102 



GENERAL CONCEPTS 



La-bcnt 



traction Relajx:ation 



jcriod -V 



Lcnoth of muscle 

 Action potential 



BirefrinOence. 



t 



Heat producecl 

 J I 



Time- (0.01 se-c. intervals) 



Stimulits 



Figure 5.10. Diagram of the changes that occur in a muscle during a single muscle 

 twitch. See text for discussion. 



has fully recovered Irom the previous one, the muscle becomes fatigued 

 and the twitches become feebler and finally cease. The fatigued muscle 

 will regain its ability to contract if allow^ed to rest. 



Muscles do not usually contract in individual twitches but in more 

 sustained contractions evoked by a volley of nerve impulses reaching 

 them in rapid succession. This state of sustained contraction is known as 

 tetanus; the individual motor units are stimulated in rotation. Thus 

 individual muscle fibers contract and relax, but do this in rotation so 

 that the muscle as a whole remains partly contracted. The strength of 

 the contraction depends on the fraction of the muscle fibers which con- 

 tract at any given moment. 



All normal skeletal muscles are in a state of sustained partial con- 

 traction, called tonus, as long as the nerves to the muscle are intact. 

 Tonus, then, is a state of mild tetanus, maintained by a constant flow 

 of nerve impulses to the muscle. 



The problem of how protoplasm can exert a pull is far from settled, 

 but it is now believed that the molecules of actomyosin shorten by fold- 

 ing and thus produce the tension of muscle contraction. The energy for 

 the contraction is derived from the energy-rich phosphate bonds of 

 adenosine triphosphate and phosphocreatine and these are renewed by 

 the energy derived from the glycolysis of glycogen to lactic acid. Tins 



