EXCITATORY AND INHIBITORY PROCESSES 225 



The rate of oxygen consumption of the illuminated axons increases by 

 205% of the rate measured in dark axons at 12°C. 



In anaerobic conditions, the rate of reduction of intracellular hydrogen 

 acceptors (such as methylene blue micro-injected in the cell) is increased by 

 400% as compared to the rate observed in the dark. 



The rate of reduction of cytochrome c in yeast cells is increased by 100% 

 under illumination as compared to the rate in the dark. 



The rate of deoxygenation of the intraneuronal heme-protein is accelerated 

 by 100% under light. 



But, how might mechanisms of the first detectable photobioelectrical 

 response, i.e. of the generator depolarization, be related, in time and space, 

 to the events in the above sequence? In nerve cells {Aplysia), the cell mem- 

 brane, site of the generator depolarization, is one micron or more distant 

 from the nearest lipochondria wherein the initial photophysical act is per- 

 formed. In the present state of our knowledge, radiationless electronic energy 

 transfer from lipochondria to the membrane cannot be proved. The transi- 

 tions in the respiratory chain, initiated on the creation of micro-oxidant and 

 micro-reducing loci, start some 10"^ sec later, being thus fairly concomitant 

 to the development of the generator depolarization. Hence, if in the mech- 

 anisms of the generator depolarization a chemical mediator has to be 

 invoked, the latter might be relevant to the respiratory metabohsm. 



As regards the inhibitory effects elicited by activation of the carotene molecules, the 

 mechanisms are not well understood. Piatt (1959) pictures the carotene as an electron 

 donor-acceptor molecule which would act as a very suitable mediator of the electron 

 transfer. If activated, this molecule might be associated with the oxidizing reducing 

 centers and thus interfere with the respiratory chain. 



In the methylene-blue or neutral-red-stained axon the dye molecules bind 

 themselves to the axon membrane and aggregate therein, by stacking into 

 polymers. This is a suitable structural device which would allow the apph- 

 cation of the exciton theory and might suggest efficient energy transfers. 

 Moreover, both the primary photophysical sequence (excitation of the dye 

 molecule, photoconductivity, migration of energy, oxidant-reducing micro- 

 spots) and the bioelectrical transitions coincide here on the same cyto- 

 structural site, i.e. the cell membrane. 



The stained axon might prove to be a very useful experimental model in our 

 analysis. The so-called lamellae of the retinal photoreceptor's outer segment 

 where the pigments are ordered should (according to recent data from Moody 

 and Robertson (I960)) be visualized as the regular, orderly repeating infold- 

 ings of a wide differentiated portion of the cell membrane itself. The stained 

 axon might thus be viewed as a rough, primitive replica of the retinal photo- 

 receptor's outer segment, but as one which provides, nevertheless, reliable 

 data. These strongly suggest that at least generator potentials might actually 

 be initiated by Ught in the retinal photoreceptor cell itself by mechanisms 



