578 ANNALS NEW YORK ACADEMY OF SCIENCES 



Neural Metabolism 



Finally, I shall recall the gross metabolic picture of neural 

 tissue. ^""^^ For nerve, in contrast to muscle, the heat and respira- 

 tion balance of rest and action were first established. Lactic acid, 

 next studied, seemed to be excluded from any role except as an 

 anaerobic end-product, but it was later shown to serve as an effective 

 substitute fuel when sugar oxidation was interfered with by iodoacetic 

 acid; and anaerobic glycolysis was similarly established as a source of 

 useful energy. Yet oxidations, by oxygen or an oxidizing reserve, re- 

 main of especial importance in nerve, for the long and large delayed 

 heat production cannot be dissociated from the initial heat by anoxia 

 or by any other maneuver tried. What fuel or fuels are oxidized, is 

 largely unknown. At rest, nerve may destroy more carbohydrate than 

 could be fully oxidized, while the R. Q. hovers at 0.8; and CHO utiliza- 

 tion can taper off to zero while O2 consumption is maintained unaltered. 

 Even brain, with a resting R. Q. of 1.0 and a CHO fuel, can shift to 

 another substrate which fully supports respiration. During activity, 

 the much increased oxidative metabolism of nerve is not supported by 

 CHO. The R. Q. of the extra respiration does rise to nearly 1.0 in 

 tetanized nerves, but CHO loss is not increased. Some rise in acid- 

 soluble phosphorus and in ammonia-liberation occurs, suggesting the 

 degradation of phospholipins or phosphoproteins; but the problem is 

 still wide open. The lipo-protein changes in rods on illumination 

 (mentioned by Wald) and the Swedish work^^ on nucleoprotein de- 

 crease in fatigued nerve (to which Schmitt called attention), are ob- 

 servations challenging a resolution of this enduring uncertainty. 



THE NERVE MACHINE 



The resting metabolism of nerve is essential to keeping the tissue 

 functionable. The normal resting potential, for example, falls when 

 respiration is prevented^'' and even more rapidly when glycolysis is 

 also blocked. ^^ When an action is evoked, the cell machinery whirs, 

 physical and chemical changes occur, an impulse is propagated, and, 

 finally, a cycle is completed and the machine fully reset. The events 

 associated with activity are known in moderate detail, and it will be 

 helpful to outline this sequence. Since so much attention has been 

 given by investigators to the early and the electrical phenomena of 

 response, may I emphasize that all the phenomena are closely coupled 

 together. A single impulse, gone by in a millisecond, is yet irrevocably 

 followed by a rise in heat liberation and in oxygen consumption which 



