02/10/2017 Making sense of sound Auditory system: anatomy and physiology (making sense of the auditory pathways) Silence is Golden Lucy Anderson UCL Ear Institute [email protected] Interlearn scientific bootcamp – September 2017 Making sense of sound Making sense of sound And can evoke an emotional response. Silence is Golden But sound informs… Why should we care? What can we do? • Hearing loss increases risk or impact of many long term conditions, including dementia To understand how we can intervene in the – HL significant risk factor for developing dementia impaired-auditory system we first need to understand – HL more than doubles risk of depression / mental health issues how the normal auditory system works – HL impact on schooling – 71% deaf children failed to reach Government GCSE benchmark • Prevalence of hearing loss increases with age • 1:6 UK population reports some form of hearing loss We can’t afford not to! 1 02/10/2017 Outline of today’s talk The Problem of Hearing Mechanical sound pressure waves have to be • Discussion of “the problem of hearing” and how peripheral converted to electrical signals in order for our system overcomes it brains to understand them • Introduction to the principle nuclei within the central But auditory system • Sound vibrations in air can be very small • Example illustrating the importance of acknowledging • Cells capable of converting mechanical signals subdivisions for accurate reporting of physiological data. into electrical signals kept in a fluid filled cavity • Sound waves are complex • Introduction to the ascending auditory pathways The Problem of Hearing Outer Ear Inner Ear • Problem 1 : Getting sound waves into the ear • Problem 2 : Transferring sound waves across air/fluid boundary • Problem 3 : Decoding the sound waves Middle Ear Outer Ear Inner Ear • Problem 1 : Getting sound waves into the ear • Problem 2 : Transferring sound waves across air/fluid boundary • Problem 3 : Decoding the sound waves Middle Ear 2 02/10/2017 Function of the pinna • Pinna funnels sound waves into the auditory canal. • Grooves and ridges in the pinna create a pattern of reflections and delays which assist in localisation of sounds. ~ 25 – 35 mm Resonant frequency ~2-4 kHz The Problem of Hearing Outer Ear Inner Ear • Problem 1 : Getting sound waves into the ear • Problem 2 : Transferring sound waves across air/fluid boundary • Problem 3 : Decoding the sound waves Middle Ear Middle ear function • To transfer movements of the tympanic membrane to the fluid filled cochlea without significant loss of energy • To protect the hearing system from the effects of loud sounds 3 02/10/2017 Middle Ear: Tympanic membrane and oval window Force = Pressure x Area • Force stays the same • Area decreases Oval Therefore Pressure must increase window Tympanic membrane 17x Difference in area between tympanic membrane and oval window compensates for impedance mismatch between air and cochlear fluid The Ossicles – levers in action Tympanic membrane vs oval window Foot vs stiletto INCUS / • Force stays the same MALLEUS / • Area decreases STAPES / Therefore Pressure must increase ON TO OVAL WINDOW Area of average foot ~ 20 square inches Thus, 100-pound person applies 100/20 = 5 pounds per square inch Area of stiletto heel ~ 0.25 square inches Thus, 100-pound person applies 100/0.25 = 400 pounds per square inch Malleus 1.3x longer than incus compound lever action Impedance Matching ° Tympanic membrane > oval window ~ 17x ° Malleus > incus ~1.3x Overall pressure change of ~ 22x (17x1.3) 4 02/10/2017 Outer Ear Inner Ear The Problem of Hearing • Problem 1 : Getting sound waves into the ear • Problem 2 : Transferring sound waves across air/fluid boundary • Problem 3 : Decoding the sound waves Middle Ear The Problem of Hearing The Problem of Hearing • Problem 3 : Decoding the sound • Problem 3 : Decoding the sound waves waves – Convert mechanical vibrations into electrical signals – Convert mechanical vibrations into electrical signals (sensory transduction) (sensory transduction) – Split complex sounds into simple components – Split complex sounds into simple components (frequency analysis) (frequency analysis) – Amplify the sound signal – Amplify the sound signal The Inner Ear: the cochlea Apex Located in the inner ear, near the vestibular canals. Coiled structure with 2-5 turns In man, 1 cm wide and 5 mm long; if uncoiled would be 35 mm long. Contains 2 fluid compartments Base = end with oval and round window 5 02/10/2017 Unrolled vibrating cochlea Cochlea cross section Scala vestibuli Reissner’s membrane Scala media Scala tympani (perilymph) (perilymph) Organ of Corti Hair cells - Stereocillia Tip links 6 02/10/2017 The Problem of Hearing • Problem 3 : Decoding the sound Site of transduction channels waves – Convert mechanical vibrations into electrical signals (sensory transduction) – Split complex sounds into simple components (frequency analysis) – Amplify the sound signal Basilar Membrane Properties Basilar Membrane Properties Cochlea: Frequency Analysis http://www.hhmi.org/biointeractive/cochlea 7 02/10/2017 The Problem of Hearing Inner Hair Cell • Problem 3 : Decoding the sound waves – Convert mechanical vibrations into electrical signals (sensory transduction) – Split complex sounds into simple components (frequency analysis) – Amplify the sound signal Outer Hair Cell Micrograph of Hair Cells Inner Hair Cells One row ~3500 Outer Hair Cell 3-5 rows ~14000 Outer Hair Cell Motility 8 02/10/2017 Outline of today’s talk Central Auditory Pathways Auditory cortex • Discussion of “the problem of hearing” and how peripheral system overcomes it V Medial Geniculate Body (MGB) • Introduction to the principle nuclei within the central auditory system Inferior colliculus • Example illustrating the importance of acknowledging subdivisions for accurate reporting of physiological data. Superior olivary complex • Introduction to the ascending auditory pathways Auditory nerve Cochlear nucleus complex aka The input Central Auditory Pathways The input Auditory cortex • Sound features received by the brain Medial – Frequency Geniculate Body (MGB) – Loudness (intensity) – Timing Inferior colliculus • Sound qualities we consider important – Localisation (where a sound comes from) – What a sound sounds like Superior olivary complex – Emotion – Analysing the auditory scene Auditory nerve Cochlear nucleus complex aka The input There are many overlapping single-fibre Auditory nerve tuning curves in the auditory nerve. Each auditory-nerve Loud fibre responds only to a narrow range of frequencies. PLACE CODING Tuning curve Action potential Audiogram Quiet Evans 1975 9 02/10/2017 The discharges of auditory nerve fibres to low-frequency sounds are not random; Central Auditory Pathways they occur at particular times ( phase locking ). Auditory cortex TEMPORAL CODING Medial Geniculate Body (MGB) Inferior colliculus Superior olivary complex Auditory nerve Cochlear nucleus complex aka Evans (1975) The input Auditory nerve Anteroventral cochlear nucleus Cell types in the cochlear nucleus Dorsal cochlear nucleus • Cochlear nucleus contains distinct classes of neurons: – Spherical bushy cells Input via endbulbs of Held = Excellent temporal fidelity – Globular bushy cells Input from few AN fibres = Good fidelity Input from multiple AN fibres = Wide frequency – Octopus cells tuning & dynamic range, time locking Narrow frequency tuning, wide dynamic – Multipolar / stellate cells range, burst firing = coding spectral shape – Fusiform /pyramidal cells Complex response areas – role in spectral contrasts, – Giant cells notch detection (elevation) University of Buffalo 2006 Cochlear nucleus: the auditory shunting yard To inferior colliculus and auditory thalamus Dorsal (DCN) and posteroventral cochlear Superior Anteroventral cochlear nucleus (PVCN) olivary nucleus (AVCN) Complex information complex Fast, precise information Projects to inferior colliculus Projects to superior olive and medial geniculate body Dorsal cochlear nucleus Ventral cochlear nucleus Adapted from Osen and Roth (1969) 10 02/10/2017 Central Auditory Pathways Superior olivary complex Auditory cortex • Lateral Superior Olive – Inter-aural level difference Medial Geniculate • Medial Superior Olive – Inter-aural time difference Body (MGB) • Nucleus of the Trapezoid Body - Inhibition Inferior colliculus LSO Superior olivary complex Medial nucleus Auditory nerve Cochlear nucleus complex of the trapezoid aka MSO The input body Inter-aural level difference (ILD) ILD Generation: Head shadow For human head sizes: acoustic shadow generated for frequencies > 1.5 kHz ILD is best for high frequency sounds Lateral superior olive • Neurons receive excitatory projections from ipsilateral bushy cells from AVCN • Neurons receive inhibitory projections from MNTB which in turn receives its input from globular bushy cells from contralateral AVCN 11 02/10/2017 Inter-aural time difference (ITD) Inter-aural time difference (ITD) occurs in MSO occurs in MSO works best for low frequency sound works best for low frequency sound Sounds arrive at right ear first Sounds arrive at both ears together Interaural time difference = Interaural time difference = 200 μs 0 μs Inter-aural time difference (ITD) Inter-aural time difference (ITD) occurs in MSO occurs in MSO works best for low frequency sound works best at low frequencies – worse for high frequencies Sounds arrive at left ear first Sounds arrive at left ear first – BUT… Interaural time difference = Interaural time difference = 200 μs 200 μs Medial Superior