van Rossum M C, Smith R G
Department of Neuroscience, University of Pennsylvania, Philadelphia 19104-6058, USA.
Vis Neurosci. 1998 Sep-Oct;15(5):809-21. doi: 10.1017/s0952523898155037.
Mammalian rods respond to single photons with a hyperpolarization of about 1 mV which is accompanied by continuous noise. Since the mammalian rod bipolar cell collects signals from 20-100 rods, the noise from the converging rods would overwhelm the single-photon signal from one rod at scotopic intensities (starlight) if the bipolar cell summed signals linearly (Baylor et al., 1984). However, it is known that at scotopic intensities the retina preserves single-photon responses (Barlow et al., 1971; Mastronarde, 1983). To explore noise summation in the rod bipolar pathway, we simulated an array of rods synaptically connected to a rod bipolar cell using a compartmental model. The performance of the circuit was evaluated with a discriminator measuring errors in photon detection as false positives and false negatives, which were compared to physiologically and psychophysically measured error rates. When only one rod was connected to the rod bipolar, a Poisson rate of 80 vesicles/s was necessary for reliable transmission of the single-photon signal. When 25 rods converged through a linear synapse the noise caused an unacceptably high false positive rate, even when either dark continuous noise or synaptic noise where completely removed. We propose that a threshold nonlinearity is provided by the mGluR6 receptor in the rod bipolar dendrite (Shiells & Falk, 1994) to yield a synapse with a noise removing mechanism. With the threshold nonlinearity the synapse removed most of the noise. These results suggest that a threshold provided by the mGluR6 receptor in the rod bipolar cell is necessary for proper functioning of the retina at scotopic intensities and that the metabotropic domains in the rod bipolar are distinct. Such a nonlinear threshold could also reduce synaptic noise for cortical circuits in which sparse signals converge.
哺乳动物的视杆细胞对单个光子的反应是产生约1 mV的超极化，同时伴有持续的噪声。由于哺乳动物的视杆双极细胞收集来自20 - 100个视杆细胞的信号，在暗视强度（星光）下，如果双极细胞线性求和信号，来自汇聚视杆细胞的噪声将淹没来自单个视杆细胞的单光子信号（贝勒等人，1984年）。然而，已知在暗视强度下视网膜保留单光子反应（巴洛等人，1971年；马斯特罗纳德，1983年）。为了探索视杆双极通路中的噪声求和，我们使用一个房室模型模拟了一系列与视杆双极细胞突触连接的视杆细胞阵列。该电路的性能通过一个鉴别器进行评估，该鉴别器测量光子检测中的错误，将其作为假阳性和假阴性，并与生理和心理物理学测量的错误率进行比较。当只有一个视杆细胞与视杆双极细胞相连时，每秒80个囊泡的泊松率对于单光子信号的可靠传输是必要的。当25个视杆细胞通过线性突触汇聚时，即使完全去除暗连续噪声或突触噪声，噪声仍会导致不可接受的高假阳性率。我们提出，视杆双极树突中的代谢型谷氨酸受体6（mGluR6）提供了一个阈值非线性，以产生一个具有噪声去除机制的突触。有了阈值非线性，突触去除了大部分噪声。这些结果表明，视杆双极细胞中的mGluR6受体提供的阈值对于视网膜在暗视强度下的正常功能是必要的，并且视杆双极中的代谢型结构域是不同的。这样的非线性阈值也可以减少稀疏信号汇聚的皮质回路中的突触噪声。
