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

1. In response to strong, large-field flashes the dark-adapted rods of Chelydra serpentina gave initial hyperpolarizing responses of 30-40 mV, declining rapidly to plateaus of 10-15 mV which lasted 20 sec or more.2. In the most sensitive cells the flash-sensitivity at 520 nm to a large illuminated area was 3-6 mV per photoisomerization (assuming an effective collecting area of 13.6 mum(2)).3. The initial response to a step of light agreed with that predicted by super-position from the flash response but even with very weak lights the step response fell below the predicted curve at times longer than about 2 sec.4. The step sensitivity defined from the initial peak of the response to a step of light was 2-6 mV photoisomerization(-1) sec, about 1000 times greater than the most sensitive cones in the turtle retina.5. The response to a small weakly illuminated spot (radius 21 mum) reached a peak later and lasted longer than the linear response to a weakly illuminated large area (radius 570 mum).6. 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.7. As others have found, strong flashes did not give an initial hyperpolarizing transient when the radius of the spot was less than about 50 mum.8. Experiments made by flashing long narrow strips of light onto the retina showed that the response spread a long way initially (lambda =... 70 mum) and then contracted down to a relatively small region (lambda =... 25 mum) at times of about 2 sec. 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.9. The results in (8) can be explained quantitatively by assuming that delayed voltage-dependent conductance changes mimic an inductance and make the rod network behave like a high-pass filter with series resistance and parallel inductance.10. In sensitive rods, flash responses varied randomly with a 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.11. The variance peak usually occurred later than the potential peak of the rod response.12. The high-pass filter characteristics of the rod-network help to explain several puzzling features of the behaviour of rods, for example (1), (5), (7), (8) and (11) of this summary.13. The high-pass filter characteristics of the rod-network may help it to optimize the signal to noise ratio by integrating over a large area for rapid signals and over a small one for slow signals.