Subject: Re: HC selectivity ... was Re: Physiological models of cochlea activity - alternatives to the travelling wave From: Andrew Bell <andrew.bell@xxxxxxxx> Date: Thu, 4 Oct 2007 10:01:02 +1000 List-Archive:<http://lists.mcgill.ca/scripts/wa.exe?LIST=AUDITORY>Dear list: Martin makes a telling point about the impossibly small magnitudes (1 pm) associated with a hydrodynamic traveling wave. Such a dimension is too small to be detected by stereocilia, keeping in mind that the stereocilia's tips deflect about 5 times less than the basilar membrane's vertical motion [Dallos, 2003]. I think the effective stimulus at such low intensities must be the pressure of the acoustic wave propagating through the cochlear fluids. I've recently published a model of the outer hair cell as a pressure sensor [Bell, 2007] and assembled circumstantial evidence that the basal body beneath the cuticular pore could be the actual pressure-sensing element. The idea is that OHCs are compressible, and they are immersed in incompressible liquid, so that when the stapes pushes in on the cochlear liquids, the energy is delivered straight to the sensors. In this case, I calculate that at 0 dB SPL in the ear canal, the motion at the pore would be 100 pm. This is still small, but at least two orders of magnitude greater than the traveling wave picture provides. For those interested, the paper is "Detection without deflection? A hypothesis for direct sensing of sound pressure by hair cells", J. Biosci. 32, 385-404, and is freely accessible at http://www.ias.ac.in/jbiosci/mar2007/385.pdf Andrew. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Andrew Bell Research School of Biological Sciences The Australian National University Canberra, ACT 0200, Australia T: +61 2 6125 5145 F: +61 2 6125 3808 andrew.bell@xxxxxxxx ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Dallos, P. (2003). Organ of Corti kinematics. JARO 4, 416-421. -----Original Message----- From: AUDITORY - Research in Auditory Perception [mailto:AUDITORY@xxxxxxxx On Behalf Of Martin Braun Sent: Wednesday, 3 October 2007 10:42 PM To: AUDITORY@xxxxxxxx Subject: Re: [AUDITORY] HC selectivity ... was Re: Physiological models of cochlea activity - alternatives to the travelling wave Dear A.J. and others, 1 pm is a subatomic magnitude. It is: 0.02 of the diameter of the hydrogen atom, the smallest atom in the universe 0.0000025 of the wavelength of blue light 0.01 of the wavelength of hard x-ray There is no known physics by which a mechanical signal of this magnitude could be transported, let alone be detected. Further, please keep in mind that the hair bundles of vertebrate hair cells are embedded in the Brownian motion of the molecules of the endolymph. It is well established that hair cell cilia are carried by this Brownian motion and thus move randomly in the order of hundreds and thousands of picometer. In mammalian outer hair cells (OHC) only the row 1 of the hair bundle is fixed by the tectorial membrane (TM). Rows 2 and 3 are freely floating in the endolymph. Because the channel gating mechanism of the tip links bridges adjacent rows, we know that the mechanical sensitivity of the ion channels in the OHC bundles is subject to Brownian motion. So, an OHC sensitivity to a 1 pm signal would not only be against all known physics, it must also be excluded on theoretical grounds. The problem that we are facing is a psychological one. Bekesy's traveling wave model was widely accepted long before useful data of the passive mechanics of the basilar membrane became available in the 1990s. Why was the model not abandoned then? It had been crystalized to marble in the majority of publications of the majority of living researchers. It's a simple as that, and Max Planck had a rather cynical view of this phenomenon in science: "A new scientific truth does not triumph by convincing its opponents and making them see the light, but rather because its opponents eventually die, and a new generation grows up that is familiar with it." Martin --------------------------------------------------------------------- Martin Braun Neuroscience of Music S-671 95 Klässbol Sweden web site: http://w1.570.telia.com/~u57011259/index.htm ----- Original Message ----- From: "A.J. Aranyosi" <aja@xxxxxxxx> To: <AUDITORY@xxxxxxxx> Sent: Tuesday, October 02, 2007 8:59 PM Subject: Re: HC selectivity ... was Re: Physiological models of cochlea activity - alternatives to the travelling wave Dear Martin, If we take Ruggero's measurements (Ruggero et al 1997, figure 16) and extrapolate back to 0 dB SPL, the resulting BM displacement is about 0.15-0.5 picometers peak, or about 0.3-1 pm peak-to-peak. That's not much, but it's certainly a physical, non-zero magnitude. As for the open probability of ion channels, you seem to be implying that there is some threshold displacement below which this probability can't be altered. My understanding of this process is that the channels are constantly flitting back and forth between open and closed states, and that any deflection of the bundle will tend to bias them toward one of these states. Taking the equation for this open probability from Howard and Hudspeth 1988, and using their estimates of parameter values based on their measurements, I calculated about an 0.01% change in open probability for a one picometer bundle deflection. Admittedly that's also not much, but we are talking about the threshold of hearing. And as the input of a feedback system with a gain of 60 dB, this would lead to about a 10% change in open probability overall. Of course, this argument assumes a perfect, noiseless system. With only 50-100 transduction channels per cell, and with each of them flipping randomly between open and closed states, the transduction current will have some "noise" associated with it. Is this noise large enough to mask small changes in the open probability? I don't know. Perhaps someone with more knowledge in this area could comment. -A.J. @xxxxxxxx{Ruggero/etal-97, author = "M. A. Ruggero and N. C. Rich and A. Recio and S. S. Narayan and L. Robles", title = "Basilar-membrane responses to tones at the base of the chinchilla cochlea", journal = "J Acoust Soc Am", volume = 101, pages = "2151-63", year = 1997} @xxxxxxxx{Howard/Hudspeth-88, author = "J.~Howard and A.~J.~Hudspeth", title = "Compliance of the hair bundle associated with gating of mechanoelectrical transduction channels in the bullfrog's saccular hair cell", journal = "Neuron", volume = 1, pages = "189-199", year = 1988}