (near oval window), more flexible/wider near apex (helicotrema)
coding of sound intensity: pitch- features
- 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
coding of sound intensity: pitch- cochlear has place code
- high freq. greatest displacement of basilar membrane at base of cochlear
- low freq. at helicotrema
coding of sound intensity: pitch- brain
- 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)
cochlear amplifier:
- additional active mechanism
- sound induced physical movements of OHCs (electromotility)
cochlear amplifier: name 2 mechanisms
- somatic motor
- hair bundle motor
cochlear amplifier: somatic motor
- voltage sensitive proteins (prestins) in OHC somatic membrane
- contract when cell depolarises (response to deflection of stereocilia and opening of MET channels)
cochlear amplifier: hair bundle motor
- OHC stereocilia deflected in response to sound wave, but actively rebound (‘twitch;) back to og position
- therefore influx of Ca through MET channels
cochlear amplifier: electromotile behaviour
- 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
primary (tonotopic) auditory pathway: features
- slow acting
- considerable processing of auditory info
primary (tonotopic) auditory pathway: cochlear nuclei
- ventral
- decode duration, intensity and frequency of sound
primary (tonotopic) auditory pathway: sup olive and inf colliculi
- help localise sound (comparing ipsilateral/ contralateral inputs from both ears)
primary (tonotopic) auditory pathway: thalamus
- prepares brain for motor response (speech)
primary (tonotopic) auditory pathway: cortex
- recognises, remembers and integrates sound signals
non-primary (reticular) auditory pathway: features
- fast acting pathway
- eg. auditory reflexes (startle)
non-primary (reticular) auditory pathway: cochlear nuclei
- dorsal
non-primary (reticular) auditory pathway: reticular formation
- selects which sensory info to pay attention to (eg. person can read book when listening to music)
non-primary (reticular) auditory pathway: thalamus
- MGN
- relays info to cortex
- connections to limbic system (eg. hypothalamus) for autonomic responses to auditory info
non-primary (reticular) auditory pathway: cortex
- auditory association cortex
