606 



Comparative Animal Physiolttgy 



potentials may be present even when muscular activity is negligible.-- ^ The 

 rhythmic contractions are superposed on a background of tonus. Conducted 

 waves, peristaltic contractions, are local reflexes initiated by distention and 

 mediated by the nerve plexuses. The activity of the intestine, whether myo- 

 eenic or myenteric reflex, is enhanced by acetylcholine or by vagus stimula- 

 tion and is diminished bv adrenalin or by sympathetic stimulation. Atropine 

 relaxes and pilocarpine excites the gut. Nicotine in low concentrations abol- 

 ishes the reflex contractions. Potassium stimulates, possibly by acetylcholine 

 release; calcium has the reverse eff^ect.'^ The eff^ects of several drugs on dif- 

 ferent visceral muscles are compared in Table 72. 



TABLE 72. RESPONSES OF VISCERAL MUSCLES TO DRUGS 



-{- indicates stimulation (increased amplitude or frequency, or both, of spontaneous 

 contractionsj; — indicates depression of activity; and indicates no effect. 



In the uterus, a muscle lacking nerve plexuses, excitablHty and tonus are 

 low and spontaneity is rare during anoestrous, but activity appears in oestrus 

 and after injections of estrogens. Uterine muscle shows waves of electrical 

 activity, usually originating in the uterine horns and possibly conducted 

 from cell to cell by protoplasmic bridges.^"'' ^"' ^- In the ureter similar 

 rhythmic waves originate at the renal end. Peristaltic contractions of the 

 ureter and stomach do not involve nervous elements and are not abolished 

 by high concentrations of nicotine and cocaine."'- Teleostean fish show re- 

 sponses of the intestine similar to those of mammals, adrenalin and atropine 

 inhibiting and acetylcholine accelerating contractions.-"*- In elasmobranchs, 

 however, adrenalin and both sympathetic and vagus impulses stimulate the 



