a o c 



/N 



A 



A /N/^ 



8. Successive Stimuli. So far, we have considered the 

 influence of a single stimulus on the shape of muscle. But 

 in nearly every muscular action the contraction of the muscles 

 must last much longer than ^th of a second. 



How is this continued contraction of muscles produced ? 

 To understand this it is necessary to study the influence of a 

 series of stimuli on muscle. 



If, to a frog's muscle which takes ^th of a second to contract 

 and relax, stimuli at the rate of five per second are applied, it 

 is found that a series of simple contractions, each with an 

 interval of j l th of a second between 

 them, is produced (fig. 27, 1). If the 

 stimuli follow one another at the rate 

 of ten per second, a series of simple 

 contractions is still produced, but now 

 with no interval between them. 



If stimuli be sent more rapidly to 

 the muscle, say at the rate of twelve 

 per second, the second stimulus will 

 cause a contraction before the contrac- 

 tion due to the first stimulus has 

 entirely passed off (fig 27, ^). The 



second contraction will thus be super- 



FIG. 27.-Effect of a series of i mposec l on the fi rst and i t i s f oun d 



, , , , 

 iiw.ii ^ ne secon d contraction is more 



complete than the first, and the third 

 than the second. But while the second contraction is markedly 

 greater than the first, the third is not so markedly greater than 

 the second, and each succeeding stimulus causes a less and 

 less increase in the degree of contraction until, after a certain 

 number, no further increase takes place, and the degree of con- 

 traction is simply maintained. 



When the contractions follow one another at such a rate that 

 the relaxation period of the first contraction has begun, but is 

 not completed, before the second contraction takes place, a lever 

 attached to the muscle, and made to write on a moving surface, 

 produces a toothed line. The contraction is not uniform, but 

 is made up of alternate shortenings and lengthenings of the 

 muscle. This constitutes " incomplete tetanus." 



If the second stimulus follows the first so rapidly that the 



Stimuli on Skeletal Muscle. 

 (See text ) 



