

CH. XIV.] NATURE OF THE NERVE IMPULSE 163 



The Nature of the Nerve Impulse. 



What is the nature of this change which we have provisionally 

 been alluding to as a molecular disturbance ? The ancients imagined 

 the nerves were tubes along which a flow of a spiritual essence 

 (animal spirits) took place. We know that this is not the case, but 

 we do not know anything else about it for certain. Theories there 

 are in plenty, but none of them are adequate to explain the pheno- 

 menon. The theories fall under two main headings, chemical and 

 physical. In a chemical theory we may compare the transmission of 

 the impulse to the propagation of a flame along a train of gunpowder ; 

 but such an analogy is very imperfect, for the gunpowder is entirely 

 consumed, and has not the power to repair itself as a nerve has. 

 Nevertheless there are certain facts which make a chemical theory 

 acceptable ; these are : 



(1) Analogy with muscle, where the propagation of the muscular 

 impulse is undoubtedly largely due to the propagation of chemical 

 disturbances. 



(2) Evidence that the nerve does undergo metabolic changes, as 

 shown by the necessity for oxygen, and the production of minute 

 amounts of carbon dioxide. 



(3) Arrhenius and van 't Hoff showed that a rise of 10 in tem- 

 perature increases the velocity of a chemical reaction to two or three 

 times its original rate. Purely physical changes are not accelerated 

 nearly so greatly by the same rise of temperature. Maxwell's recent 

 experiments show that a rise of 10 C. approximately doubles the 

 velocity of nerve conduction, and the conclusion is drawn that, 

 therefore, the nerve impulse is a chemical phenomenon. Keith 

 Lucas confirmed this observation. Woolley obtained the same figure 

 from the influence of temperature on the rate of conduction in 

 muscle, so probably the conduction process is of a similar nature in 

 both tissues. 



The physical theories in relation to this question compare the 

 nerve impulse to the way in which an electrical change is propagated 

 along a wire. When the electrical accompaniment of nervous 

 activity was first discovered this view was unhesitatingly accepted 

 by many physiologists, and the current of action was regarded not as 

 an accidental concomitant of the impulse, but as the change which 

 really constitutes the essence of the impulse, and which serves to 

 excite the chemical and other changes in the tissues to which the 

 nerve is distributed. Two facts, however, stood out at once which 

 rendered the adoption of this simple view difficult ; one of these is 

 the slow rate of conduction in nerve ; and the other is the pheno- 

 menon of inhibition ; it is quite conceivable that an electrical dis- 

 turbance, feeble though it be, can fire off an excitable tissue and lead 



