Tessier-Lavigne M, Attwell D
Department of Physiology, University College, London, U.K.
Proc R Soc Lond B Biol Sci. 1988 Jul 22;234(1275):171-97. doi: 10.1098/rspb.1988.0043.
Electrical coupling of vertebrate photoreceptors is well known to improve the signal: noise ratio in the photoreceptor layer for large-area stimuli. For example, if N photoreceptors are perfectly coupled to each other, the signal: noise ratio is improved for stimuli illuminating more than a number M = square root of N of the receptors but is made worse for small-area stimuli illuminating less than M of the N receptors. Using the model of Lamb & Simon (J. Physiol., Lond. 263, 257 (1976], which treats the photoreceptor layer as a square array of cells, each coupled through a resistive gap junction to the four cells around it, we show that the signal:noise ratio for small-area stimuli is much greater than would be expected from a model in which receptors are assumed to be perfectly coupled. Contrary to predictions made assuming perfect coupling, receptor coupling should not prevent rods from detecting single photons, but whether the single photon signal can be detected at the bipolar cell level depends on how signals are read out of the receptor layer. The signal:noise ratio in bipolar cells postsynaptic to the photo-receptor layer is determined partly by synaptic convergence and nonlinearity in synaptic transmission from receptors. If the synaptic gain decreases with light-induced receptor hyperpolarization, as is found experimentally, then receptor coupling can improve the postsynaptic signal:noise ratio for stimuli illuminating only one receptor, even though coupling decreases the presynaptic signal:noise ratio for such stimuli. Moreover, increasing the number of coupled receptors projecting to a bipolar cell can improve the signal:noise ratio for localized stimuli if the synapse is sufficiently nonlinear (although, for the degree of nonlinearity seen in lower vertebrates, synaptic convergence makes the ratio worse for the single photon event). The fact that receptor coupling and synaptic convergence can, under some circumstances, improve the signal:noise ratio in bipolar cells suggests a principle of retinal design that may compete with the requirements of high spatial resolution.
脊椎动物光感受器的电耦合能够提高大面积刺激下光感受器层的信号噪声比，这是广为人知的。例如，如果N个光感受器彼此完美耦合，那么对于照亮超过M = √N个感受器的刺激，信号噪声比会得到改善；而对于照亮少于N个感受器中M个的小面积刺激，信号噪声比则会变差。利用Lamb和Simon的模型（《伦敦生理学杂志》，第263卷，第257页，1976年），该模型将光感受器层视为细胞的方形阵列，每个细胞通过电阻性间隙连接与其周围的四个细胞耦合，我们发现小面积刺激的信号噪声比远高于假设感受器完美耦合的模型所预期的值。与假设完美耦合所做的预测相反，感受器耦合不应阻止视杆细胞检测单个光子，但单光子信号能否在双极细胞水平被检测到取决于信号如何从感受器层读出。光感受器层突触后双极细胞中的信号噪声比部分由突触汇聚以及感受器突触传递中的非线性决定。如果突触增益随光诱导的感受器超极化而降低，正如实验所发现的那样，那么即使耦合会降低此类刺激的突触前信号噪声比，感受器耦合仍可改善仅照亮一个感受器的刺激的突触后信号噪声比。此外，如果突触足够非线性，增加投射到双极细胞的耦合感受器数量可以改善局部刺激的信号噪声比（尽管对于低等脊椎动物中观察到的非线性程度，突触汇聚会使单光子事件的信号噪声比变差）。感受器耦合和突触汇聚在某些情况下能够改善双极细胞中的信号噪声比这一事实，暗示了一种视网膜设计原则，该原则可能与高空间分辨率的要求相互竞争。
