Howard J, Blakeslee B, Laughlin S B
Department of Neurobiology, Research School of Biological Sciences, Australian National University, Canberra, A.C.T.
Proc R Soc Lond B Biol Sci. 1987 Sep 22;231(1265):415-35. doi: 10.1098/rspb.1987.0053.
The function of the intracellular pupil mechanism is examined by comparing the responses of photoreceptors in normal flies with those from white-eyed flies that lack the pupil. In white-eyed flies the response to an intensity increment of fixed contrast decreases at high background intensities. There is a smaller decrease in noise amplitude so that the signal:noise ratio falls. The intensity dependence of the photoreceptor signal:noise ratio fits a simple model in which activated photopigment molecules compete for 3 X 10(4) transduction units. The signal:noise ratio decreases at high intensities because the transduction units are saturated. This model is supported by a noise analysis, which provides three estimates of the number of events generating photoreceptor responses. In white-eyed flies the event number saturates at high background intensities, suggesting that a maximum of 2 X 10(4) events can be simultaneously active. Wild-type flies do not exhibit saturation effects over the range of intensities studied. The signal:noise ratio rises with intensity to reach a stable asymptote, close to the maximum observed for white-eyed flies. Pupil attenuation is calculated from measurements of signal:noise ratio in white-eyed and wild-type flies. The pupil is progressively activated over a two log unit intensity range and when fully closed attenuates the effective intensity by 99%. The threshold of this pupil effect coincides with the threshold of pupil activation measured optically. We conclude that the intracellular pupil attenuates the light flux to prevent receptor saturation and to extend the range of intensities at which fly photoreceptors operate close to their maximum signal:noise ratio. This upper limit is determined by the number of transduction units generating a cell's response.
通过比较正常果蝇与缺乏瞳孔的白眼果蝇中光感受器的反应，来研究细胞内瞳孔机制的功能。在白眼果蝇中，对固定对比度强度增加的反应在高背景强度下会降低。噪声幅度的下降较小，因此信号噪声比下降。光感受器信号噪声比的强度依赖性符合一个简单模型，即活化的光色素分子竞争3×10⁴个转导单位。在高强度下信号噪声比降低是因为转导单位饱和。该模型得到了噪声分析的支持，噪声分析提供了对产生光感受器反应的事件数量的三种估计。在白眼果蝇中，事件数量在高背景强度下饱和，表明最多2×10⁴个事件可以同时活跃。野生型果蝇在所研究的强度范围内未表现出饱和效应。信号噪声比随强度增加达到一个稳定的渐近线，接近白眼果蝇观察到的最大值。根据白眼果蝇和野生型果蝇信号噪声比的测量计算瞳孔衰减。瞳孔在两个对数单位的强度范围内逐渐被激活，当完全关闭时，有效强度衰减99%。这种瞳孔效应的阈值与光学测量的瞳孔激活阈值一致。我们得出结论，细胞内瞳孔衰减光通量以防止感受器饱和，并扩展果蝇光感受器在接近其最大信号噪声比下工作的强度范围。这个上限由产生细胞反应的转导单位数量决定。
