Figure 7.

Simulations of the expected time course of photoexcited VP and PDE in nonmammalian rods and cones at room temperature. The left panels illustrate the response to a single excited VP molecule and those on the right the response to 10 molecules. Simulations are based on the lifetime of active PDE in rods and cones derived from dark noise analysis by Rieke and Baylor (1996) or this report (Table II). The lifetime of PDE is 10 times shorter in cones than in rods (54 ms and 555 ms, respectively). As a consequence, it can be expected that when the same number of visual pigment molecules are excited in the photoreceptors, the total number of activated PDE molecules will be less in cones than rods, and this number will be reached more rapidly in cones than in rods.