84 PHYSIOLOGY OF MUSCLE AND NERVE 



The Character of the Contraction. — Even the most casual observa- 

 tion of the peristaltic wave of the stomach, intestine or ureter must 

 show that smooth muscle reacts in a very sluggish manner, but it would 

 be going too far to state that its irritabiUty is less than that of striated 

 tissue. Practically all the different types of stimuH are effective; in 

 fact, in the case of the iris of the eye of frogs and other animals it is 

 possible to produce constrictor reactions even with light. ^ Obviously, 

 this phenomenon cannot be explained by saying that it is due to reflex 

 causes, because the same results may be obtained with small pieces 

 of excised iris. It is also evident that smooth muscle is very suscep- 

 tible to mechanical and thermal stimuli, but rather insensitive to 

 electrical stimuli. The latter peculiarity necessitates the use of some- 

 what stronger induction shocks than are ordinarily required to activate 

 striated muscle. 



Different types of smooth muscle differ in their speed of reaction, 

 but, broadly speaking, it may be said that their latent period is from 



Fig. 51. — Contraction of Smooth Muscle (Cat's Bladder.) 

 L, latent period; C, period of contraction; R, period of relaxation; time in seconds. 



100 to 500 times as long as that of striated muscle. But inasmuch 

 as the amplitude of the reaction of smooth muscle is directly propor- 

 tional to the strength of the stimulus, it forms no exception to the 

 general law and gives, therefore, an ascending series of minimal and 

 maximal contractions which increase with the strength of the current. 

 The chief peculiarity of the curve of contraction of smooth muscle 

 is its great length. Thus, if a preparation of the frog^s stomach, the 

 bladder of a cat,^ or a segment of intestine is stimulated with a current 

 of moderate strength, minutes usually elapse before it again regains its 

 normal form. In the case of striated muscle, on the other hand, 

 the same quality of stimulus evokes a contraction which is generally 

 completed in less than a second. This difference is dependent upon 

 the fact that the periods of contraction and relaxation of plain muscle 

 are greatly prolonged, so that the entire curve really acquires the char- 

 acteristics of a contracture of striped muscle. Inasmuch as its short- 

 ening is always accomplished in a much briefer time (10-15 sec.) than 

 its relaxation (60 sec), it is claimed by Winkler^ that the strength of 



iGuth, Pfliiger's Archiv, Ixxxv, 1901, 118. 

 • « C. C. Stewart, Am. Jour, of Physiol., iv, 1900, 185. 

 » Pfliiger's Archiv, Ixxi, 1898, 386. 



