60 E. F. MACNICHOL, jr. 



going to work with Dr. Hartline during the next year and I hope he will get 

 some more answers. It looks as if the cross fibers fan out into kind of a brush 

 over the region where they surround the bundle of axons. There do not seem 

 to be any connections between the ommatidia directly. They appear to occur 

 entirely along the axons. It may be that these slow potentials, this electronic 

 spread that I have been measuring in the nerve is conducted through the cross 

 connections over to the axons of other eccentric cells producing the inhibitory 

 effect, but we do not have enough evidence yet to state this positively. 



Dr. Stark: It seems to me that your explanation of the relationship between 

 the spikes and slow potential is very similar to the local response of a nerve 

 but different from what Dr. Fuortes was suggesting with the electrotonic slow 

 change in the current in a different region from the spikes. Now, one point 

 which might settle this is: Do your thresholds for light stimulation go up when 

 you get the slow potential changes? 



Dr. MacNichol: We always get the slow potential changes when we stim- 

 ulate. 



Dr. Stark: You always do? 



Dr. MacNichol: Yes. 



Dr. Stark: In other words, you would assume that your are injuring your 

 membrane and therefore you would be getting a smaller safety factor and a 

 larger slow potential before you got spike potentials; is that your reason? 



Dr. MacNichol: No, I don't think so. Where we have gotten the large 

 spikes, the preparation is quite stable. Sometimes we get spontaneous activity, 

 but this is usually due to injury and often subsides after awhile. 



Dr. Blum: It seems we have a system in which you have a sort of a total 

 maximum discharge, that is, some of the potential, the slow potential, in the 

 spike is a sort of constant potential — you have a small D.C. potential and you 

 get a large spike. You also, I believe, showed on the slide that the frequency of 

 discharge was proportional to the slow potential and likewise the slow potential 

 was proportional to the intensity. So you have two constants, Ki and K 2 . If 

 you study these two constants at, say, different KCl's, or you produce calcium 

 and you get spontaneous activity, you will get different constants. It seems to 

 me that the relation between these constants may be used to obtain some clue 

 as to the actual mechanism of the transduction that was going on. Because, 

 assuming that the total change in potential, total change of slow potential, is 

 a constant, then measuring the changes in Ki and K 2 , as a function of potas- 

 sium or calcium, would give you some detailed clues as to what was happening 

 in the inter-relation between the two. 



Dr. MacNichol: I abandoned the work on the use of ions temporarily, I 

 hope, when it looked as if the microelectrode technique was going to be useful 

 and I have not gotten back to using that technique with the changes in ionic 



