226 INFLUENCE OF TEMPERATrRE ON BIOLOGICAL SYSTEMS 



stance and, consequently, the magnitude of the retinal action potential 

 (fig. 3j. 



The effect of temperature on magnitude is not consistent under all con- 

 ditions of stimulation, nor for all photoreceptors. Both the Limulus lateral 

 eye and the frog eye show slight temperature effects on the response mag- 

 nitude; but even here the effect of temperature on the magnitude of the 

 retinal action potential is much less marked than is the effect of tempera- 

 ture on the latent period. 



The model suggested for the latent period is defined by the equation 

 dp/dt = hp + nl and it contains a photochemical term and an autocata- 

 lytic term, both positive. These processes cause the accumulation of a 

 factor P to a critical concentration Pc . When this critical level is reached 

 the latent period ends and the electrical response begins. Here again, one 

 may assume that the photochemical term is relatively temperature stable 

 whereas the autocatalytic term is temperature labile; i.e. with increasing 

 temperature the rate of the autocatalytic process speeds up, pc is reached 

 sooner and the latent period is shortened. 



The latency model predicts that the rate constant, h, of the autocatalytic 

 process should increase with increasing temperature. This constant may be 

 evaluated from the data using the expression, m = —log 10/h, and it does, 

 indeed, increase with temperature, exhibiting an average Qio of 1.5. The 

 model permits the evaluation of the ratio Pc/n, from the relation Ii = hpc/n. 

 The quantity Pc/n varies markedly with temperature, decreasing as the 

 temperature increases. Between 10° and 20°C this quantity decreases by a 

 factor of 156 and between 20°C and 30°C it decreases by a factor of 15. 

 Since n is the rate constant of the photochemical process in the latency 

 model, it may be assumed not to vary greatly with temperature. Conse- 

 quently, the changes in the ratio Pc/n may largely be attributed to changes 

 in the ciuantity, Pc . 



Although it is impossible at this time to draw quantitative relationships 

 between the electrical events in photoreceptors and the photochemical 

 events, it is perhaps of interest to speculate. The bleaching process both 

 in the intact photoreceptor (3) and in rhodopsin solutions (9, 10) is the 

 only transient event thus far observed, which has the time course and 

 temperature sensitivity compatible with the characteristics of the latent 

 period of the retinal action potential. The relative insensitivity to tem- 

 perature of the transient absorption bands which appear during photo- 

 excitation (9, 10) suggests a possible association with the events leading 

 to the generation of the retinal action potential. 



SUMMARY 



The effect of temperature upon the photochemical and electrical events 

 which occur in photoreceptors or their parts is discussed on the basis of 



