STIMULATION OF MUSCLE 



741 



Stimulation by the Voltaic Current. While the current continues to 

 pass through a nerve without any sudden or great change in its in- 

 tensity, there is no stimulation, and the muscle connected with the 

 nerve remains at rest. The same is true of striated muscle when a 

 weak current is passed directly through it. But in muscle the con- 

 stancy of the rule is more and more frequently broken by exceptional 

 results as the current is strengthened, a state of permanent contrac- 

 tion being very apt to show itself during the whole time of flow (Wundt) 

 (Fig. 246). Above a certain intensity of current a greater or less 

 degree of permanent contraction is invariably produced. This is some- 

 times called the ' closing tetanus.' It is, however, not a true tetanus, 

 but a tonic contraction, which is strongest in the neighbourhood of the 

 kathode, and does not spread far from it. A similar condition, the 

 so-called galvanotonus, is normally seen in human muscles when they or 

 their motor nerves are 

 traversed by a stream 

 of considerable inten- 

 sity. Under certain con- 

 ditions, too e.g., when 

 a strong current is 

 allowed to flow for a 

 comparatively long time 

 through a muscle the 

 muscle remains contrac- 

 ted after the opening of 

 the current (so-called 

 ' opening or Hitter's tet- 

 anus '). Smooth muscle 

 is excited to contraction 

 even when a voltaic cur- 

 rent is very gradually 

 passed into it and slow- 

 ly increased, and again 

 when it is caused very 

 gradually to disappear. 

 But striped muscle is 

 not stimulated under 



Fig. 2471 Tonic Contraction during and after Flow 

 of Voltaic Current. Curve from frog's gastroc- 

 nemius. At M constant current closed, at B broken. 

 Contracture continues after opening of current. 

 Time trace, two-second intervals. 



these conditions. 



For nerve, and with these qualifications for muscle, too, the law 

 holds that the voltaic current stimulates at make and at break, but not 

 during its passage. Or, generalizing this a little, since it has been 

 shown that a sudden increase or decrease in the strength of a current 

 already flowing also acts as a stimulus, we may say that the voltaic 

 current stimulates only when its intensity is suddenly and sufficiently 

 increased or diminished, but not while it remains constant.* 



When a strong current is closed through a muscle there is an im- 

 mediate sharp contraction (initial contraction). The muscle then 

 promptly relaxes, but incompletely. When the current is opened, 

 there is another contraction (Fig. 247). The force of the initial con- 

 traction, as measured by the resistance necessary to prevent it, is 

 greater than that of the tonic contraction which follows it. 



A second law of great theoretical importance is that of polar stimula- 

 tion. At make the stimulation occurs only at the kathode ; at break only 

 at the anode. This is true both for muscle and nerve, but it is most 



* This law of Du Bois-Reymond has been questioned by Hoorweg and others. 

 It seems to need modification, but the subject cannot be discussed here. 



