ELECTRICAL PHENOMENA IN VISION 



589 



ondly, PII (not PHI) appears to be influenced in the same manner by 

 anoxia and drugs as are the nerve impulses of vertebrates. 



Although PII in the vertebrate eye has the polarity and other charac- 

 teristics of the monophasic invertebrate electroretinogram, Granit has 

 cited evidence that it does not originate in either sensory or ganglion 

 cells. Granit and Helme (1939) found that the vertebrate electroretino- 

 gram remained normal, while antidromic volleys were aroused electri- 

 cally in the optic nerve, i.e., in the axons of the ganglion cells. This 



Table 13-1. Principal Characteristics of the Retinal Action Potential 

 AS Related by Graxit to Three Underlying Processes 



procedure might be expected to depress the action potential if it did in 

 fact originate in these cells. The evidence against rods and cones as 

 originators of the vertebrate action potential lies chiefly in the fact that 

 the principal electroretinogram component (the 6-wave) has the charac- 

 teristics of synaptically mediated activity. The 6-wave, accounted for 

 chiefly by PII, is easily depressed by potassium chloride and by anoxia, 

 but it is augmented by strychnine. Hence the suggestion by Granit that 

 the bipolar cells are a likely source of this principal wave in the verte- 

 brate electroretinogram. 



A further extension of this analysis involves Granit's distinction (1935) 

 between excitatory and inhibitory (E- and I-) retinas. E-retinas are con- 

 sidered to be those of mammals, whereas I-retinas are found in all other 

 vertebrate eyes. The I-retina responds strongly to the cessation of light 



