508 



COLLEGE ZOOLOGY 



tic acid is formed, and changes occur in 

 certain organic phosphates which are pres- 

 ent. There is much evidence that the energy 

 used in the shortening of a muscle comes 

 from the breakdown of adenosine triphos- 

 phate (ATP). It appears that all animal 

 movements such as those of the arms, legs, 

 wings, and fins are traceable to ATP. In the 

 recovery process, glycogen is broken down 

 into lactic acid; some of this acid is oxidized 

 to carbon dioxide and water, and the re- 

 mainder is reconverted to glycogen. There 

 are other intermediate reactions involved. 



A summary of some of the better-known 

 chemical reactions in muscle contraction, in 

 the sequence in which they occur, may be 

 expressed thus: 



1. Adenosine triphosphate (ATP) -* adeno- 

 sine diphosphate (ADP) + phosphate -j- 

 energy for muscle contraction. 



2. Glycogen -^ complex intermediate stages -^ 

 lactic acid + energy for resynthesis of ATP. 



3. About y^ of the lactic acid + oxygen ->• 

 carbon dioxide + water + energy, for resyn- 

 thesis of the remaining % of the lactic acid 

 to glycogen. 



Some of the reactions in the summary are 

 reversible, and an unknown number of 

 enzymes are involved in the chemical reac- 

 tions. 



Oxygen is not used in the actual shorten- 

 ing of a muscle; a muscle can contract for a 

 time without oxygen, for it is required only 

 in the chemical reactions associated with 



recovery. In strenuous muscular exercise, 



such as in a 100-yard dash, lactic acid ac- 

 cumulates because oxygen cannot be sup- 

 plied as rapidly as it is used. This results in 

 what is called an oxygen debt, which is re- 

 paid bv rapid deep breathing for some time 

 after the race. 



True fatigue is caused, in part at least, by 

 the effects of waste products (carbon diox- 

 ide, lactic acid, acid phosphate) which ac- 

 cumulate during exercise. The loss of nutri- 

 tive substances may also be a factor in 

 fatigue, but the accumulation of wastes is 

 probably of major importance. 



Rest is necessary after violent exercise to 

 enable the blood to carry the metabolic 

 waste substances to the excretor)' organs and 

 nutritive materials to the muscles. Exercise 

 stimulates circulation and brings about an 

 increase in size, strength, and tone of the 

 muscles. 



SELECTED COLLATERAL 

 READINGS 



Goss, G.M. (ed.). Grafs AnatoTny, Lea & 

 Febiger, Philadelphia, 1954. 



Gray, }. How Animals Move. Cambridge Univ. 

 Press, London, 1953. 



Heilbrunn, L.V. An Outline of General Physi- 

 ology. Saunders, Philadelphia, 1952. 



Stiles, Kad A. Handbook of Histology. Mc- 

 Graw-Hill, New York, 1956. 



Toldt, Cad. An Atlas of Human Anatomy. 

 Macmillan, New York, 1928. 



