EXCITATION AND INHIBITION 



407 



tissue would naturally become less negative, that is, it would appear to become 

 electro-positive. It is of some importance, therefore, to note that Gaskell was 

 unable to detect the slightest change in tonus, although using a very sensitive 

 lever to magnify any possible movement. It is evident that inhibitory action is 

 accompanied by some kind of change, which is of the opposite nature to that 

 which is responsible for the negativity of a tissue in the state of excitation. 



With regard to some doubts that have been expressed as to these results, I should like 

 to state chat I have myself on two separate occasions shown the fact %,s a lecture experiment 

 and have obtained the electro-positive change without difficulty. It should be borne in mind 

 that the magnitude of the change is much less than that of the negative change occurring on 

 contraction, so that observers who have attempted to observe it with the capillary electrometer 

 have been unable to do so. Recently, however, both Meek and Eyster (1912) and Samoilov 

 (1913) have photographed it by 

 aid of the string galvanometer, 

 made especially sensitive. The 

 advantage of the use of this in- 

 strument is that the positive 

 effect can be seen in the beating 

 heart, both of the frog and of 

 the tortoise (see Fig. 116). 



Gaskell has also shown 

 that, after the heart of the 

 tortoise has been brought to 

 a standstill by the applica- 

 tion of the alkaloid 

 muscarine to the sinus, 

 stimulation of the vagus 

 nerve produces the same 

 change as that recorded 

 above ; moreover, stimula- 

 tion of the augmentor 

 nerves of the toad, which 

 in the beating heart causes 

 increase in the rate or 

 height of the contractions of 

 the ventricle, produces in 

 the ventricle of the heart at 

 rest under the action of 

 muscarine or otherwise, an 

 electrical change of the 

 same sign as that associated 

 with contraction of the 

 muscle (Fig. 117). 



Another effect of the 

 state of inhibition is to 



I I I 



FIG. 115. ELECTRICAL CHANGE PRODUCED IN THE 

 QUIESCENT AURICLE OF THE TORTOISE BY STIMULATION 

 OF THE VAGUS NERVE BETWEEN THE DOTTED LINES 

 ON THE CURVE. At the arrow, muscarin was applied 

 to the sinus to arrest its contractions. This has no 

 effect on the subsequent stimulation of the vagus. 



Note that the electrical change is opposite in sign to 

 that associated with contraction, and also to that 

 produced by the augmentor nerve in quiescent muscle 

 (see Fig. 117 below). 



(Gaskell, Journ. of Physiol, 8 (1887), 404-415.) 



depress the excitability of 



the heart muscle with regard 



to the response to direct stimulation. M'William showed (1885, 1) that the sinus, 



auricles and ventricle of the newt's heart are inexcitable under vagus inhibition, 



and (1885, 2, p. 226) that the auricle of the eel's heart behaves in the same way. 



Although this phenomenon could not be demonstrated in other animals, it shows 



that the action of inhibitory nerves is excited on the muscle itself. 



The work of Dorothy Dale and G. R. Mines (1913) shows that the action of 

 the vagus on the heart, as investigated by the string galvanometer, is to produce 

 increased resistance to transmission from auricle to ventricle and to shorten the 

 duration of the electrical disturbance. That of the accelerator nerves is to improve 

 the rate of conduction and to increase the duration of the electrical response in 

 the ventricle. 



Since the effects of the excitatory and inhibitory nerves on smooth muscle are 

 of an opposite nature, it is to be expected that the effect of one could be balanced 

 by the simultaneous stimulation of the other. Experiments of this kind were 



