Stochastic Treatment of Bump Latency and Temporal Overlapping in Limulus Ventral Photoreceptors

We present quantum bum ps obtained from flash experim ents at the Limulus ventral nerve photoreceptor under voltage clam p conditions. The results are shown and discussed in form o f histogram s for the latency, am plitude and net charge transfer (current time integral) o f the bum p current responses. We argue tha t the experim ental latency histogram s cannot be de­ scribed satisfactorily by chemical models if one assum es tha t not m ore than one photon is cap­ tured per flash. Instead of, one has to take into account the Poisson statistics o f the captures o f 0 ,1 ,2 ,... photons released by a single flash. We show that the inclusion o f Poisson statistics makes the effective latency histogram s o f flash responses typically asym metric and skewed to ­ wards short latencies as com pared to tha t o f model histogram s for one-photon responses. O ur conjecture also implies tha t under ou r experim ental conditions a fraction o f up to 20% of the bum p responses evoked by a flash should be suspected to be superpositions o f two ore m ore one-photon responses which cannot be separated by any kind o f evaluation analysis. C onse­ quently, the average values o f am plitudes and net charge transfers o f the light-evoked bum p responses are expected to be overestim ated as com pared to that o f true one-photon responses. This hypothesis is confirm ed by a num erical sim ulation o f light-evoked bum p responses using experimentally recorded spontaneous bum ps (at times larger than 1 s after the flash) as the simulation m aterial. We show tha t the superposition o f one-photon events in the light-evoked bump responses due to Poisson statistics settles the question why their am plitudes and net charge transfers are found to be larger than tha t o f the spontaneous bumps. We suggest that true one-photon responses evoked by a light flash and spontaneous bum ps start from the same activated rhodopsin state and take the same biochem ical pathw ay.