THE NERVE-IMPULSE OR PROPAGATED DISTURBANCE 761 



as to the effect produced on a nerve by excitation at a given point from 



the resultant effect on the muscle to which the nerve is attached (or 



on a galvanometer or electrometer if we are following the effect by 



means of the electrical changes), we must know whether the change 



set up in the nerve at the 



point of excitation can pass 



freely along the nerve to the 



muscle or to the point at 



which it is led oil to the 



galvanometer. Now, changes 



of conductivity are certainly 



produced in a nerve by the 



constant current, which even 



outlast its flow. For all 



currents above a certain 



strength the conductivity at 



the kathode and in its neigh- 

 bourhood is eventually dim- 

 inished, and with currents 



still only moderately strong 



the block deepens into im- 



passability. The conduc- 

 tivity at the anode is, during 



this stage, higher than at the 



kathode, so that at the time 



of full kathodic block the 



nerve - impulse still passes 



through the region around 



the positive pole. With still 



stronger currents the con- 268 ._ D iagram of Changes of Excitability 



ductivity here, too, dimm- ^ conductivity produced in a Nerve by a 



Voltaic Current. E, changes of excitability 

 during the flow of the current, according to 

 Pfliiger. These are seen most typically with 

 the weaker currents. In particular the in- 

 creased excitability at and around the kathode 

 when the current is strong very quickly gives 

 place to depression. The ordinates drawn 

 from the abscissa axis to cut the curve repre- 

 sent the amount of the change. C(i), changes 

 of conductivity found shortly after the closure 

 and during the flow of a moderately strong 

 current. Conductivity greatly reduced around 

 kathode; little affected at anode. C(a), 

 changes of conductivity during flow of a very 

 strong current. Conductivity reduced both in 

 anodic and kathodic regions, but less in the 

 former. C 1 , changes of conductivity just after 

 opening a moderately strong current. Con- 

 ductivity greatly reduced in region which was 

 formerly anodic; little affected in region for- 

 merly kathodic. 



ishes, until the anode as well 

 as the whole intrapolar re- 

 gion is blocked. After the 

 opening of the current, the 

 relation between kathodic 

 and anodio conductivity is 

 reversed, for now the post- 

 kathodic region conducts the 

 nerve-impulse relatively bet- 



ter than the post-anodic. It 

 will be seen that these 

 changes of conductivity up- 

 on the whole run parallel to 

 the (secondary) changes of 

 excitability, depression of 

 excitability correspond ing to 

 depression of conductivity, 

 and vice versa. With the 

 relatively strong currents re- 

 quired to produce decided 



to 



effects on the conductivity, 

 any preliminary change in the same sense as the (so-called primary) 

 effects on the excitability (increase at kathode, decrease at anode) 

 might be expected to be fleeting, and thciefore less easy to detect. 



The above facts serve to explain the manner in which the effects of 

 stimulation of a nerve with the constant current vary with the strength 



