908 CENTRAL NERVOUS SYSTEM 



of any of the others. This may be shown by applying an electric stimu- 

 lus to a single fiber, by means of the very delicate technic devised by 

 Pratt, 23 when it will be observed that this fiber alone responds to the 

 excitation (Fig. 224). The fibers of the muscle are insulated from one 

 another in such a way that the disturbance set up by the stimulus in 

 one of them does not spread to any other. In this respect skeletal mus- 

 cle differs markedly from cardiac muscle (see page 177). The unit of 

 function in skeletal muscle is consequently the twitch of the individual 

 fiber. 



The All-or-none Law. The question obviously arises how the strength 

 of contraction of the fiber is affected by the strength of the stimulus ap- 

 plied to it. As soon as it was realized that a single nerve impulse did 

 not vary in strength, except under conditions which affected the con- 

 ducting power of the nerve fiber, it was difficult to imagine how the 

 nerve impulse could be made to alter the degree of contraction of the 

 muscle fiber which it excited. It was consequently suspected that the 

 all-or-none law applied to the activity of the individual fiber of skeletal 

 muscle just as it does to heart muscle as a whole. The final direct proof 

 of this view is supplied by the experiments of Pratt and Eisenberger 

 who showed that when a single muscle fiber is excited its response is 

 maximal if it responds at all. This fact is shown in Fig. 225, in which 

 the movement of a droplet of mercury placed on the contracting fiber 

 has been photographed. On increasing the strength of the stimulus 

 no change occurs in the amount of contraction until the current strength 

 becomes strong enough to affect an adjoining fiber. At this point the 

 amount of movement increases by a definite step, and then continues 

 at the new level until a third fiber is brought into action and another 

 step-like rise in the record occurs. As the strength of stimulus is de- 

 creased again the contractions fall off through the same series of steps. 

 It is consequently believed that if a skeletal muscle fiber contracts at 

 all, it does so to the full extent to which it is capable. Graded series of 

 muscular contractions in response to graded strengths of stimuli, such 

 as are shown in Fig. 45, are due to the fact that as the stimulus is in- 

 creased in strength more and more fibers, each contracting maximally, 

 are brought into play until finally all are excited and the contraction 

 of the muscle as a whole becomes maximal and cannot be further in- 

 creased. The adjustment of the strength and degree of muscular move- 

 ment consequently depends on bringing into action the proper num- 

 ber of muscle fibers. If the fibers were not insulated from one another, 

 so that one could contract without the others joining in, graded muscular 

 movements would be impossible and our skeletal muscles as a whole 

 would act in an all or none way just as the heart does. 



Although the skeletal muscle fiber contracts to the utmost if it con- 



