Muscle and Electric Organs 595 



excitation spreads over a ner\'e network, and facilitation may be neuro-neural 

 as well as neuromuscular. Relaxation is slow, and as more stimuli are 

 given more of the muscle responds because more junctions are facili- 

 tated^^*'' "*' (Figs. 227-228). In both jellyfish and sea anemones a long re- 

 fractory period in the conducting units prevents any fusion of the mechani- 

 cal response (Fig. 227). ^■'**'- "" 



The body wall muscle of the earthworm responds to single shocks but 

 excitation spreads with increasing frequency and fusion occurs at about 14 

 per second. After a brief tetanus, the responsiveness is enhanced for as long 

 as 30 seconds.^' 



Long-fibered smooth muscles appear to summate less than short-fibered 

 smooth muscles do. The long retractor of Thyone and the proboscis re- 

 tractor of Phascolosoma develop maximum tension (tetanus-twitch ratios 

 of 7 to 10) at low frequencies (10 per sec). 



Two general types of mechanism permit increasing contraction with in- 

 creasing frequency of excitation: the overcoming of mechanical resistance 

 (or greater contractility), and junctional (including interfiber) facilitation. 

 A comparati\'e study of responses to different frequencies would elucidate 

 the basic differences in the importance of these mechanisms. 



Fig. 228. Responses of jellyfish (Rhopallia) to repetitive stimuli, intervals between 

 separate stimuli given in seconds. Bullock."" 



MULTIPLE INNERVATION; SUMMATION AND INHIBITION 



Many muscles receive dual or multiple inner\'ation. In some the two 

 inner\'ations are antagonistic. The x'crtebrate heart, blood vessels, and vis- 

 ceral muscles are innervated from the antagonistic sympathetic and para- 

 sympathetic systems. Here one system is excitatory, the other inhibitory. 

 The intestine is made to contract by parasympathetic and to relax by sym- 

 pathetic impulses; the systemic arteries are aflfected oppositely. The retrac- 

 tor penis is contracted bv a nerve of svmpathetic origin and relaxed by 

 the nervus erigens arising in the sacral cord. A given tvpe of nerve fiber 

 may be excitatory for one muscle and inhibitory for another. The effects 

 may even be reversed by appropriate drugs; hence the state of the effector 

 cells may decide the nature of the nervous effect. 



Another type of multiple inner\'ation is found where two excitatory 

 (motor) fibers go to a single muscle fiber. There js evidence^^*^' ^^^^ indi- 

 cating that some skeletal muscles of the frog also have, in addition to nor- 

 mal motor innervation, innervation by small nerve fibers which elicit slower, 

 smaller responses. Stimulation of the large-diameter (15 /x) fibers sets up 

 propagated impulses and fast all-or-none twitches. Stimulation of the 



