VISUAL RECEPTORS AS BIOLOGICAL TRANSDUCERS 59 



these 30 or 40 millivolt spikes. 1 think this is probably just a difference in the 

 location of the two things. 



The distal process of the eccentric cell, whatever it is, is quite long and thin. 

 If the slow potential occurs here and if this is where our electrode is we get a 

 large slow potential. But if we have an electrode in this large cell body, due to 

 current flow through the membrane, the attenuation of this slow potential is 

 likely to be very great. 



Dr. Mullixs: There are two interesting implications. One is the spikes are 

 not conducted backward. We can see spikes up there. 



Dr. MacNichol: Apparently they do not invade the distal process. 



Dr. Mullixs: Secondly, the electronic plane is very sharp. 



Dr. MacNichol: I have been studying this recently, putting electrodes on 

 the nerve back a ways, back several millimeters and we find that we can get 

 slow potentials with the spikes superimposed upon them. And the records ob- 

 tained back here look exactly like those we get with the micropipette except 

 that the slow potentials are small. The spikes will go way off scale if we have 

 sufficient amplifier gain to see the slow potentials. But the slow potentials 

 seem to have the same quantitative relationship to the light intensity and to 

 the frequency of discharge that they do further back in the system. We have 

 also been able to pick up sub-liminal slow potentials. That is, if we put a short 

 flash of light in, we will get this rise in slow potential below threshold for the 

 firing off of spikes. The slow potential starts up and then decays, or if it is a 

 long flash the slow potential stays up. If you increase the light intensity, the 

 slow potentials will get bigger and bigger and bigger and pretty soon we get 

 spike. That looks as if there is evidence of a sub-liminal process. 



In reply to, I think it was your first question about the electrical polarization, 

 we did do a couple of experiments last summer in which we tried to relate the 

 change in spike frequency to light and to current. That is, we had a calibrated 

 amplifier and measured the resistance of our electrode, so that we could cal- 

 culate for a given deflection in the base line produced by applying electrical 

 polarization to the electrode how much the applied potential inside the cell 

 had changed. We then applied different amounts of direct current and meas- 

 ured the spike frequency corresponding to various potentials between the in- 

 side and outside of the cell. We measured the change in potential across 

 the cell produced by varying light intensity. What we found was, that with 

 the particular location of our electrode that we had in this experiment, that 

 about five times as much potential change was required to produce a given 

 change in spike frequency in response to applied electric current as was required 

 to get the same spike frequency by stimulating with light. 



I think all this says is that we were just not stimulating at the most favora- 

 ble place. 



Dr. Miller has done a good bit of histology on the cross connections. He is 



