The relation between intercellular coupling and electrical noise in turtle photoreceptors.

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Abstract
Intracellular recordings from cones and rods in the retina of the turtle, Pseudemys scripta elegans, revealed that in darkness the cell voltage fluctuated spontaneously about its mean level. The fluctuations were reduced during bright steady illumination of the cell often to a level close to that obtained with the electrode outside the cell where the noise did not change significantly during illumination. The magnitude of the intrinsic dark noise (voltage variance in darkness minus voltage variance in strong light) varied widely from cell to cell. In the noisiest cones it was about 0.4 mV2 while in quiet cones it was often as low as 0.01 mV2. The noise appeared random and could be fitted by a Gaussian probability density function. The spread of voltage in the network of coupled photoreceptors was estimated by measuring the spatial profile of the response to a brief flash of constant intensity moved across the retina. For a light stimulus in the form of a long narrow slit, the peak flash response usually decayed exponentially with displacement from the centered position. For maximum responses less than about 5 mV in cones, the length constant of exponential decay, .lambda. varied from less than 10 .mu.m to greater than 35 .mu.m, and the values obtained in opposite directions were often unequal. Background illumination did not significantly change .lambda.. In cells with extremely narrow spatial profiles, an exponential fit to the decay could not be made reliably. Occasionally the spatial profiles had definite secondary peaks. In the most pronounced examples in a red-sensitive cone and in a rod, the maxima were separated by about 20 and 50 .mu.m respectively; for each, 1 peak was approximately as sharp as the optical stimulator while the 2nd was broader.