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 [email protected] 1 2 1 21/05/2019 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 3 sound hearing aid basilar membrane tectorial membrane auditory inner hair outer hair cells fibres to cell brainstem 4 2 21/05/2019 The hair cell building block transduction of the (today) cochlea, vestibular & lateral line systems basolateral membrane (Lecture 3) synapse (Lecture 4) 5 Dobzhansky: Biology can only be understood in the context of evolution Grothe & Pecka Front Neural Circ 2014 (After Manley) 6 3 21/05/2019 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 7 8 4 21/05/2019 The ear you don’t see 9 Sound is a compressional longitudinal wave: Amplitude credit: ISVR, Southampton 10 5 21/05/2019 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 11 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 12 6 21/05/2019 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) 13 Answer Yup Nope Sound Level 14 7 21/05/2019 A behavioural experiment: non-human hearing 100 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. 15 Result: an audiologram Threshold (dB SPL) Frequency (kHz) Owren et al. (1988). Journal of Comparative Psychology, 102(2), 99-107. 16 8 21/05/2019 human hearing range 17 18 9 21/05/2019 Beginnings Hermann von Helmholtz Georg von Békésy 1821-1894 1899-1973 19 Cochlear structure: orientation scala media Organ of Corti Scala media: an internal compartment containing ‘endolymph’: hi K+ , lo Ca++ 20 10 21/05/2019 What is psychoacoustics measuring? behaviour middle ear muscles efferent system Outer Middle Inner CNS ear ear ear 21 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 22 11 21/05/2019 An important approach: mouse models of hearing Stage E14.5. Endolymphatic compartment filled with dye D Wu, NIH 23 Maintaining hearing: the potassium circulation hypothesis 24 12 21/05/2019 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) 25 26 13 21/05/2019 27 30 µm TM IHCs OHCs 28 14 21/05/2019 From mechanics to neural signal tectorial membrane basilar membrane nanometres! 29 15.