SUPPORT AND MOVEMENT 



389 



the stimulus is sufficient to initiate a re- 

 sponse. No matter how much the stimulus 

 is increased, the resulting contraction re- 

 mains the same. This fact has led to the 

 establishment of the so-called "all or none" 

 principle, which means simply that if the 

 heart muscle contracts at all, it will do so 

 to its greatest extent. The question arises as 

 to whether or not this applies to striated 

 muscle. Obviously such muscles contract in 

 graded amounts because one can contract 

 any of his muscles as much or as little as he 

 likes. Here the principle does not apply to 

 whole muscle, but to individual fibers or 

 to motor units ( about 100 fibers ) . Although 

 there still seems to be some question about 

 it, the available evidence points to the 

 fact that motor units do obey the "all or 

 none" principle. Hence, the force with 

 which a muscle contracts depends on how 

 many motor units are stimulated. A mild 

 contraction would result when only a very 

 few were stimulated; a maximal contrac- 

 tion, when all of the units received a 

 stimulus. 



Just how muscles contract is still an un- 

 solved mystery, although a great deal is 

 known about the chemical and physical 

 changes that take place. The movement of 

 a human body does not differ from the 

 movement of a car alono; a street with 

 respect to the basic requirements. Both 

 require energy to accomplish the feat and 

 that energy comes from oxidation, a process 

 with which we are already familiar. Muscles 

 require oxygen indirectly in burning a 

 series of energy-rich organic compounds. 

 It was once thought to be a rather simple 

 process, because when the leg muscle of 

 a frog was stimulated continuously lac- 

 tic acid accumulated, which subsequently 

 burned to carbon dioxide and water. Since 

 glycogen simultaneously disappeared from 

 the muscle, it was considered to be the 

 source of energy. Someone, not satisfied 

 with this simple answer, discovered that 

 after stopping the formation of lactic acid 

 from glycogen (using the specific poison, 



iodoacetic acid), the muscle continued to 

 contract. It was also found that the muscle, 

 if denied oxygen, would contract with just 

 as much force as in the presence of an 

 abundance of the gas. 



From where, then, did the energy come? 

 Since there was no glycogen breakdown, 

 there could be no lactic acid to burn to COo 

 and HoO. This meant, of course, that hid- 

 den in the muscle were some other sub- 

 stances that released energy in a manner 

 resembling that of oxidation. A diligent 

 search revealed the presence of an organic 

 phosphate, adenosine triphosphate (ATP 

 for short), which is formed through oxida- 

 tion and which changes suddenly to phos- 

 phoric acid and another compound with 

 the release of large quantities of energy. 

 This is done anaerobically, that is, without 

 oxygen. Located in tlie muscle fibrils is an- 

 other substance called myosin, which is 

 known to consist of long protein molecules, 

 and it is thought that the actual shorten- 

 ing of the fiber is due to a folding or con- 

 traction of these myosin molecules. The 

 energy for such an action is obtained from 

 the adenosine triphosphate breakdown. 

 There seems, then, to be a series of reac- 

 tions, a chain reaction, that makes the con- 

 traction of a muscle possible. The sub- 

 stances involved have been enumerated but 

 perhaps their roles may be made clearer if 

 we put their reactions in the form of equa- 

 tions, similar to those used in expressing 

 chemical reactions: 



Contraction Phase 



Adenosine triphosphate -^ phosphoric acid 

 + adenosine diphosphate + energy (A) 



Relaxation or Recovery Phase 



Glycogen -^ lactic acid (20%) -^ CO2 + 



H2O + energy (B) 

 (B) energy + phosphoric acid -f adenosine 



diphosphate -^ adenosine triphosphate 

 (B) energy + lactic acid (80%) -> glycogen 

 (B) energy -^ heat (body heat) 



From this it is seen that during contrac- 

 tion the adenosine triphosphate breaks 



