SECTION II 

 PROPAGATION ALONG NERVE FIBRES 



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

 m 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 gastroc- 

 nemius is prepared, with a long piece of sciatic nerve attached. The muscle 

 is arranged (Fig. 103) so that its contraction may be recorded on a rapidly 

 moving surface, on which are also recorded, by means of electro-magnetic 



FIG. 103. 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 bs 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.) 



signals, the moment at which the stimulus is sent into the nerve, and also a 

 time-marking showing ^| 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 stimulus 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 

 estimated at from 60 to 120 metres per second. The higher of these figures 

 is probably nearer the truth. 



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