Muscle and Electric Organs 607 



stomach, while adrenalin inhibits the spiral valve and rectum, structures 

 which are stimulated by the splanchnic nerve.^"''' Similarly in a cyclostome, 

 where there are no sympathetic nerves, the vagus has no effect on intestinal 

 muscle, but atropine, adrenalin, and nicotine inhibit contractions initiated 

 by acetylcholine.""' The drug effects are thus not dependent on sympathetic 

 innervation. 



The digestive tracts of insects and crustaceans contain circular and longi- 

 tudinal muscles which are normally active in digestion. Spontaneous rhythms 

 in the crayfish intestine are accelerated by dilute solutions of either acetyl- 

 choline or adrenalin (Table 72). Stimulation of the abdominal nerve cord 

 of the lobster elicits rhythmic contractions of the intact intestine and anal 

 muscles.'"" The gut muscles of the honeybee and probably of many other 

 insects are striated. The cockroach crop and gizzard show much activity in 

 vivo, the gizzard movements stopping when the hrst thoracic ganglia are 

 removed.-^*' The foregut and intestine of Dytisciis beat well when isolated 

 in saline; acetylcholine, nicotine,-'-' and potassium'-^ are excitant, adrenahn 

 is without effect.-'" 



In some molluscs adrenalin increases the tone and normal rhythm (crop 

 of Aplysia,^-"^ rectum of Sepia^-^'). In Helix and Aplysia acetylcholine and 

 potassium increase tonic activity of the intestine, but the Sepia intestine is 

 insensitive to ACh except at high concentrations which lower tonus and 

 stop automatic beats.'*- 



In the tube-dwehing polychaete Arenicola the proboscis (extrovert) con- 

 sists of buccal mass, pharynx and esophagus; the stomatogastric nerve plexus 

 extends over the pharynx and esophagus and connects to the circumesopha- 

 geal ring of the central nervous system. A variety of experiments--^^ show 

 that the esophagus is a pacemaker region from which rhythmic bursts arise 

 at intervals of about 6 minutes. When the extrovert remains attached to 

 a piece of the body wall with the anterior nerve cord intact, the body wall 

 shows rhythmic activity so long as the nen'es to the esophagus are intact. 

 The buccal mass can show small-amplitude asynchronous waves when re- 

 moved from the esophagus; this activity is inhibited by adrenalin. Esophageal 

 contractions and those induced in the buccal mass -and body wall from the 

 esophageal pacemaker are greatly stimulated by adrenalin. Acetylcholine 

 also stimulates the esophagus. The stomatogastric nervous system apparently 

 initiates rhythmic movements of the entire proboscis region including body 

 wall; both adrenaUn and ACh stimulate this pacemaker. In the free-swim- 

 ming Glycera, also, waves of activity originate in the stomatogastric system 

 of pharynx and esophagus, but, in addition, impulses from the central nerv- 

 ous system can invade the buccal tube eliciting a different rhythm.--^ 



In holothurians the isolated intestine is stimulated by adrenalin, relaxed 

 by atropine. Both pilocarpine and physotigmine cause an increase in tone.^^ 



Table 72 shows that only in the muscles of gastrointestinal tracts of verte- 

 brates is there strict antagonism between acetylcholine and adrenalin. 



Muscle cells in general have an inherent spontaneity of movement. How- 

 ever, adult muscles rarely manifest spontaneity; nervous pacemakers usually 

 gain control. 



