153 



ON THE PATH AND VELOCITY OF THE EXCITATORY IMPULSE 

 WITHIN STRIATED MUSCLE FIBRES. 



By O. W. TiEGS^ D.Sc, Zoology Department, University of Adelaide. 

 [Read May 10, 1923.] 



In previous papers (1922, 1923) I have pointed out that the "striations" 

 of voluntary and cardiac muscle fibres are always arranged in the form of double 

 spirals (double helicoids) which travel from one end of the fibre to the other. 

 I have also found that in insect wing muscle, which has a wholly different type 

 of embryonic development, the complex multicellular fibres have adopted this 

 same arrangement of "striations." 



The fact that two such tissues as ordinary skeletal muscle and insect wing 

 muscle, which have evolved from wholly different sources, should have found it 

 necessary to adopt this same arrangement of their "striations," seems to show 

 that the spirals are of some fundamental importance for the successful function- 

 ing of these tissues ; and it would seem that the slightly more even distribution 

 of the strain within the fibres, which would result from this, is not sufficient to 

 account for its occurrence. 



The function of the spirals is to be looked for in a wholly different direction, 

 and there is considerable evidence to show that it is along the spirals that the 

 excitatory impulse travels within the fibre. The evidence for this statement is 

 twofold : — 



(1) Let us consider how the excitatory impulse can reach the various 

 fibrils (or sarcostyles) within a minute fibre. The impulse travels to each fibre 

 by the terminal branch of a motor nerve; the actual motor end plate does not 

 penetrate the fibre, but lies in close contact with the sarcolemma. While, there- 

 fore, it is easy to see that those fibrils, lying close to the end plate might receive 

 the stimulus, it is difficult to conceive how those on the opposite side of the 

 fibre should be affected by it. There are three ways in which this might take 

 place : — 



(a) The impulse might travel directly across the striation ; since the 



striation is not a disc, but a spiral band which runs right along 

 the fibre, the impulse would not cease when it reached the "other 

 side" of the fibre, but continuing its path, would eventually travel 

 right along the fibre. 



(b) On the other hand, the impulse might diffuse through the sarco- 



plasm ; or 



(c) It might travel to the opposite side across the "striation," and thence 



longitudinally along the fibrils. 



Now, neither the second nor the third method is very probable; the sarco- 

 plasm in very slowly-moving muscles (e.g., sluggish insect larvae) is very 

 unevenly distributed; and moreover, there is never a direct continuity of the 

 sarcoplasm (except along the spiral), successive portions being always separated 

 by the membranes of Krause. Moreover, if the sarcoplasm was the 

 transmitting agent, then we should expect the contractile wave, which is pro- 

 duced by the impulse, to travel in such a way that the fibrils in the neighbourhood 

 of the motor end plate would be contracted along a greater part of their length 

 than those on the opposite side of the fibre. The wave of contraction should 

 travel not perpendicularly to the long axis of the fibre, but at a sharper angle 



