42 



E. F. MACNICHOL, JR. 



5 10 15 20 



POTENTIAL- MILLIVOLTS 



Fig. 8. The relationship between the frequency of discharge and the potential 

 change within a Limuhis photoreceptor cell in response to prolonged illumination. 

 After dark adaptation records were taken with progressively increasing illumination 

 (0.25 log. unit steps). Sufficient time was allowed between records to prevent light 

 adaptation. The number of impulses during the third second of illumination was 

 plotted as a function of the average increase in potential over its resting value during 

 that time. (The top line of Fig. 7 is one of the records from which the data were ob- 

 tained.) 



of the light intensity during the third second of a prolonged flash. As you can 

 see the relation is approximately linear over a range of about 4.75 log units, a 

 change in illumination of about 50,000 times. 7 



By counting the number of impulses during the third second of discharge 

 the steady state frequency was obtained. Fig. 9 shows the approximately 

 linear relationship between the slow potential and the frequency of discharge 

 measured from the records used to prepare Fig. 8. 



Thus we have a logarithmic relationship between light intensity and the 

 slow depolarization of the receptor cell and a linear relationship between de- 

 polarization and spike frequency. From the well known properties of electric 

 currents in stimulating nervous tissue it is tempting to postulate that the 

 action of light amplified by some electrochemical process that we do not yet 

 understand causes a graded electrical depolarization of the receptor cell. The 

 currents thus produced would stimulate the pacemaker mechanism. That the 

 slow potential is the precursor of the spikes is supported by several lines of 

 evidence. 



7 A similar linear relationship between depolarization and spike frequency has 

 been found in frog muscle spindles (Katz, 1950). 



