SECTION IV 



CONDITIONS AFFECTING THE PASSAGE OF A 

 NERVOUS IMPULSE 



TEMPERATURE. Below a certain temperature the propagation of the 

 excitatory process in the nerve is absolutely abolished. The exact tempera- 

 ture at which this occurs varies according as we use a warm- or a cold- 

 blooded animal. In the frog it is necessary 

 to cool the nerve below C. before con- 

 duction is abolished, whereas in the mammal 

 it is sufficient to cool the nerve to somewhere 

 between and 5 C. Since cooling the 

 nerve does not excite it, this procedure forms a 

 convenient method for blocking tEe passage 

 of impulses along a nerve without using 

 the irritating procedure of section. On 

 warming the nerve again the conductivity 

 returns. The rapidity with which the excita- 

 tory process is propagated along either a nerve 

 or a muscle fibre depends on the temperature. 

 Thus the mean rate of conduction in the 

 frog's nerve at 8 to 9 C. is about 16 metres 

 per second. The temperature coefficient 

 of the velocity of nerve propagation, i.e. 



velocity at Tn + 10 , , 



has been found by Lucas 



velocity at Tn 



to be about 1-79. The same value was 

 found by Maxwell for conduction in molluscan 

 nerve, and in frog's striated muscle Woolley 

 found the temperature coefficient for con- 

 duction of the excitatory process to vary between 1-8 and 2. 



An ingenious method (Fig. 105) has been used by Keith Lucas for the determination 

 of the conduction rates in nerve at different temperatures. The glass vessel repre- 

 sented in the figure is filled with Ringer's solution, in which the whole nerve-muscle 

 preparation is immersed. The muscle used was the flexor longus digitorum, so that 

 the whole length of the sciatic, tibial, and sural nerves could be used. The nerve is 

 passed up through the constrictions in the inner glass vessels at c and D, and is attached 

 to the thread E. p, I, and G are three non-polarisable electrodes composed of porous 



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