The Muscular System 121 



through another series of compounds which are present in muscle 

 tissues. Chief of these is glycogen. Glycogen passes through a series 

 of intermediate stages to break down to lactic acid along with a release 

 of energy. This energy is then made available for the resynthesis of 

 the organic phosphates such as the phosphocreatine and ATP. This 

 reaction still is an anaerobic one, that is, oxygen is not used. It is in 

 the synthesis of glycogen from lactic acid that oxygen is used ; thus this 

 latter is an aerobic reaction. Four-fifths of the lactic acid produced is 

 reconverted to glycogen by the energy released from the oxidation 

 of the remaining one-fifth. Again this is best illustrated by a series 

 of abbreviated equations. 



Glycogen ^ intermediate stages ^ lactic acid + energy (an 



anaerobic reaction). 



1/5 of lactic acid + 0. > CO. + H.O + Energy 



This complex series of reactions is actually made possible through 

 an equally complex series of enzymes. For our present discussion, how- 

 ever, these abbreviated summaries sufifice. 



As the glycogen is gradually used up through the oxidation of the 

 lactic acid, it is renewed from the sugars brought in by the circulatory 

 system. 



The actual contraction of the muscle cells seems to be brought 

 about by the unusual properties of the material known as actomyosin. 

 Actomyosin is a complex formed of two colloidal materials, actin and 

 myosin. Myosin is a protein composed of rods, while actin, also a pro- 

 tein, forms long continuous threads. The union of these two proteins 

 results in the actin thread having rodlets of myosin attached to it. 

 Each protein when alone shows no ability to contract, but together, 

 they are able to utilize the energy available in the organic phosphates, 

 and thus contract. 



During strenuous muscular activity, it is impossible for the cir- 

 culator}' system to supply enough oxygen to the muscles to allow for 

 the resynthesis of the various compounds. It is possible, however, for 

 these reactions to continue for some time without the resynthesis. In 

 this manner, an oxygen debt, is acquired by the muscle. This is evi- 

 denced by the accumulation of lactic acid in the muscle tissues. Once 

 exercise is stopped, the rapid respirations continue for some time until 

 this debt is repaid. An oxygen debt is accumulated more rapidly at 

 the beginning of exercise than later. This is due to the fact that the 

 circulatory and respiratory systems have not immediately adjusted to 



