Temporal and spatial characteristics of the voltage response of rods in the retina of the snapping turtle

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Abstract
In response to strong, large-field flashes, the dark-adapted rods of C. serpentina gave initial hyperpolarizing responses of 30-40 mV, declining rapidly to plateaus of 10-15 mV which lasted 20 s or more. In the most sensitive cells, the flash-sensitivity at 520 nm to a large illuminated area was 3-6 mV/photoisomerization (assuming an effective collecting area of 13.6 .mu.m2). The initial response to a step of light agreed with that predicted by superposition, but even with very weak lights the step response fell below the predicted curve at times longer than about 2 s. The step sensitivity defined from the initial peak of the response to a step of light was 2-6 mV/photoisomerization s, about 1000 .times. greater than the most sensitive cones in the turtle retina. The response to a small, weakly illuminated spot (radius 21 .mu.m) reached a peak later and lasted longer than the linear response to a weakly illuminated, large area (radius 570 .mu.m). The difference in sensitivity between large and small spots was reasonably consistent with the apparent space constant of the rod network obtained from the exponential decline of the flash response on either side of an illuminated strip. Strong flashes did not give an initial hyperpolarizing transient when the radius of the spot was less than about 50 .mu.m. Flashing long narrow strips of light onto the retina spread the response a long way initially (.lambda. .apprxeq. 70 .mu.m). The response contracted down to a relatively small region (.lambda. .**GRAPHIC**. 25 .mu.m), at times of about 2 s. When the line source was at some distance from the impaled rod, the response reached a peak earlier and was shorter than when the source was close. Delayed voltage-dependent conductance changes apparently quantitatively mimic an inductance and make the rod network behave like a high-pass filter with series resistance and parallel inductance. In sensitive rods, flash responses had a random variance which was about 1/30 of that expected in an isolated cell; this reduction in noise is satisfactorily explained by electrical coupling between rods. The variance peak usually occurred later than the potential peak of the rod response. The high-pass filter characteristics of the rod-network help to explain several puzzling features of the behavior of rods. The high-pass filter characteristics of the rod-network may help optimize the signal to noise ratio by integrating over a large area for rapid signals and over a small one for slow signals.