204 



PHYSIOLOGY 



PROPAGATION OF CONTRACTION. THE CONTRACTION WAVE 

 The whole muscle does not as a rule contract simultaneously. When 

 excited from its nerve the contraction begins at the end-plates and spreads 

 in both directions through the muscle. The rate of propagation of the con- 

 traction wave can only be measured by employing a curarised muscle, so as 

 to avoid the wide spreading of the excitatory change by means of the intra- 

 muscular nerve-endings. For this purpose a curarised sartorius muscle 

 is taken, stimulated at one end, and the thickening of the muscle recorded 

 by means of two levers placed, one near the exciting electrodes and the second 

 at the other end of the muscle, as shown in the diagram (Fig. 64). The 



FIG. 64. Diagram of arrangement for recording the contraction wave in a 

 curarised sartorius. 



difference between the latent periods of the two curves represents the time 

 taken by the contraction wave in travelling from a to 6. By measurements 

 carried out in this way it is found that the rate of propagation of the con- 

 traction in frog's muscle is 3 to 4 metres per second ; in the muscle of 

 warm-blooded animals it may amount to 6 metres. 



The actual duration of the shortening at any given point is necessarily 

 smaller than that of the whole muscle, and amounts in frog's muscle to only 

 '05-0 - 09 sec., about half the duration of the contraction of a whole muscle 

 of moderate length. The length of the wave is obtained by multiplying the 

 rate of transmission by the duration of the wave at any one point. It varies 

 therefore in frog's muscle between 3000 X -05 (= 150) and 4000 X -09 

 (= 360) millimetres. Thus the muscle fibres in the frog are much too short 

 to accommodate the whole length of the wave, and the contraction of the 

 whole muscle must be made up of the summated effects of the contraction 

 wave as it passes from point to point. Hence the longer the muscle, the 

 more must the contraction be lengthened by the time taken up in propagation 

 from one end to another. 



