124 The Vertebrate Organ Systems 



First it can be noted that there is a slight pause between the applica- 

 tion of the stimulus and the beginning of the contracting phase. This 

 is the latent period which lasts about 0.01 second in the isolated frog 

 muscle. The period of contraction lasts but 0.04 second, while the 

 relaxation period takes 0.05 second. During this latter stage, the 

 muscle returns to its former condition. Following this contraction, the 

 muscle cannot respond immediately to a new stimulus. This short 

 period which lasts about 0.005 second in the frog is known as the 

 refractory period. 



Smooth muscle does not show this same type of simple muscle 

 twitch. A contraction of smooth muscle which may be brought about 

 by a single stimulus can last as long as 20 seconds and have a latent 

 period of some 3 seconds. 



Cardiac muscle differs in its action from both smooth and skeletal 

 muscle. It has a refractory period of from 0.1 to 0.2 second, thus 

 cannot be maintained in a contracted state by rapid stimulation. 



Effect of Weight on the Muscle Twitch. — If the isolated muscle 

 has a weight placed on it in such a manner that the muscle is somewhat 

 stretched, the effect of this on the muscle contraction can be studied. 

 Immediately it is apparent that the muscle does more "work" when 

 loaded than when it is not. The amount of work done will increase 

 with increasing loads until the weight is too heavy. Then the muscle 

 will be unable to contract at all. 



Tetanus. — The muscle twitch as just described seldom if ever 

 occurs in the living organism. Instead, the normal skeletal muscle 

 action is one of sustained contraction brought about by a rapid series 

 of stimuli from the central nervous system. Experimentally this can 

 be demonstrated by applying a series of electrical stimuli to the isolated 

 muscle. By tracing this reaction on the kymograph drum, it will be 

 seen that a single prolonged contraction results. The height of this 

 contraction is greater than that of the individual twitch. This is referred 

 to as summation of response and the sustained contraction is known 

 as tetanus (Fig. 38). 



Tetanus is thought to be due not to the contraction of all the 

 muscle fibers at any one time, but rather to the asynchronous con- 

 traction of various fibers, that is only 5 to 50 per cent of the fibers are 

 contracting at any one time. 



