Donner K, Hemilä S, Koskelainen A
Department of Biosciences, University of Helsinki, Finland.
Vision Res. 1998 Jan;38(1):19-36. doi: 10.1016/s0042-6989(97)00144-2.
The sensitivity and time scale of the dominant (562 nm) cone system of the frog, Rana temporaria, were studied as functions of steady adapting illuminance (IB). Photoreceptor responses to brief flashes of light were recorded as aspartate-isolated ERG mass potentials from the isolated retina. The characteristics of the cone signal after transmission through the retina were derived from response thresholds and stimulus--intensity-response--latency functions for extracellularly recorded spike discharges of single ganglion cells in the eyecup. At 14 degrees C, the single-photon response of dark-adapted cones, extrapolated from ERG intensity-response functions, had an amplitude of 0.5% of the saturated response (Umax) and peaked at tp approximately 0.4 sec. Steady background illumination decreased both tp and flash sensitivity (SF), starting from apparent "dark lights" of, respectively, less than 10 (for time scale) and about 100 (for sensitivity) photoisomerisations per cone per second [Psec-1]. From there upwards, two distinct ranges of background adaptation were apparent. Under moderate backgrounds (up to IB approximately 10(4) - 10(5) Psec-1), sensitivity fell according to the relation SF alpha IB-0.64 and time scale shortened according to tp alpha IB-0.16. Under brighter backgrounds, from approx. 10(5) Psec-1 up to the limit of our light source at 10(7) Psec-1, the decrease in SF was significantly stronger than predicted by the Weber relation (SF alpha IB-1), while the decrease in tp levelled out and even tended to reverse. All these changes were virtually identical at the photoreceptor and ganglion cell levels, although the absolute time scale of cone signals apparent at the latter level was 2-fold longer. Our general conclusion is that photoreceptors have several distinct regimes for light adaptation, and traditional descriptions of functional changes (in sensitivity and kinetics) relevant to vision need to be restated with higher resolution, in view also of recent insights into the diversity of underlying mechanisms.
研究了欧洲林蛙（Rana temporaria）优势（562纳米）视锥系统的敏感度和时间尺度与稳定适应光照度（IB）的函数关系。通过从离体视网膜记录天冬氨酸分离的视网膜电图（ERG）群体电位，来记录光感受器对短暂闪光的反应。通过对眼杯中单个神经节细胞细胞外记录的动作电位发放的反应阈值和刺激强度-反应-潜伏期函数，得出光信号经视网膜传递后的特征。在14摄氏度时，从ERG强度-反应函数外推得出的暗适应视锥细胞的单光子反应，其幅度为饱和反应（Umax）的0.5%，峰值出现在tp约为0.4秒时。稳定的背景光照降低了tp和闪光敏感度（SF），从明显的“暗光”开始，分别为每个视锥细胞每秒少于10次（对于时间尺度）和大约100次（对于敏感度）的光异构化[P秒-1]。从那里向上，明显出现了两个不同的背景适应范围。在中等背景下（直至IB约为10(4)-10(5)P秒-1），敏感度根据SFαIB-0.64的关系下降，时间尺度根据tpαIB-0.16缩短。在更亮的背景下，从约10(5)P秒-1直至我们光源的极限10(7)P秒-1，SF的下降明显强于韦伯关系（SFαIB-1）的预测，而tp的下降趋于平稳，甚至趋于逆转。所有这些变化在光感受器和神经节细胞水平上几乎相同，尽管在后者水平上视锥信号的绝对时间尺度长了2倍。我们的总体结论是，光感受器有几种不同的光适应模式，鉴于最近对潜在机制多样性的深入了解，与视觉相关的功能变化（在敏感度和动力学方面）的传统描述需要以更高的分辨率重新阐述。
