26 



PHYSIOLOGICAL CHARACTERISTICS OF MUSCLE 



Effect of Two or More Successive Stimuli. If a muscle 

 receives two successive stimuli a sufficient length of time 

 apart, two curves of contraction are produced, the second 

 being a little higher than the first (beneficial effect of con- 

 traction). However, if the second stimulus arrives before 

 the period of relaxation is complete, a secondary rise is pro- 

 duced which is called superposition or summation of effects. 



n 



FIG. 20. 



I, Two successive submaximal contractions. //, A series of contractions in- 

 duced by 12 induction-shocks in a second. ///, Marked tetanus induced by 

 rapid shocks. (Landrois.) 



If the two stimuli occur close enough together, the result will 

 be one curve which is greater than either would have pro- 

 duced separately. This is called summation of stimuli. If, 

 instead of just two stimuli, a number of stimuli are applied 

 very close together, we get the effect shown in (II). If 

 these stimuli occur still closer together the effect shown in 

 (III) is produced which is called tetanus. When the stim- 

 uli occur so as to allow partial relaxation between each 

 stimulus, (II) the effect is called incomplete tetanus, but 

 when no relaxation occurs as in (III) the effect is complete 

 tetanus. 



