Mechanoelectrical transduction assisted by Brownian motion: a role for noise in the auditory system
Entry by Richie Tay for AP 225 Fall 2012
Authors: Fernán Jaramillo, Kurt Wiesenfeld
The ear is potentially capable of detecting nanometer-scale vibrations, but Brownian motion of the sensory apparatus generates noise that is at least an order of magnitude larger. In this paper the authors showed that, rather than limiting our threshold of hearing, this thermal noise actually enhances the ear’s sensitivity to weak signals.
The mechanics of hearing
Inner hair cells in the cochlear each have hundreds of cylindrical stereocilia protruding from their apical surface; collectively these form hair bundles, the mechanosensitive organelles that gather and transmit auditory information. The tips of the stereocilia are joined by filamentous tip links, which are believed to be connected to mechanosensitive ion channels. Mechanical stimuli (in the form of sound waves) cause the stereocilia to slide along one another, thus altering the tension of the tip links and initiating signal transmission.
Stochastic resonance refers to a nonlinear cooperative phenomenon where the addition of low-level noise to a system improves the system’s ability to detect weak (information-carrying) signals. To illustrate, imagine a double-well potential curve which could correspond, for instance, to the low-energy “open” and “closed” states of an ion channel (Figure 1). The wells are separated by an energy barrier. A very weak signal will not excite a particle sitting in one well into the neighboring well, but the addition of some noise will occasionally allow transitions between the wells. Stochastic resonance occurs when these exit events become correlated with the weak signal, so that transitions become more regular, and the regularity/coherence improves with the addition of more noise. Beyond an optimum background noise, the frequency of transitions no longer correlates with the weak signal and coherence deteriorates.