58 E. F. MACNICHOL, jr. 



Dr. Stark: We are in agreement then? 



Dr. McElroy: Well, I still think that it tits in with ATP as being an im- 

 mediate energy source. 



Dr. Mullixs: I would like to ask Drs. McElroy and MacNichol about two 

 of the points that arose in the discussion. Dr. McElroy says that you have to 

 release this pyrophosphate in order to stimulate the reaction and presumably 

 you have to have this thing taken up in order to get the thing back to normal 

 again. That just pushes the problem of where the excitation is off to what 

 releases pyrophosphate into the system, and so we should start, then, at that 

 level, rather than at this advanced one. 



Dr. McElroy: Dr. Mullins has brought up the point that perhaps retinene, 

 when it is liberated, is the thing which is responsible for activation. I think 

 that Dr. Wald feels that it is more likely the protein, such as the sulfhydryl 

 groups that are uncovered and which are then reactive and combined with 

 something, perhaps, in the membrane which triggers off the photoelectro- 

 chemical mechanism. The only reason for postulating this is because of the 

 delay in response. If one is at threshold conditions and ultimate sensitivity 

 the reaction is limited by the diffusion time from wherever it happens — wher- 

 ever you happen to break off such a single molecule — it may be deep in the 

 receptor or it may be near the surface and so you should find some variability 

 in that time; it shouldn't always be just one second. 



Dr. Fuortes: In your data you found that there were small spikes, or large 

 spikes and small slow potentials. I thought the slow potential was perhaps large 

 enough even when you had the small spikes. Is there a way to explain this? 

 Are the two things generating on a different basis? Why are the spikes small 

 where the slow potential is large? Is that, by any chance, damage due to a 

 particular structure? Concerning the effect of direct current stimulation, are 

 direct currents active in changing the figures of the charge wherever the direct 

 current is located? Or, are they active when it is repeated in structures that 

 produce the slow potential? My third question concerns the cross connections 

 that give inhibition in the one receptor. Is anything known about where these 

 terminations go on the receptacles except for the physiologicals? 



Chairman Gerard: May I ask another question? Have you any idea what 

 the retinula cells are doing there? 



Dr. MacNichol: I wish I knew. The axons, as far as we see, do not conduct 

 anything and they may be purely some kind of an evolutionary accident. They 

 may have been functional in some ancestor of the Limulus. 



Chairman Gerard: I am sorry, I didn't mean to divert you. 



Dr. MacNichol: Let me see if I can remember these three questions, now. 

 When we get in some regions of the cell we get these enormous slow potentials; 

 they are tens of millivolts and where we have the large spike activity we do not 

 see more than a millivolt or two of what might be a generator potential below 



