CH. XV.] ACTION CURRENT OF NERVE 171 



control the nutrition of the nerve-fibres are situated within the cord 

 for the anterior roots, and within the spinal ganglia for the posterior 

 roots. 



Changes in a Nerve during Activity. 



When a nerve is stimulated, the change produced in it is called a 

 nervous impulse ; this change travels along the nerve, and the pro- 

 pagation of some change is evident from the effects which follow : 

 sensation, movement, secretion, etc. ; but in the nerve itself very little 

 change can be detected. There is no change in form ; the most deli- 

 cate thermo-piles have failed to detect any production of heat, and 

 we are also ignorant of any chemical changes. The only alteration 

 which can be detected as evidence of this molecular change in a nerve 

 is the electrical one. Healthy nerve is iso-electric, but during the 

 passage of a nervous impulse along it there is a very rapid diphasic 

 variation, which travels at the same rate as the nervous impulse. 

 This is similar to the diphasic change in muscle, which we have 

 already studied, and can be detected in the same way. 



Waller regards the current of action of any excitable tissue as an index of the 

 magnitude of action, and records the movement of the galvanometer by photograph- 

 ing the excursion of the spot of light on a moving photographic plate. He has in 

 this way obtained records from muscle, nerve, retina, skin, plant tissues, etc. He 

 points out that the only available index of action within the nerve itself is the 

 electrical sign of activity, whereas in muscle the mechanical action can be compared 

 with its accompanying electrical changes. The amount of contraction in a muscle 

 caused by excitation of its nerve is only a very rough, or even a fallacious, indica- 

 tion of the excitability of the nerve, because the nerve is connected to the muscle by 

 motor end-plates, and these, as we have already seen, are fatigued long before the 

 nerve shows any sign of fatigue. 



Using this method, Waller has obtained a number of interesting results on the 

 variation in nerve action produced by drugs and other agents. He finds that the 

 effect of carbonic acid is to cause a diminution, and finally disappearance of the 

 galvanometric response ; when this gas is replaced by air the nerve recovers, and the 

 action-currents increase. Ether acts similarly ; but with chloroform recovery is 

 difficult to obtain. Small doses of carbonic acid increase the action-currents, and 

 Waller considers that the staircase effect in muscle (p. 117), and the similar progres- 

 sive increase noted in the action-currents of nerve as the result of repeated stimula- 

 tion are due to the evolution of this gas during activity. 



This hypothesis has been recently confirmed by some experiments of Bseyer 

 and Frohlich. They have shown that peripheral nerves participate in respiratory 

 exchanges, using up oxygen and producing carbonic acid in measurable amounts. 



There can be no doubt that the existence of the electrical variation is as a rule 

 the index of the excitatory alteration in a nerve. In the isolated nerve it is in fact 

 the only change that can be detected. But in the present state of our knowledge 

 we are not justified in assuming that it gives an absolutely faithful record. The 

 electrical variation can be detected in a nerve for many days after its removal from 

 the body. Although the electrical change is a concomitant of the real excitatory 

 process, the former may be therefore perceptible when other evidence of the 

 existence of the latter fails. Moreover, Gotch and Burch have obtained further 

 evidence of the dissociation of the electrical response from the excitatory process. 

 In the frog's sciatic nerve, it is possible with two stimuli in rapid succession to 

 obtain only one electrical response near the seat of excitation which has been cooled, 

 while two such responses occur in a more peripheral warmer region. 



