SECTION II 

 PROPAGATION ALONG NERVE FIBRES 



THE velocity of propagation along a nerve fibre may be measured, 

 although in early times it was thought to be as instantaneous as the 

 lightning flash. To measure the velocity of propagation in a motor 

 nerve, a frog's gastrocnemius is prepared, with a long piece of sciatic 

 nerve attached. The muscle is arranged (Fig. 104) so that its con- 

 traction may be recorded on a rapidly moving surface, on which are 



m 



bn 



FIG. 104. Diagram of arrangement of experiment for the determination of 

 the velocity of transmission of a motor impulse down a nerve. 



The battery ^current passes through the primary coil of the inductorium 

 c, and a ' kick over ' key k. By means of the switch s, the break shock 

 in the secondary circuit can be sent through the nerve n, either at b or 

 at a. The muscle m is arranged to write on the blackened surface of a 

 trigger or pendulum myograph, and is excited during the passage of the 

 recording surface by the automatic opening of the key k. (The time- 

 marker is not shown.) 



also recorded, by means of electro- magnetic signals, the moment at 

 which the stimulus is sent into the nerve, and also a time-marking 

 showing o-g-Q sec. Tracings are now taken of the contraction of the 

 muscle : first, when the nerve is stimulated at its extreme upper 

 end ; secondly, as close as possible to the muscle. It will be found that 

 the latent period, which elapses between the point at which the stimu- 

 lus is sent into the nerve and the point at which the lever begins to rise, 

 is rather longer in the first case than in the second. The difference 

 in the two latent periods gives the time that the nervous impulse has 

 taken to travel down the length of nerve between the two stimulated 

 points. Calculated in this way the velocity of propagation in frog's 

 nerve is about 28 metres per second. 



In man and in warm-blooded animals the velocity has been variously 



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