Statistics and Quantum Bumps in Arthropod Photoreceptors

摘要

Psychophysical experiments with humans more than 40 years ago suggested that a photoreceptor could detect single-photon absorptions by the visual pigment, but these experiments did not determine what changes these absorptions produced in the receptor. Some years later, by serendipity, discrete waves of depolarizing membrane potential changes, originally called bumps, were discovered with intracellular electrodes in dark-adapted single photoreceptors of the lateral eye of Limulus, the horseshoe crab. An essential property of bumps is their random occurrence. Discrete waves of depolarizing membrane potential in arthropod photoreceptors, called quantum bumps, appear to result from single-photon absorptions of the visual pigment. Statistical analysis of bump records suggest a model for bump occurrence in dark-adapted receptors at low levels of illumination. This model assumes that a photon that isomerizes a visual pigment molecule can trigger a stochastic process that can produce no more than one bump under normal conditions, and that the stochastic processes triggered by different isomerized visual pigment molecules are independent of each other.