Muscle and Electric Origans 



603 



contract when distended (Ch. 15, p. 547). In vivo hormonal agents, par- 

 ticularly acet\lcholine and adrcnin, are also important in regulating smooth 

 muscle tone. Local, non-conducted impulses from a "pacemaker" region may 

 result in tonus in an organ such as the uterus.-'- 



Between these two extremes of striated skeletal and the smooth muscle 

 of hollow organs are many muscles in which rapid rcHex responses may be 

 superimposed upon a tonic background intrinsic in the muscle. Squid chro- 

 matophore muscles, for example, show a tonic state upon which twitches 

 are super]:)osed."'' Relaxation curves of numerous molluscan and mammalian 

 smooth muscles are similar to the curves of release of tension after a stretch 

 (Fig. 232).^"- ^' However, if in addition the muscle develops tonus on 



Fig. 233. Responses ot the byssus retractor ot Mytilus to stimulation with alternating 

 current and direct current. A, Three contractions in response to stimulation by AC (50 

 cps) for 10 sec. each, and response to stimulation by DC for 10 sec. B, Response to AC 

 14 sec. tor each minute. C, Response to DC 14 seconds per minute. Note delayed 

 relaxation, summation with DC, fast relaxation and fatigue with AC. After Winton.""*' 



Stretching, the relaxation from a contraction may be faster than release of 

 tension. ^■'*- '"'' |ordan and his students in many papers, summarized by Jor- 

 dan,^'"' have shown that tonus in the molluscan foot is in part a response 

 of the muscle to stretch but is regulated by the nervous system. It is sug- 

 gested that the tonic and contractile mechanisms are separate. The pedal 

 ganglion decreases the resistance to stretching but does not affect the con- 

 tractility of the foot to an electrical stimulus, whereas the presence of the 

 cerebral ganglion is said to increase the contractility without having any 



