Jump to: Page Content, Section Navigation, Site Navigation, Site Search, Account Information, or Site Tools.
|
|
Articles
Distributed Relaxation Processes in Sensory AdaptationSpatial nonuniformity in receptors can explain both the curious dynamics and logarithmic statics of adaptation
1 Max-Planck-Institut für Verhaltensphysiologie, 8131
Seewiesen, Germany
Dynamic description of most receptors, even in their near-linear ranges, has not led to understanding of the underlying physical eventsin many instances because their curious transfer functions are not found in the usual repertoire of integral-order control-system analysis. We have described some methods, borrowed from other fields, which allow one to map any linear frequency response onto a putative weighting over an ensemble of simpler relaxation processes. One can then ask whether the resultant weighting of such processes suggests a corresponding plausible distribution of values for an appropriate physical variable within the sensory transducer. To illustrate this approach, we have chosen the fractional-order low-frequency response of Limulus lateral-eye photoreceptors. We show first that the current "adapting-bump" hypothesis for the generator potential can be formulated in terms of local first-order relaxation processes in which local light flux, the cross section of rhodopsin for photon capture, and restoration rate of local conductance-changing capability play specific roles. A representative spatial distribution for one of these parameters, which just accounts for the low-frequency response of the receptor, is then derived and its relation to cellular properties and recent experiments is examined. Finally, we show that for such a system, nonintegral-order dynamics are equivalent to nonhyperbolic statics, and that the efficacy distribution derived to account for the small-signal dynamics in fact predicts several decades of near-logarithmic response in the steady state. Encouraged by the result that one plausible proposal can account approximately for both the low-frequency dynamics (the transfer function sk) and the range-compressing statics (the Weber-Fechner relationship) measured in this photoreceptor, we have described some formally similar applications of these distributed effects to the vertebrate retina and to analogous properties of mechanoreceptors and chemoreceptors.
THIS ARTICLE HAS BEEN CITED BY OTHER ARTICLES:
|
Science. ISSN 0036-8075 (print), 1095-9203 (online)