190 



N. CHALAZONITIS 



In the same way that one often relates the instantaneous value of the 

 resting potential to the excitability of the membrane, one can relate the latter 

 to the instantaneous values of the ratio [02]/[C02]. 



Among the three functional types described, namely, the A soma of regular 

 frequency, the Br soma autoactive on slow waves, and the arhythmic somata, 

 let us first consider the A soma because of its relative simplicity. 



The behaviour of the A cell is easily predictable if we consider the diagram 

 relating its instantaneous frequency vs. its membrane potential (see Fig. 24). 



100-1 



F 



50- 



0^„^liL_20 :^p 40.,5fi 60 70 mV 



IV ID U I mp 



Fig. 24. Curve relating the instantaneous frequency (F) to the membrane potential 

 imp), for an ideal A type cell. I, II, III, IV, are line segments of the abscissa 

 into which the frequency of the electrical activity may receive predictable values 

 (see the discussion), a, b, c, d, e, possible initial values of the somatic resting 

 potential from which any further shift issued from any change of the ratio 

 lO-il/lCOo], may determine a predictable evolution to the instantaneous 

 frequency of the ceil (see discussion). 



The axis representing the possible values of the membrane potential can 

 be divided into four segments, each segment being related to a distinct elec- 

 trical activity. Segment I corresponds to zero frequency, being the zone of 

 highest polarization. Segments II and III correspond to the firing zones of 

 the cell. The point dividing these two segments corresponds to the maximum 

 frequency (m.F.). Segment IV corresponds to the maximum depolarization 

 of the cell. After the insertion of a microelectrode into the cell the value of 

 its membrane potential may be represented by the points a or /> if the cell 

 is in good physiological condition. 



Thus, if the initial point is situated at a the decrease of the ratio [02]/[C02] 

 will bring it to b and perhaps even further from h to c\ c being the point 

 corresponding to the maximum firing frequency. Such an effect is an excita- 

 tory one. If the ratio [O2]/ [CO2] is small enough to decrease the membrane 

 potential past c to J or even to e, then this effect is an inhibition through 

 excessive depolarization. 



