MUSCLE 



227 



finally the most powerful stimuli produce no effect. Cooling 

 has thus practically the same effect on contraction as con- 

 tinued exercise (fig. Ill) {Practical Physiology). 



(3) Many drugs modify muscular contractions, e.g. veratrin 

 enormously prolongs the relaxation period. 



4. Strength of Stimulus- — A stimulus must have a certain 

 intensity to cause a contraction. The precise strength of this 

 minimum effective stimulus depends upon the condition of the 

 muscle. The application of stronger and stronger stimuli to 

 the nerve causes the muscular contraction to become more 

 and more rapid, more and more complete, and more and 

 more powerful, by involving more and more fibres. Each 



fibre when stimulated undergoes the 



all or nothing " change 



— Con. 



A 8 



Fig. 112. — Influence of increasing the Strength of the Stimulus upon the con- 

 traction of Skeletal Muscle. St., the stimulus; Con., the resulting 

 contraction. A, a subminimal stimulus ; B, the minimum adequate 

 stimulus ; C, the optimum stimulus ; , stimuli ; - - -, contractions. 



already described (p. 68). But increase in the contraction is 

 not proportionate to the increase in the stimulus. If the 

 stimidus is steadily increased, the increase in contraction 

 becomes less and less. This may be represented diagram- 

 matically in the accompanying figure, where the continuous 

 lines represent the strength of the stimuli and the dotted 

 lines the extent of the contractions (fig. 112). 



After a certain strength of stimulus has been reached, 

 further increase of the stimulus does not cause any increase 

 in the muscular contraction, since all the fibres have been 

 made to contract. This smallest stimulus which causes the 

 maximum muscular contraction may be called the optimum 

 stimulus. 



Increasing the strength of the stimulus shortens the 



