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 temperature 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 conduction 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, 



o 



this procedure forms a convenient method 

 for blocking the passage of impulses along 

 a nerve without using the irritating pro- 

 cedure of section. On warming the nerve 

 again the conductivity returns. The 

 rapidity with which the excitatory pro- 

 cess is propagated along either a nerve or 

 a muscle fibre depends on the tempera- 

 ture. 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 propa- 



velocity at Tn 4- 10 , 

 gation, i.e. - has been 



velocity at Tn 



found by Lucas 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 conduction of the excitatory process to 



vary between 1-8 and 2. 



An ingenious method (Fig. 106) has been used by Keith Lucas for the determina- 

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

 represented in the figure is filled with Ringer's solution, in which the whole 

 nerve-muscle preparatio'n 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 



289 19 



