386 ANNALS NEW YORK ACADEMY OF SCIENCES 



tive cation permeability is abolished. Correspondingly, it has been 

 assumed that, during excitation, the potential fall is as great as is the 

 resting potential, measured at best with the impaled nerve or muscle. 

 But this is not true. Impedance measurements have shown the resist- 

 ance to persist, to some extent, during excitation (Curtis and Cole) . In 

 other words, the resting potential could be expected to be larger than 

 the action potential. However, the contrary is true. Hodgkin and 

 Huxley,- and Curtis and Cole," inserting a microelectrode into the axon, 

 detected the potential change, during activity, to be even larger than 

 that due to injury. For example, in the experiments of Curtis and 

 Cole, the resting potential average is 51 mV, the action potential 

 108 mV. 



Before discussing this interesting situation, attention will be turned 

 briefly to a special problem. The word, breakdown, suggests leakage, 

 and for this reason, activity could be expected to be accompanied by 

 leakage, especially from the large surplus of well-diffusible potassium 

 .normally retained in the axoplasm. However, such an escape from 

 frog nerve, though often investigated, is doubtful, except following 

 very prolonged stimulation (for example, 60 stimuli per second, for 

 1-3 hours, in the experiments of Arnett and Wilde, with Fenn).^^ 

 However, this may be accounted for, by assuming that only a very 

 small area of the surface of a myelinated nerve, the Ranvier nodes, 

 is available for diffusion. This can be correlated with the experiments 

 of Cole and Curtis, ^^ regarding impedance and membrane capacity 

 of the squid nerve. Notwithstanding the fact that, during excitation, 

 the resistance of the squid nerve falls off from 1000 ohm/square-cm. 

 to only 25 ohm/square-cm., not more than 2% of the area is involved 

 in the increase of permeability. This means that the remainder, 

 about 98%, would be inactive. Another point is the fact that the 

 state of excitation, in general, lasts only a very short time, measured 

 in milliseconds. Very slowly reacting cells, therefore, may offer a 

 greater chance to detect an ion escape. As a matter of fact, the con- 

 ductivity of the water on the outside of the surface of a Nitella cell 

 rises perceptibly, after several excitation waves have passed the slowly 

 responding object, the excitation time being measured in tenths of a 

 second (Cole and Curtis). Since depolarization is followed by re- 

 polarization, the question arises, whether and how the ions which 

 escape through the leaky membrane are recovered. It becomes in- 

 creasingly clear that, in one way or the other, energy is utilized for 

 this purpose. In other words, the physiological membranes are more 

 than labile structures. Rather, they are, or can be, acting machineries. 



