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THE VERTEBRATE BODY 



muscle physiology. The calf muscle together with its connecting nerve 

 may be dissected out to study the muscle nerve reaction. When the 

 nerve is stimulated with a weak electric current the muscle will contract ; 

 as the strength of the current is increased the contraction becomes more 

 vigorous due to the stimulation of greater numbers of the motor units. 

 When all of them are being stimulated, however, the response cannot 

 become greater even though the current is greatly increased. If the 

 stimulation is repeated over and over the response will diminish and 

 finally cease altogether. It might be assumed that this cessation is due 

 to muscle fatigue, but if the wires are applied directly to the muscle, it 

 will contract. We also know that a nerve fiber does not easily become 

 fatigued. We may conclude, therefore, that it must be the connections 

 between the end of the nerve and the muscle, the neuromuscular junc- 



Fig. 22.9. Histological structure of muscle. At the left, striated muscle fibers from 



the skeletal muscle; in the center, smooth muscle cells from the visceral muscle; at 



the right, cardiac muscle fibers from the heart. 



tion, which becomes fatigued first and the muscle fibers fail to receive 

 the stimulus transmitted by the nerve. If the muscle fibers are stimu- 

 lated directly with the electric current, the amount of their contractions 

 will eventually become reduced due to the accumulation of waste prod- 

 ucts, chiefly lactic acid. When the concentration of lactic acid reaches 

 0.5 per cent, contractions cease entirely. 



If a single muscle fiber is stimulated with a very weak current, it 

 will not respond. When the current is increased until it is just strong 

 enough to set a reaction, the muscle fiber will contract to its maximum 

 capacity. A much stronger current will not increase the amount of con- 

 traction. This is known as the all or none law. The reaction is either 

 all the way or none at all. It is a law which applies to many phases of 

 physiology. It can be compared to the shooting of a rifle — if the trigger 

 does not hit the cartridge hard enough it will not explode, but when suf- 

 ficient force is applied to explode the cartridge it will throw the bullet 

 just as far as if a hundred times as much force were applied in pulling 



