588 



RADIATION BIOLOGY 



fiber may serve a number of bipolars, each bipolar may in turn be con- 

 nected with several rod or cone receptor cells, and horizontal inter- 

 connecting fibers may carry inhibition or facilitation from one retinal 

 location to another. 



Components of the Electroretinogram. Many attempts have been made 

 (see Kohlrausch, 1931) to interpret the typical vertebrate electroretino- 

 gram on the basis of hypothetical component processes. The most 

 comprehensive of such attempts is that of Granit (1933, 1947). In 

 Granit's analysis (see Fig. 13-6) there are three fundamental processes — 

 PI, PII, and PHI — some of whose properties may be adduced from 



0.7 

 0.6 

 05 

 04 



3 



> 



E 0,2 



^"oi 



o 

 > 



03 

 0.2 

 0.1 







05 



I 



I 5 



2.0 25 



TIME, sec 



Fig. 13-6. Analysis of the vertebrate electroretinogram according to Granit (1947). 

 Upper heavy line is the electroretinogram in response to a bright light; lower heavy 

 line is that for a weaker light. PI is primarily responsible for the c-wave, PII for the 

 6-wave, and PHI for the o-wave and the rf-wave (off -response). 



Table 13-1. This table is intended, in the interests of brevity, to sum- 

 marize the most important features of Granit's discussion (1933; 1947, 

 Chaps. 3 and 4). 



The assignment of PHI to the sensory cells agrees with the fact that 

 the a-wave is often of extremely short latency. Also to be noted is the 

 observation by Pieron and Segal (1939) that this wave is not affected 

 by changes in temperature — a fact suggesting a photochemical origin. 

 Origination in the receptor cells might suggest that PHI represents the 

 event leading to the discharge of nerve impulses in the vertebrate retina. 

 There are two main objections to this suggestion. First, the polarity is 

 wrong, since the invertebrate action potential is a simple monophasic 

 wave showing negativity at the terminal portion of the receptors. Sec- 



