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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

j.ashmore@ucl.ac.uk

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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

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100Pe

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tage

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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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