Hearing inner ear Outer ear: Mechanical protection of the middle ear Diffracts

Hearing inner ear Outer ear: Mechanical protection of the middle ear Diffracts www.phwiki.com

Hearing inner ear Outer ear: Mechanical protection of the middle ear Diffracts

Joey Boy,, Host has reference to this Academic Journal, PHwiki organized this Journal Outer ear: Mechanical protection of the middle ear Diffracts in addition to focuses sound waves (pinna) The ear canal acts as a resonator (3-5 kHz enhancement) The end of the canal has an eardrum which vibrates with sound inner ear Middle ear: Converts impedance of the air to the impedance of the cochlear liquid ZAIR:ZLIQ = 1:4000 99.9% loss of energy if no impedance match Protects inner ear Reactions to intense sounds (but rather slow 60-120 ms reaction time) Low-pass filter 15 dB/oct from 1 kHz Characteristic acoustic impedance of a tube filled with gas or fluid Z0= rc/A, r-the density of the medium c-the velocity of sound A-cross-sectional area of the tube air outside salty liquid (cochlear fluid) inside stirrup Inner ear: Mechanical frequency analysis of the incoming sound Converts mechanical movements to electrical pulses Changes in acoustic pressure => movement of bones in middle ear => movement of membrane on oval window => vibrations in the cochlear liquid => vibrations of basilar membrane oval window round window 0rgan of Corti basilar membrane tectorial membrane hairs

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Basilar membrane as a mechanical frequency analyzer Cochlea as frequency analyzer How selective is the basilar membrane Movement of the basilar membrane in dead animal observed by a microscope von Bekesy 1960

Selectivity very different after the death of the animal! Cochlea is most likely an active system with a positive feedback loop that accounts as long as the high cochlear sensitivity. small piece of radioactive material glued on basilar membrane Doppler shift in emitted g-rays indicates amplitude of the membrane vibrations Nonlinear system! (curves vary with intensity) Code as long as the brain Sensory neurons produce spikes Spike rate increases with an increase in the stimulus intensity (here it was a weight on a muscle) Adaptation: after a while, the firing rate decreases even when the stimulus intensity stays the same

Shapes of five individual action potential (spikes) Stimulus at t=0 (sudden change of the scene that fly sees) From movements to electrical pulses The basilar membrane contains ~15,000-20,000 hair cells (sensory cells) Inner hair cells transduce vibration into electrical signal in addition to send them to the brain Outer hair cells receive signals from the brain, which could change mechanical properties of the organ of Corti

basilar membrane movements => bending of hair cells => electrical pulses inner hair cells ~ 40 hairs/cell ~ 140 hairs/cell outer hair cells auditory nerve fiber auditory nerve fiber tectorial membrane basilar membrane tunnel of corti inner hair cells – in as long as mation outer hair cells – govern cochlear mechanics Intracellular voltage changes in an inner hair cell as long as different frequencies of stimulation

Spikes on the auditory nerve are in phase with the signal Only in one half of the cycle One-way rectification Coding of the stimulus intensity Tuning curves

Reverse correlation technique B in addition to widths of tuning curves increase with frequency (frequency resolution decreases with frequency) Place Theory of Hearing Tones of certain frequencies excite certain areas of the cochlea that are connected to certain auditory fibres. the fibres are distributed tonotopically (by their best frequencies) in the auditory nerve this tonotopical organization is preserved throughout the higher areas of hearing all the way to the brain

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Place theory of peripheral auditory processing B in addition to widths of tuning curves increase with frequency (frequency resolution decreases with frequency) characteristic frequency b in addition to width firing rate depends on sound intensity

Response of horseshoe crab’s visual neuron to change in light Two-tone suppression (lateral inhibition) tone elicits certain response (firing rate) second tone in the + area increases the firing rate second tone in the – area decreases the firing rate

2-dimensional “receptive field” in vision Receptive field on your skin

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