rHFMOPOTFNTIALS IN GIANT NERVE CELLS 191 



Conversely, the increase of the ratio [02]/[C02] will cause a hyperpolari- 

 zation and if initially the cell was at c or l\ its spike frequency will tend to 

 decrease (inhibitory effect). But. if the cell's membrane potential is at e or c/ 

 the spike frequency will increase (excitatory eflfect on initially depolarized cells). 



Prediction of the effects of the [O2]/ [CO2] ratio changes on the activity 

 of the B cell is more difficult than the prediction of these effects on the A 

 cell: no i.p.s.p.'s are impinging on the latter during CO2 action. However, the 

 firing of the B cell is periodically inhibited, obviously by the firing of a 

 second cell, inhibitory, which controls its activity. So the behaviour of the B 

 type cell depends not only on the initial level and further displacement of its 

 membrane potential into the firing zone, but also on that of the monitoring 

 inhibitory interneuron being itself under the effect of the [02]/[C02] changes. 



As regards the behaviour of the Br cell, more work is necessary in order 

 to relate it to the [02]/[C02] ratio changes, because the depolarization re- 

 quired for the transformation of the slow waves into a continuous activity is 

 higher when CO2 is used than when anoxia is used. 



Let us now examine the factors which miglit control the ratio [O2]/ [CO2] 

 in the somatic membrane. These factors may be exogenous or endogenous. 

 Exogenous factors are, for instance, the composition of the blood (concentra- 

 tions of O2 and CO2) and the diameter, at a given instant of the capillaries 

 surrounding the soma. Endogenous factors are for instance, the densely accu- 

 mulated mitochondria near the synaptic portions of the somatic membrane. 

 Arvanitaki and Chalazonitis (1949. p. 549) and more recently Palay (1958). 

 emphasized the possible chemical control of mitochondrial products on the 

 synaptic membrane.* Among such products. CO2 and other small meta- 

 bolites, because of their high diffusibility and because of the relative small 

 concentration changes required for a significant shift of the membrane poten- 

 tial, are the first to be considered. 



As a general conclusion it is certain that chemosensitivity, recognized in 

 higher animals as a specific property of certain nerve cells (chemoreceptors) is 

 also a general property of any soma. Furthermore, results to be published 

 concerning the higher chemosensitivity of the soma relative to the axon 

 hillock area and to its own axon will give further support to this conclusion. 



REFERENCES 



Arvanitaki, A. and Chalazonitis, N. (1949) Prototypes d'interactions neuroniques et 



transmissions synaptiques. Donnees bioelectriques de preparations cellulaires. Arch. 



sci. physiol. 3 : 547-566. 

 Arvanitaki, A. and Chalazonitis, N. (1952) Repartition de qiielques catalyseurs res- 



piratoires dans Fespace cellulaire des neurones geants (Aplysia et Torpedo). Arch .sci 



physiol. 6 : 213-232. 

 * DeRobertis (1958) and co-workers have extensively studied the presence of the synaptic 

 vesicles and their changes. Unfortunately the biochemical function of these particles is 

 not yet investigated. Relative to some other possible cytochemical specificities on synaptic 

 terminals, see also Couteaux (1958). 



14 



