120 The Vertebrate Organ Systems 



lips, the dilator naris dilates the nostrils, and the levator scapulae raises 

 the scapula. A region in which a muscle is found also may be the 

 source of the name : the temporalis is found in the region of the temple. 



THE PHYSIOLOGY OF MUSCLE ACTIVITY 



When a movement is brought about by the contraction of a muscle, 

 the most obvious change occurring is a physical change in the position 

 of the part involved. To bring about this physical change in position 

 of a part or structure, certain changes of a chemical, thermal, physical, 

 and electrical nature occur in the muscle itself. 



Chemical Changes. — The transfer of chemical energy into me- 

 chanical energy is an extremely complicated process involving a large 

 number of dififerent chemical reactions. The energy which initiates 

 this series of reactions in muscle tissue is apparently derived from the 

 breaking up of organic phosphates. The most important of these ap- 

 pears to be the compound known as ATP, adenosine triphosphate. The 

 energy of this compound is apparently used directly for the muscular 

 contraction. 



The initial release of the energy of the organic phosphates is pre- 

 sumably due to the presence of calcium ions in the immediate neighbor- 

 hood of ATP. Another compound, creatine, is also found in verte- 

 brate muscle. This material when combined with organic phosphate 

 becomes phosphocreatine. This compound also is rich in easily avail- 

 able energy. Following the ATP breakdown to ADP, adenosine diphos- 

 phate, phosphocreatine breaks down to produce creatine and phosphoric 

 acid. This latter reaction releases even more energy than did the first. 

 These two reactions can be summarized as follows : 



Creatine + Adenosine triphosphate (ATP) > Adenosine di- 

 phosphate (ADP) -|- phosphocreatine -f- energy 

 Phosphocreatine ^ Phosphoric acid -\- creatine -f energy 



As can be seen, the energy used for muscle contraction is released 

 without any need for the presence of oxygen. The energy is available 

 from the organic phosphate. This condition contrasts with the more 

 familiar reactions in which energy is released through the oxidation 

 of various compounds such as the simple sugars. 



Ultimately, however, the energy for muscular contraction is from 

 a reaction similar to that of the oxidation of sugars. This occurs 



