Auditory System Feedback

Transcription

1 Feedback Auditory System Feedback Using all or a portion of the information from the output of a system to regulate or control the processes or inputs in order to modify the output. Central control of input: Auditory processing The Nature of Sound The brain uses its own input to modify the machinery responsible for transducing that input Stereo speaker Compressed air Automatic gain control through negative feedback circuits in the middle ear Amplification of signals by positive feedback circuits (the cochlear amplifier) Gain control of the cochlear amplifier through the olivocochlear feedback path One cycle Rarefied air Distance BCP Chapter 11 1

2 ntensity waves as loud. Low frequency High frequency Low intensity High intensity Distance Distance Measuring Sound Pressure (and intensity) What is a Decibel (db)? The decibel scale is used to express relative power or pressure on a logarithmic scale For sound pressure: db = 20 x log(p/p ref ) Threshold audible level (20 mpascals) Range: ~0 100 decibels (db) At threshold : 20 x log(p ref /p ref ) = 20 x log(1) = 20 x 0 = 0 db SPL 2

3 Understanding Decibels Sound Pressure Levels and Frequencies Relevant to Mammalian Hearing 0 db means that the intensity or pressure level is equal to the reference db > 0 gain or amplification with respect to reference db < 0 loss or attenuation with respect to reference Decibel (SPL) = 20 x log(p/p ref ) examples: 1) Pressure 100 times as intense reference: = 20 x log(100) = 40dB 2) Pressure ½ as intense as reference: = 20 x log(0.5) = -6dB Outer ear Middle ear Inner ear Ossicular Chain Ossicles Oval window Vibration of the ossicles transmits sound signals from one medium (air) to another (cochlear fluids) Outer Ear Middle Ear Inner Ear Pivot Incus Scala Vestibuli Oval Window Tympanum Stapes Cochlea Malleus Basilar Membrane Round Window Pinna Auditory canal Tympanic Bone AIR Scala Tympani Fluid 3

4 Impedance Matching by the Ossicular Chain Molecules of air are more easily displaced than molecules of cochlear fluid, i.e. air has lower acoustic impedance than fluid The Acoustic Reflex (attenuation reflex in BCP) Attenuation of the vibratory input delivered to the cochlea during exposure to intense sounds The middle ear facilitates this transfer by: Tensor Tympani Stapedius V VII Organ of Corti VIII Contraction of the stapedius muscle pulls the stapes slightly sideways, pressing it snugly in the oval window Concentrating pressure Use of a lever Brainstem Contraction of the tensor tympani muscle pulls the eardrum inward Stapedius Seventh Nerve Tensor Tympani Trigeminal Nerve The Acoustic Reflex (cont d) The Cochlea Protect the cells of the inner ear (not for single sounds) In humans, and bats, the muscles may contract prior to vocalizations V Reissner s Scala vestibuli Scala media Negative feedback circuit providing automatic gain control Tensor Tympani Stapedius VII Organ of Corti VIII Brainstem Tectorial Stria vascularis Loss of reflex Hyperacusis: oversensitivity to sound Organ of Corti Basilar Scala tympani 4

5 Deflection of the Basilar Membrane Place Coding Along the Basilar Membrane Tonotopic Map The Traveling Wave 5000Hz Sinusoidal Input Base Apex Distance along the Basilar Membrane 5

6 The Organ of Corti Hair Cells The Ear s Sensory Receptors Each hair cell contains ~ stereocilia Synapse onto bipolar neurons whose cell bodies are in the spiral ganglion In humans, about 3500 inner hair cells, and 15,000-20,000 outer hair cells Of the afferent fibers, >90% innervate the inner hair cells Stereocilia Tectorial Reticular lamina Stereocilia bending out (a) Outer hair Basilar Rods of Inner hair (b) Basilar cells Corti cell deflected up Corti cell Deflection of the stereocilia in preferred direction opens potassium channels, which depolarizes cells due to the high external K+ Cochlear Fluids and the Endocochlear Potential The endolymph is similar to intracellular ionic composition, BUT has a +80mv resting potential Perilymph: low [K + ], high [Na + ] This electrochemical gradient is maintained by the stria vascularis (high metabolic cost) Endolymph: high [K + ], low [Na + ] scala vestibuli scala media scala tympani The large endocochlear potential is critical for effective transduction of mechanical vibrations (sounds) by the inner ear 6

7 Deflection of the Basilar Membrane Log Displacement Amplitude Loss of Outer Hair Cells Reduces Cochlear Response OHC alive Outer hair cells and the cochlear amplifier Electromechanically coupled to the basilar Positive feedback constituting an electo-mechanical cochlear amplifier OHC dead Position on Basilar Membrane (mm) 1) Length varies from base to apex 2) Contain contractile motor proteins that cause the cells to contract and extend upon depolarization/repolarization The Traveling Wave With Amplification 5000Hz Sinusoidal Input Outer Hair Cells Mechanical Tuning Passive Cochlea Active Cochlea bat (base) human (apex) Base Apex Base Apex Distance along the Basilar Membrane human (base) 7

8 Echo amplitude Electrical Oscillation in Hair Cells K + Low [ ] o K + [ K + High ] o K + Ca [ Ca ] Mitochondrial i Ca 2+ Calcium Pump Uptake Voltage-dependent Channel K + Calcium-dependent Channel lower [K + ] from perliymph seeping up Electrical Oscillation in Hair Cells 1. Cilia bend, K + enters, cell depolarizes Ca enters through voltage sensitive calcium channels, cell depolarizes more 3. K + exits through voltage-dependent and calcium-dependent channels, cell repolarizes Ca sequestered in mitochondria and extruded by pump 5. Cycle repeats Motile Outer Hair Cells Otoacoustic Emissions Sounds generated by the ear as a result of positive feedback circuitry of the cochlear amplifier Can be either spontaneous (SOAE) or evoked (TEOAE) In damaged ears, evoked otoacoustic emissions can reveal missing characteristic response in particular frequency ranges J Ashmore (1987) Infants are now regularly tested using OAE techniques to identify hearing deficits that may be addressed as early as possible Click Contractile proteins tune the oscillations of the traveling wave Time after click (ms) 8

9 Echo amplitude Auditory cortex Controlling the Cochlear Amplifier: The Olivocochlear Bundle Cochlea Ponto-medullary junction Medial geniculate Inferior colliculus OHC IHC Superior olive Cochlear nuclei OCB VIII Afferent axons Olivocochlear axons Superior olivary complex Dynamic control of cochlear sensitivity by negative feedback Control of Sensory Input Olivocochlear bundle activation by noise in contralateral ear Sound + Basilar + + IHC Brainstem Control Time after click (ms) + + OHC - Olivocochlear Bundle Activation of the olivocochlear bundle (by contralateral noise) can reduce the amplitude of the EOAE The brain uses its input to modify the machinery responsible for transducing that very input 9

10 Effects of cutting a nerve Soma Cut here Terminals Retrograde changes Anterograde changes Proximal Distal Cochlea Spiral ganglion VIII afferents? X Olivocochlear bundle S.O. VIII efferents Brainstem 10