SECTION III 



EVENTS ACCOMPANYING THE PASSAGE OF A 

 NERVOUS IMPULSE 



IN muscle we saw that the passage of an excitatory wave was accompanied 

 or followed by electrical changes, production of heat, and mechanical change, 

 all pointing to an evolution of energy from the explosive breaking-down of 

 contractile material. 



In nerve however which serves merely as a conducting medium, we 

 should not expect so much expenditure of energy, or in fact any expenditure 

 at all. All that is necessary is that each section of the nerve should transmit 

 to the next section just so much kinetic energy as it has received from 

 the section above it. And experiment bears out this conclusion. The 

 most refined methods have failed to detect the slightest development of 

 heat in a nerve during the passage of an excitatory process, and we know 

 already that there is no mechanical change in the nerve. The only physical 

 change in a nerve under these circumstances is the development of a current 

 of action. A nerve becomes, when excited at any point, negative at this 

 point to all other parts of the nerve and, just as in muscle, this ' negativity ' 

 is propagated in the form of a wave in both directions along the nerve. 



That the excitatory process in nerves is probably accompanied by certain 

 small chemical changes is indicated by the facts that, in the complete 

 absence of oxygen, the nerve fibres lose their irritability, and that this loss 

 of irritability is hastened by repeated stimulation of the nerve. When the 

 irritability has been abolished by stimulation in the absence of oxygen, it 

 may be restored within a few minutes by readmission of oxygen to the nerve. 



If we connect a galvanometer to two points of an uninjured nerve, no 

 current is observed, all points of a living nerve at rest being isoelectric. On 

 making a cross-section of the nerve at one leading-of? point, a current is at 

 once set up, which passes from the surface through the galvanometer to the 

 cross-section. This is a demarcation current, set up at the junction between 

 living and dying nerve. This current rapidly diminishes in strength and 

 finally disappears, owing partly to the fact that the dying process started 

 in the nerve by the section extends only as far as the next node of Ranvier and 

 there ceases, so that after a short time the electrode applied to the cross- 

 section is simply leading off an intact living axis cylinder through the dead 

 portion of the nerve, which acts as an ordinary moist conductor. On making 

 a fresh section just above the previous one, the process of dying is again set 



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