Responses of Higher Animals: The Effectors - 435 



Fig. 24-5. Fatigue. Many successive twitches of an 

 excised muscle, stimulated about 30 times per minute, 

 are more or less superimposed. The earlier contrac- 

 tions reach higher levels, and the relaxations occur 

 more quickly. Finally the capacity to contract is abol- 

 ished completely. (From Gerard, The Body Functions. 

 Permission of John Wiley and Sons, Inc.) 



series of action potentials can be detected in 

 the muscle, so long as the tetanus continues. 



Gradation of Contractions. If a single fiber 

 (Fig. 24-6) is isolated from the many that 

 compose even the smallest whole muscle, 

 the fiber contracts with maximal force each 

 time it is excited successfully, regardless of 

 whether the exciting stimulus is relatively 

 strong or weak. This all-or-none law holds 

 true for the single fiber, but it does not 

 apply to the muscle as a whole. The intact 

 muscle gives graded contractions according 

 to the strength of the stimulus applied. 

 Weaker stimuli do not arouse all the fibers 

 of the muscle; and a maximal contraction, in 

 the case of either a tetanus or a twitch, is ob- 

 tained only when the stimulus is strong 

 enough to bring all the fibers of the whole 

 muscle into play. 



Chemical Aspects of the Contractile Process. 

 Even when completely deprived of oxygen, 

 a muscle can perform many twitches and can 

 sustain a fairly long tetanus. But without 

 oxygen, fatigue sets in prematurely; or to 

 state the matter more precisely, under aerobic 

 conditions a muscle can do about four times 

 more work than under anaerobic conditions. 



Chemical analysis of a totally fatigued mus- 

 cle reveals a number of changes in the tissue. 

 The glycogen of the tissue, which in a rested 

 muscle amounts to some 3 percent by weight, 

 has declined considerably, and the quantity 



Myofibrils (t-2.« 

 ^ESKjJ^Mycle't of niuAclt fvi.^i* 



1 S .:sfrf!lf£& — 



_Musc!e fiber crushed 



Showing 

 -SsrcoUmma ( IaO 



sC-rmaetive tissue « lb 



, perl-myali 



Blum end of fiber 



Fig. 24-6. Group of fibers of skeletal muscle. For- 

 merly it was thought that the striations of the fibers 

 were due to markings on the myofibrils (see p. 438). 

 (Courtesy of Department of Art as Applied to Medi- 

 cine, Johns Hopkins Medical School.) 



of inorganic phosphate compounds has un- 

 dergone considerable increase. Under anaer- 

 obic conditions, there is also an accumula- 

 tion of lactic acid (C 8 H 6 3 ), in an amount 

 equivalent to the missing glycogen. 



One reaction known to occur while a mus- 

 cle is working is the breakdown of glycogen 

 into lactic acid. This process of glycolysis 

 does not consume oxygen, as may be seen in 

 the equation at the top of the next page. 



Under aerobic conditions the muscle tissue 

 oxidizes part (about V5) of the lactic acid as 

 it is formed, and thus the muscle obtains 

 energy whereby the other part (about %) of 

 the lactic acid is reconverted into glycogen. 

 This reconversion accounts for the fact that 

 lactic acid does not accumulate while a mus- 

 cle has ample oxygen; and also it explains 

 why glycogen is used up four times faster 



