Chap. 10 MOVEMENT MUSCLES 163 



traction and relaxation phase and recovery, all together termed a muscle 

 twitch. This is the reply to any one of the stimuli which come into muscle cells 

 in rapid succession during muscular action. 



Contraction is accompanied by the explosive breakdown of an unstable 

 organic compound, phosphocreatine, into phosphate and creatine. The separa- 

 tion of the creatine and phosphate liberates the energy taking part in the con- 

 traction, plus some energy in heat. 



Relaxation is also associated with a series of chemical changes. Through 

 the action of an enzyme in the muscle, glycogen breaks down, ultimately into 

 lactic acid. This energy takes part in reuniting phosphate and creatine into 

 their previous state as the unstable organic compound, phosphocreatine. The 

 muscle is then ready for another breakdown at the next contraction. The fore- 

 going series of changes does not require oxygen and constitutes the nonoxida- 

 tive or anaerobic phase of muscle action. 



Anaerobic respiration of mammalian muscle is a chain of chemical reac- 

 tions during which the muscle uses glucose, which it derives from its store of 

 glycogen. These anaerobic reactions release the energy used by the muscle in 

 doing work. The process results in the by-product of lactic acid. Part of this is 

 eliminated by oxidation and the energy thus released rebuilds the remainder 

 of it into glucose and glycogen. The muscle is then ready to do more work. It 

 loses some of its store of glycogen with each contraction because the lactic 

 acid that is burned in oxidation turns into carbon dioxide and water which are 

 eliminated. Strenuous exercise may run up a debt of several quarts of oxygen. 

 Lactic acid accumulates, diffuses into the blood, and makes it acid and this 

 acidity is a demand for oxygen. Its increase in the blood, modified by the 

 buffering salts, stimulates the respiratory center of the brain which sends out 

 impulses that lead to vigorous breathing. Forced breathing continues until 

 enough oxygen has accumulated to burn the lactic acid and reinstate the glu- 

 cose. The blood is no longer unusually acid and ceases to stimulate forced 

 breathing. 



Recovery occurs following the changes in the relaxation phase of the mus- 

 cle. One-fifth of the lactic acid previously produced in the anaerobic phase is 

 now oxidized, and water, carbon dioxide, and energy are released. Of the 

 energy thus freed part is heat and part becomes active in the resynthesis of the 

 remaining four-fifths of the lactic acid in glycogen. These changes constitute 

 the oxidative, the recovery or aerobic phase of the muscle action. 



At first, it may seem as if there would be an advantage if oxygen came into 

 the chemical changes earlier. Muscular action however actually starts more 

 quickly because it does not. The blood is constantly bringing oxygen to the 

 muscles, but they collect no supply above their momentary use. There is no 

 extra oxygen to spend, on a sudden action like snatching away one's hand 

 when it touches a nettle or a hot iron. Although no supply of oxygen is ready. 



