Basilar Membrane: Part I

compression or condensation

When the stapes moves into the vestibule, the cochlear partition deflects downward, called -footplate goes down moves in large deflection

rarefaction

When the stapes moves out of the vestibule, the BM moves up and it is called -wave bulges up

Bekesy experiments

•1. vibration of the stapes gives rise to a traveling wave of displacement on the BM •2. For a vibration of a particular frequency, vibration grows in amplitude as the wave goes to a certain point, then dies rapidly •3. Wave travels more slowly as it goes further up the cochlea •4. Low frequencies peak near apex, high frequencies near base 5. BM acts as a low pass filter

when stapes moves in and out of oval window

•A pressure difference is set up between the SV and ST •The Basilar membrane (and cochlear partition) are elastic •Because fluid is incompressable, the BM will deflect -Fluid (perilymph) will bulge out of the RW

basilar membrane characteristics

•Basilar membrane is narrower at the base, and becomes increasingly wider towards the apex of the cochlea. •The stiffness of the basilar membrane changes along its length. It is stiff near the base, and becomes less stiff near the apex. -Narrow and stiff at base -Toward apex gets wider and more flexible

early measurements by George Von Bekesy

•Examined cochlear partition (scala media) in human and animal cadavers •Temporal bones were removed soon after death and were immersed in saline solution •Rubber windows replaced round window membrane and placed in oval window •Cochlear wall was opened under water for observations of cochlear partition (scala media) •Silver particles put on Reisner's membrane •Microscopic and stroboscopic observations of displacement --First to realize you stimulate at the base and there is displacement that gets bigger and gets to a part of the cochlea and the displacement then dies out

criticism of VB's experiments

•Had to be done at very high signal levels - 130 dB SPL, because the movements had to be on the order of a wavelength of light •Had to be performed on cadavers. Living processes cannot be observed

important experiment

•Showed there was a 'travelling wave' •The wave starts at the base and 'travels' to a particular place on the BM before dying out.

important

•The frequency that produces the maximum displacement for a particular place is called the 'characteristic frequency' (CF) or 'best frequency' (BF)

vibration of the basilar membrane in response to pure tone

-Footplate of stapes a pressure wave will start to bulge at the oval window --Travel of wave is press. Diff btwn the scala vestibuli and scala tympani

simulation of BM

-Freq the position as they move down the BM -Low freq slower -High freq faster -Magnitude displacement (height of wave) depends on BM stiffness -High freq peak at base -Low freq at apex -Period 1 ms for one cycle at 1000 Hz 0.25 ms at 4000 Hz

another experiment

-Opened the cochlea at certain points and measured the displacement at That point as frequency was varied. -6 points on BM -stapes displacement held constant -shallow slope on low frequency side, steep slope on high frequency side - concluded BM acts as a low-pass filter --Basilar membrane acts as a low pass filter won't let high freq in --Each curve (6) is from a different window , 6 windows measuring the displacement and putting sound in at diff. freq. 6 Transfer functions represent a different window on the cochlea (think of it as a filter)

travel of waves along cochlea

Experimental Procedure: -Silver dust along a length of Reisner's membrane -Could measure displacement as a function of distance -waves always starts at base -The wave moves up and down -Goes up at the apex as it travels toward the base goes lower -Plays in the cochlea depending on freq. -Look at the envelope (dotted line) - wavs will happen btwn this outline

tonotopic organization

The place-frequency map is called -we are all the way to the cortex