76 PHYSIOLOGY OF MUSCLE AND NERVE 



CHAPTER VII 



THE FACTORS VARYING THE CHARACTER OF THE 

 CONTRACTION 



The Strength of the Stimulus. — In general it may be stated that 

 the height of the contraction is proportional to the strength of the 

 stimulus. A very convenient way of illustrating this rule is to permit 

 a muscle to record its contractions upon a stationary drum while being 

 stimulated with single make or break induction shocks. By varying 

 the distance between the secondary and primary coils of the induc- 

 torium the strength of these stimuli may be accurately graded. If 

 this experiment is begun with the coils far apart, no contractions are 

 obtained at first, although it may be surmised that the different 

 stimuli then give rise to certain chemico-physical alterations in the 



(f i f , ( ^ 1 



B 



I i 3 t S- 6 7 8 9 to II 12. JJ I<h 15 



Fig. 42. — Successive Make and Break Contractions. 

 The strength of the current is gradually diminished by more widely separating the 

 secondary from the primary coil. The figures indicate this separation in centimeters 

 of distance. M, threshold of make; B, threshold of break. 



muscle which, however, are still too weak to produce visible mechanical 

 energy. These stimuli are said to be subminimal in character. If a 

 number of these subminimal stimuli are passed into the muscle in 

 quick succession, they eventually give rise to a contraction. This 

 phenomenon is known as summation of subminimal stimuli. 



If the strength of the current is now gradually increased by bringing 

 the coils closer together, a point will finally be reached when the 

 muscle gives a just barely perceptible reaction. This is the threshold 

 contraction. Moreover, since the break induction shock constitutes 

 a stronger stimulus than the make shock (page 63), the first contraction 

 must appear when the current is interrupted. If the strength of the 

 current is increased still further, these break contractions gradually 

 increase in height and become associated with the first make contrac- 

 tion. Additional increases in the strength of the current lead to the 

 production of the highest possible contractions, beyond which point 

 their height generally decreases somewhat. Beginning with the thresh- 



