STIMULATION OF MUSCLES AND OF NERVES 



429 



In summated contractions the ascending limb of the second contraction curve 

 is steeper than that of the first, hence the summit of the second appears earlier 

 than would be expected if its course were the same as the first (v. Kries). The 

 latent period of the superimposed contraction is also said to be very much 

 shorter than that following the first stimulus (Fick). 



In order that successive stimuli may produce a summation they must not 

 follow one another too rapidly. The smallest interval possible for any given 

 preparation depends upon the temperature and the strength of the stimuli : for 

 the nerves of the frog at ordinary room temperature it may be estimated at 

 about 0.001 to 0.005 second. We have a refractory period therefore in nerves 

 and skeletal muscles just as we have in heart muscle (cf. page 183). 



If more than two stimuli affect the nerve or muscle at sufficiently short 

 intervals the contraction of the muscle becomes still greater, and its curve 

 is perfectly continuous, showing no separate summits (cf. Fig. 166). This 

 form of contraction is called tetanus. 



Complete tetanus appears only when the stimuli follow one another so rap- 

 idly that the interval between them is less than the time occupied by the active 

 shortening of the muscle when that is maximal. The frequency depends there- 



FIG. 166. Tetanus curve of the frog's gastrocnemius, after Bohr. 



second. 



Twenty-seven stimuli per 



fore primarily upon the behavior of the muscle to single stimuli ; the more 

 rapidly a single contraction runs its course, the more frequently must the stimuli 

 be given to produce complete tetanus. This is beautifully shown by the behavior 

 of muscles of warm-blooded animals composed mainly of red or white fibers. 

 The red soleus muscle of the rabbit falls into almost complete tetanus with ten 

 stimuli per second, wjiile the white gastrocnemius medius with the same fre- 

 quency of stimulation gives very evident single contractions. A. frequency of 

 six stimuli per second permits the white muscle to relax almost completely 



