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Neurobiology of Hearing

Salamanca, 20th May 2015

The cochlea: sound, psychoacoustics and cochlear structure

Jonathan Ashmore Neuroscience, Physiology and Pharmacology

University College London

j.ashmore@ucl.ac.uk

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Abdus Salam 1922-1996 Director ICTP, Trieste, Italy 1964-1996

Nobel Prize for Physics 1978

quantum science neuro science

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Paul Fatt 1924-2014 at a Physiological Soclety meeting 1979

(photo: Martin Rosenberg)

1. Sound, psychoacoustics and cochlear structure

2. Cochlear mechanics and transduction in hair cells

3. Outer hair cells : cochlear amplification

4. Inner hair cells: the ribbon and efferent synapses

Outline

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the sensory hair cell

the building block of the hearing system

Transducer (today & tomorrow)

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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Sound is a compressional longitudinal wave:

credit: ISVR, Southampton

Amplitude

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Physics of Sound: Phase

Two waves of the same amplitude and frequency but peaking at different times are referred to as having a different phase. The difference in phase is measured in angles where a phase shift that brings the wave in antiphase is a 180° shift.

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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Physics of Sound: Intensity Range

140 120 100 80 60 40 20 0

Loud Rock group Car Horn at 7 m Busy Street Normal Conversation Quiet conversation Whisper Rustling leaves Auditory Threshold (20 μPa)

dB S

PL

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?

In particular: 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

100 Pe

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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CNS Inner ear

Middle ear

Outer ear

behaviour

middle ear muscles efferent 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 > 70 Total number may be > 100

• ~40 % of hereditary hearing losses in humans are connexin- related DFNB1: CX26 (recessive) DFNA2: CX31 DFNA3: CX30 DFNA3: CX26

Genetics of human deafness

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Alfonso Corti (1822–1888) Herman von Helmholtz (1821-1894)

Georg von Bekesy 1899-1973 Hallowell Davis (1896-1992)

Organ of Corti

Cochlear structure: orientation

scala media

Scala media: an internal compartment containing ‘endolymph’: hi K+ , lo Ca++

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The potassium circulation hypothesis

Stage E14.5. Endolymphatic compartment filled with dye D Wu, NIH

Mouse models of hearing

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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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50 µm

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Andy Forge , UCL Ear Inst

2 µm

From mechanics to neural signal

basilar membrane

tectorial membrane

nanometres!