EXCITABILITY OF NERVE 257 



when the posterior root gangUon is diseased, an eruption 

 occurs along the cutaneous distribution oi the nerve (herpes 

 zoster). 



The Excitability of the Nerve Fibre 



The nerve-fibre, hke any other part of the neurone, is 

 highly irritable; it responds to various stimuU, such as 

 heat, or the action of chemicals by an internal disturbance 

 which is propagated throughout the neurone and cul- 

 minates in a subjective impression or a motor effect. Of 

 all the stimuli or exciting agents the most convenient to 

 employ for experimental purposes is electricity, for although 

 this is a form of energy which but rarely affects nerves 

 under normal conditions, yet it is the only one which in 

 this connection can be measured. 



When a constant current is passed through a nerve, 

 excitation occurs at make and again at break. While the 

 current is passing no visible result is produced. In nerve, 

 as in muscle, the state of excitation begins at the kathode 

 on make and at the anode on break. Change of potential, 

 then, rather than potential itself, is the stimulating agent. 



On inquiring further into the effect of change of 

 potential upon the development of the excitatory state, 

 it is found that there are two separate factors concerned — 

 the intensity of the current and the rate of change of 

 potential. As to the latter there is for nerve as for every 

 irritable tissue an optimum rate of change or gradient 

 which is effective. This is known as the " characteristic." 

 The high-frequency current, for instance, is harmless to the 

 body, since the rate of change is too rapid to influence 

 any of the tissues. The single induction shock, while an 

 efficient stimulus to nerve, is too rapid for less irritable 

 tissues, such as intestinal muscle. 



Yet though no excitation occurs while the constant 



current is passing, duration of 'current is an important 



factor. There is between duration and intensity of current 



a reciprocal relation ; the smaller the current, the longer 



17 



