436 - Multicellular Animals, Especially Man 



(C G H 10 O,)x + xH 2 O 



glycogen water 

 0.9 g 0.1 g 



xC 6 Hin0 6 



glucose 

 1.0 g 



2xC 3 H 6 3 



lactic acid 

 1.0 g 



than normally, when a muscle is deprived of 

 oxygen. 



Prior to 1930, glycolysis was thought to 

 supply energy directly to the contraction 

 process; but about this time several poisons 

 were found that completely inhibit glycoly- 

 sis without blocking the contractions of the 

 muscle. In fact when a muscle is exposed to 

 such glycolysis inhibitors, it behaves quite 

 like one that is deprived of oxygen, except 

 that poisoned muscles fatigue somewhat 

 sooner than those that are asphyxiated. But 

 the I act that a muscle can do considerable 

 work, when no glycolysis is occurring, elimi- 

 nates this reaction as the primary source of 

 energy in contraction. 



After 1930 it was discovered that the first 

 chemical event to occur following excitation 

 in muscle is the breakdown of a series of 

 organic phosphate compounds. Indeed, the 

 most direct source of energy in muscle con- 

 traction appears to be the hydrolysis of ATP 

 (p. 143), a reaction that generates some 8 

 Cal per mole of phosphate liberated. 



Also it is now known that glycolysis pro- 

 vides the energy through which the phos- 

 phate componds of the tissue are resynthe- 

 sized, subsequent to their decomposition. 

 Thus a recharging of the contractile mecha- 

 nism, which follows each contraction, depends 

 upon glycolysis; and the oxidative reactions, 

 which extend throughout the long recovery 



period, are important not only because they 

 conserve the glycogen stores of the muscle, by 

 fostering the resynthesis of glycogen from 

 lactic acid, but also because they prevent 

 lactic acid from accumulating to a toxic level. 



Two main proteins have been extracted 

 from the myofibrils, namely actin and myo- 

 sin. Also it is important to realize that myo- 

 sin itself exerts a powerful catalytic action 

 on the decomposition of adenosine triphos- 

 phate. Therefore, the recent discovery that 

 artificial threads prepared of actin and myo- 

 sin shorten energetically when treated with 

 adenosine triphosphate has aroused great 

 interest. 



The fact that the restoration of the muscle 

 to full efficiency depends upon glycolysis and 

 other anaerobic reactions is of importance, 

 especially for athletes. Frequently our mus- 

 cles are called on for tremendous work, and 

 despite the acceleration of respiration and 

 circulation that accompanies exertion, the 

 supply of oxygen to the muscles cannot keep 

 up with the increased requirements. During 

 such exertions, the muscles are being re- 

 charged largely by glycolysis, and lactic acid 

 accumulates because it is not oxidized as fast 

 as it is formed. But a well-nourished muscle 

 has a good reserve of glycogen, and such a 

 muscle can continue to function until the 

 oxygen debt becomes excessive. After a hun- 

 dred-yard dash, for example, the runner con- 

 tinues to breathe heavily for a number of 

 minutes, and his muscles continue to utilize 

 extra oxygen for almost half an hour. Dur- 

 ing this time the accumulation of lactic acid 

 gradually subsides, as part is oxidized, and 

 the remainder is rebuilt into glycogen. Thus 

 the net result of muscular exertion is that a 

 certain quantity of the glycogen store of the 

 muscles has been consumed. The muscle may 

 oxidize substrates other than lactic acid dur- 

 ing the resynthesis of glycogen, in which case 

 a greater proportion of the lactic acid is re- 

 converted into glycogen. 



Heat Production. The maximal efficiency of 

 a muscle as a machine for delivering me- 

 chanical energy is about 40 percent, which 



