VISUAL RECEPTORS AS BIOLOGICAL TRANSDUCERS 



41 



I MV 



wiuuau L 



02 SEC 



40 MV 



rr 



15 MV. 



10 MV 



ru 



i sec 



Fig. 7. Typical responses from micropipettes inserted into Limulus ommatidia. 

 (Upward deflection indicates that the pipette became more positive.) 



Sometimes we obtained very large spikes like those from visible eccentric 

 cells as shown in the middle trace. On other occasions large slow potentials 

 like those shown in the bottom two traces were obtained. Very frequently small 

 spikes superimposed on small slow potentials were observed as shown on the 

 top line. Which type of response was obtained seems to have depended upon 

 the location of the electrode tip. We have never seen large spikes and large 

 slow potentials together. 6 There have as yet been no positive identifications of 

 the origin of the slow potentials, unless we assume by elimination of other 

 structures that they arise in the distal process of the eccentric cell. 



When we obtained both slow potentials and spike activity, we believed that 

 we were recording from a location in which the electrode intercepted currents 

 leaking from the separate sites of origin of the slow and spike potentials. 



We have investigated the quantitative relationships between light intensity, 

 the slow potentials and the spike frequency in preparations in which we were 

 able to get both potentials simultaneously (MacNichol et al., 1953). The top 

 record of Fig. 7 is one of the series used in this investigation. We were able 

 to measure the amplitude of the slow potential on the record and to measure 

 the frequency of discharge. The slow potentials were positive, indicating a de- 

 polarization of the receptor cells and the frequency of the spikes appears to be 

 a function of both the amplitude of the depolarization and of its rate of change. 

 Notice that the spikes were much faster in the beginning as the slow potential 

 rose. For simplicity we have thus far studied only the steady state conditions 

 during the third second of illumination when transient effects had subsided. 



Fig. 8 shows the relationship between the slow potential and the logarithm 



6 An alternative hypothesis is that the large slow potential is an injur}' effect in 

 which a spike is initiated but the cell cannot recover its resting potential until the 

 light has been turned off. 



