VISUAL RECEPTORS AS BIOLOGICAL TRANSDUCERS 



45 



-2 



2 4 6 8 



AMPS X 10° 



Fig. 11. Relation between electrical polarization and change in frequency of 

 discharge of a spontaneously firing photoreceptor. The records of Fig. 10 are some of 

 those used in preparing this figure. 



process was repeated with various test and control solutions until failure of 

 conduction in the nerve fiber terminated the experiment. 



The figure was obtained from a sequence of four test runs. The ordinate 

 represents the number of impulses discharged in response to a flash and the 

 abcissa the logarithm of the light intensity. The first run represented by the 

 filled circles was a control. In the second, represented by the open circles, more 

 than the physiological concentration of potassium was used in the bathing 

 fluid. In the third, represented by the solid triangles, excess calcium was used, 

 and the fourth run, indicated by the open triangles, was taken after the eye 

 was returned to the control solution. 



The effect of potassium was to decrease the threshold so that impulses were 

 discharged in response to weaker light, and to increase the number of impulses 

 added by a given change in light intensity. The effects of calcium were just the 

 reverse which is not surprising in view of the well known antagonism between 

 potassium and calcium in other tissues. Potassium-free solutions increased the 

 threshold and decreased the slope of the curve; whereas, in calcium-free solu- 

 tions the receptor behaved very much as it did in increased potassium. Both 

 increase of potassium and withdrawal of calcium caused spontaneous activity 

 in the dark. 



An unexpected result of these experiments was a specific effect of potassium 



