21/05/2019
Neurobiology of Hearing Salamanca, 21st May 2019
1. The cochlea: sound, psychoacoustics and structure Jonathan Ashmore Neuroscience, Physiology and Pharmacology University College London
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Outline
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
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sound hearing aid
basilar membrane tectorial membrane
auditory inner hair outer hair cells fibres to cell brainstem
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The hair cell building block transduction of the (today) cochlea, vestibular & lateral line systems basolateral membrane (Lecture 3)
synapse (Lecture 4)
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Dobzhansky: Biology can only be understood in the context of evolution
Grothe & Pecka Front Neural Circ 2014 (After Manley)
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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
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Sound is a compressional longitudinal wave:
Amplitude
credit: ISVR, Southampton
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Physics of Sound: units
Sound pressure level (dB SPL) = 20 log10 P / Pr
P = amplitude of pressure wave Pr = threshold (=just audible) 20 Pa
Sound pressure 10 times Pr = 20 log10 10 x Pr / Pr = 20 dB Sound pressure 100 times Pr = 20 log10 100 x Pr / Pr = 40 dB Sound pressure 1000 times Pr = 20 log10 1000 x Pr / Pr = 60 dB
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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?
T.G. Fechner (1860). “Elements of Psychophysics” Groundbreaking!
Idea was to measure the psyche via physical measurement
Psychophysics: Functional abilities and performance specifications of sensory systems
(Psychoacoustics = psychophysics of hearing)
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Answer
Yup
Nope Sound Level
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A behavioural experiment: non-human hearing
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4 kHz 250 Hz 40 kHz 63 Hz
50 Percentage Detected Percentage
0 20 40 60 80
Sound Level (dB SPL)
Owren et al. (1988). Journal of Comparative Psychology, 102(2), 99-107.
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Result: an audiologram
Threshold (dB SPL)
Frequency (kHz)
Owren et al. (1988). Journal of Comparative Psychology, 102(2), 99-107.
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human hearing range
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Beginnings
Hermann von Helmholtz Georg von Békésy 1821-1894 1899-1973
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Cochlear structure: orientation
scala media
Organ of Corti
Scala media: an internal compartment containing ‘endolymph’: hi K+ , lo Ca++
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What is psychoacoustics measuring?
behaviour middle ear muscles efferent system
Outer Middle Inner CNS ear ear ear
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Genetics of human deafness
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- related DFNB1: CX26 (recessive) DFNA2: CX31 DFNA3: CX30 DFNA3: CX26
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An important approach: mouse models of hearing
Stage E14.5. Endolymphatic compartment filled with dye D Wu, NIH
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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 turns 15000 hair cells. In mouse 7mm long Coiled 1.5 turns 3000 hair cells
(In Elephant 60mm long) Contains 3 fluid compartments (2 connected via helicotrema)
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30 µm
TM IHCs OHCs
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From mechanics to neural signal
tectorial membrane
basilar membrane
nanometres!
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