GERARD: NERVE METABOLISM AND FUNCTION 581 



earboxyl groups form a negatively charged layer in the aqueous phase, 

 this could shift the outer membrane surface charge from positive to 

 negative, but could hardly shift the inner surface charge from nega- 

 tive to positive.) 



The well-known high temperature coefficients of excitation also speak, 

 though admittedly in an uncertain voice, for chemical components in 

 the process. If elongated molecules in a loose palisade in the mem- 

 brane are merely bent about, during stimulation, then they must make 

 quite a sudden fall when a threshold is reached, and must also start a 

 vigorous series of changes. For, whether the main chemical reactions 

 of metabolism accelerate during, or only after, the explosive membrane 

 response, they are locked to it in an essentially invariable sequence. 

 And, finally, the important and complex impedance, and potential, 

 variations which accompany or follow the spike surely indicate proc- 

 esses beyond simple ion movements or dielectric strains. The action 

 potential spike represents more than a passive depolarization of a pre- 

 viously charged membrane. It is an active physico-chemical process, 

 still unknown in its details. 



Local Currents 



Whatever the events in an activated membrane region, there remains 

 no doubt as to the mechanism of projjagation along a nerve or muscle 

 fiber. Voltage differences between active and not-yet-active areas 

 must lead to current flow between them and to catelectronic depolariza- 

 tion of the latter. That such currents are a sufficient mechanism for 

 propagation is certain from the experiments in which the nerve im- 

 pulse is made to jump a block one or two millimeters Iqng.^^- ^'^' ^^ Even 

 normally, propagation is probably by similar saltations from node to 

 node, in medullated fibers.'*' '^^' ^® 



Immediate Recovery 



During the absolute refractory period, often under a millisecond, the 

 membrane must at least recover toward its normal potential, impedance, 

 and other properties, so that it is again activable. While the anodal 

 action of the eddy currents sweeping on ahead may contribute to this 

 restoration, this is obviously insufficient. Energy has been dissipated 

 and must be made good from sources beyond the currents which help 

 dissipate it. There can be no reasonable doubt that the complex of 

 initial and immediately-subsequent recovery, with the reversing thresh- 

 olds and potentials already well known,'' is dependent on one or more 

 of the energj^-yielding metabolic reactions; perhaps on ATP breakdown. 



