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Neurobiology of Hearing
Salamanca, 21st May 2019
1. The cochlea: sound, psychoacoustics and structure
Jonathan AshmoreNeuroscience, Physiology and Pharmacology
University College London
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1. The cochlea: sound, psychoacoustics and structure
2. Cochlear mechanics and transduction in hair cells
3. Cochlear amplification: outer hair cells
4. Cochlear coding : inner hair cells and the synapses
Outline
basilar membrane
tectorial membrane
outer hair cellsinner hair cell
auditory fibres to brainstem
sound hearing aid
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The hair cell
building block of the cochlea, vestibular &lateral line systems
transduction (today)
basolateral membrane(Lecture 3)
synapse(Lecture 4)
Grothe & Pecka Front Neural Circ 2014 (After Manley)
Dobzhansky: Biology can only be understood in the context of evolution
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Starting points:
•The ear is small and relatively inaccessible
•The ear works at high frequencies
•The structure of the cochlea is critical for function
•The cochlea converts sound into an electrical signal
•The central nervous system extracts significance from this signal
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The ear you don’t see
Sound is a compressional longitudinal wave:
credit: ISVR, Southampton
Amplitude
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Physics of Sound: units
Sound pressure level (dB SPL) = 20 log10 P / Pr
P = amplitude of pressure wavePr = threshold (=just audible) 20 Pa
Sound pressure 10 times Pr = 20 log10 10 x Pr / Pr = 20 dBSound pressure 100 times Pr = 20 log10 100 x Pr / Pr = 40 dBSound pressure 1000 times Pr = 20 log10 1000 x Pr / Pr = 60 dB
Basic facts about hearing:
psychoacoustics
Frequency range (human) 40 - 20,000 Hz (8.5 octaves)
JND for intensity 3% (0.2 dB)
JND for pure tones 0.3% (1/20th tone)
Threshold for hearing: 20 μPa (= 0 dB SPL = 2x10-10 atmos.)
Operating range: 0-80 dB SPL (10000x in amplitude)
The cochlea distorts
The cochlea emits sound
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What is psychoacoustics?
(Psychoacoustics = psychophysics of hearing)
T.G. Fechner (1860). “Elements of Psychophysics”
Psychophysics: Functional abilities and performance specifications of sensory systems
Idea was to measure the psyche via physical measurement
Groundbreaking!
Nope Sound Level
Yup
Answer
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Owren et al. (1988). Journal of Comparative Psychology, 102(2), 99-107.
A behavioural experiment: non-human hearing
4 kHz 250 Hz 40 kHz 63 Hz
Sound Level (dB SPL)
20 40 60 80
0
50
100Pe
rcen
tage
Det
ecte
d
Owren et al. (1988). Journal of Comparative Psychology, 102(2), 99-107.
Result: an audiologram
Threshold (dB SPL)
Frequency (kHz)
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human hearing range
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Beginnings
Hermann von Helmholtz1821-1894
Georg von Békésy1899-1973
Organ of Corti
Cochlear structure: orientation
scala media
Scala media: an internal compartment containing ‘endolymph’: hi K+ , lo Ca++
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CNSInner ear
Middle ear
Outer ear
behaviour
middle ear musclesefferent system
What is psychoacoustics measuring?
About 1 per 800 at birth, rising to 1 per 500 by late teens
Currently identified genes for non-syndromic hearing loss > 100
~40 % of hereditary hearing losses in humans are connexin- relatedDFNB1: CX26 (recessive)DFNA2: CX31DFNA3: CX30DFNA3: CX26
Genetics of human deafness
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Stage E14.5. Endolymphatic compartment filled with dyeD Wu, NIH
An important approach: mouse models of hearing
Maintaining hearing: the potassium circulation hypothesis
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The mammalian cochlea: gross structure
Located in the inner ear a compartment in the temporal bone
In man, 35 mm long.Coiled structure with 4 turns15000 hair cells.
In mouse 7mm longCoiled 1.5 turns3000 hair cells
(In Elephant 60mm long)
Contains 3 fluid compartments(2 connected via helicotrema)
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30 µm
IHCs OHCsTM
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From mechanics to neural signal
basilar membrane
tectorial membrane
nanometres!
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