CHEMOPOTENTIALS IN GIANT NERVE CELLS 189 



Stimulated firing of inhibitory interneurons. In this case, of course, if the 

 CO2 depolarization is permitted to proceed, further spiking will be stopped. 

 The slopes and the duration of the slow hyperpolarizing waves suggest 

 that they may correspond to high changes in the membrane's conductances, 

 probably of endogenous origin. 



5. CHEMICAL CHANGES DURING CO2 INJECTION 



Under the experimental conditions adopted, the mean time required for 

 the appearance of the maximuni excitatory effect by CO2 (maximal fre- 

 quency) was about 40 sec. Since anoxia does not produce such effects within 

 40 sec, it seems that the effects of CO2 may be brought about by direct 

 action upon the membrane. To what extent is the effect of CO2 attributable 

 to protons produced by the hydrolysis of carbonic acid? 



We observed that in the connective tissue surrounding the A cell (in other 

 words, in the tissue close to the A cell membrane), a 0-5 pH unit lowering 

 occurs after 40 sec exposure to CO2. (Techniques for intracellular pH measure- 

 ments in nerve cells are given in the literature (Arvanitaki and Chalazonitis, 

 1954; Caldwell, 1958; Spyropoulos, I960).) However, it is impossible to 

 attribute the entire action to H+ ions, because NH3 vapours, producing OH^ 

 ions in the cell, depolarize and act similarly to CO2. Thus, the contribution 

 of this amount of H+ produced by CO2 to the depolarization is not yet known.* 

 The change in concentration of the CO2 required for unit depolarization 

 in the Aplysias cells also remains unknown. But admitting, on the basis of 

 the results of Kolmodin and Skoglund, that 10 mV depolarization in the 

 motoneurons is due to 1 min accumulation of CO2 after the clamping of the 

 vessel, corresponding to a respiratory rate of 1800 fd/g (wet) per hr (Ciiala- 

 zonitis and Otsuka. 1956) we estimated the extra CO2 accumulated per 

 gramme (wet) of cells, during 6 sec clamping, to be 10 '^ m. This amount 

 corresponds to a depolarization of 1 mV. 



DISCUSSION AND CONCLUSIONS 



Although the relation of O2 and CO2 concentration changes to the mem- 

 brane potential changes and the associated electrical activity could not be 

 analysed in a rigorous quantitative manner, it is already possible to con- 

 sider the ratio [02]/[C02] as highly important in determining the excitability 

 and the resting potential of the somatic membrane. By increasing this 

 ratio the resting potential tends to increase, and vice versa. 



* Some pH-effect data are already available from Spyropoulos's (1960) work in the case 

 of the squid giant fibre: the resting potential may decrease by about 5 mV for a - ApH of 

 2 units. If we admit that it is possible to get a depolarization of 1 mV with a pH lowering 

 of 0-5 unit, such an effect would be of high importance in the case of spontaneous active 

 cells such as Aplysia neurons or the chemoreceptors of higher animals. 



