582 ANNALS NEW YORK ACADEMY OF SCIENCES 



Full Restoration 



For completeness, although it is far removed from the direct problem 

 of propagation, I mention, finally, the delayed recovery processes. We 

 often forget that the increased respiration of activity persists a half- 

 hour or more after a brief tetanus of nerve,^^ that the delayed heat 

 production is similarly prolonged,^" and that considerable after-poten- 

 tials may endure for comparable periods.''" And, as late recovery lags 

 progressively further behind in a continuously-driven nerve, its re- 

 sponse capacity falls to a lower equilibrium level. Irritability, velocity, 

 chemical, thermal, and electrical response per impulse, etc., fall during 

 a maintained tetanization.'*^ 



Now, with this outline of nerve action before us, I should like to con- 

 sider the questions around which so much of this symposium has re- 

 volved: (1) How does excitation engage metabolism; (2) what is the 

 roh of the acetylcholine system; and (3) in what respects does junc- 

 tional transmission differ from that in a fiber? 



THE LINKAGE OF ACTION TO METABOLISM 



Given the externally-applied stimulus energy, given even the propa- 

 gated membrane response, the insistent question remains as to how one 

 event induces the next and, especially, how chemical changes are made 

 to follow the physical ones. This was asked by Grundfest, discussed 

 by Green, and exemplified by Ochoa's contribution. It was considered 

 for nerve, in some detail, a decade ago,^'' and is today being clarified in 

 the case of muscle.*^ In muscle, the extra step of mechanical response 

 offers both an additional problem, of how the membrane response leads 

 to the myosin response, and an additional line of attack on the general 

 case. Since metabolic details are far more numerous for muscle than 

 for nerve, I shall choose illustrations freely also from the former 

 material, in confidence that the principles they illustrate are equally 

 valid for both tissues. 



Ion Action 



In the homogeneous liquid phase of a heterogeneous system like tis- 

 sue, electric currents, applied as external stimuli or generated in the 

 course of the active response, mean ion movements and only ion move- 

 ments. Where these ion streams encounter interfaces — membranes, 

 micelles, molecular palisades, etc. — ions can accumulate or decrease. 

 A local change in ion concentration at molecular or structural surfaces 

 of a cell must be the initial consequence of an electric stimulus and ap- 



