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HANDBOOK OF PHYSIOLOGY -^ NEUROPHYSIOLOGY I 



is not a property of the nerve net, for, following a 

 single shock, the excitation spreads over the entire 

 bell (13); Horridge (31) has shown that this excitation 

 consists of a single all-or-nothing nerve impulse. It 

 has not been proved, however, that the facilitation is 

 not a property of the excitation-contraction coupling 

 within the muscle fibers. 



There have been several histological studies of the 

 nerve net which presumably distributes the excitation 

 of the swimming nio\ement. Schafer (69) described 

 large bipolar neurons in the subumbrellar epithelium. 

 The nerve fibers were more or less straight, were 

 usually unbranched and were comparatively thick 

 (15 /i). This description has recently been confirmed 

 by Horridge (32) who found that the nerve fibers were 

 in the range 6 to 12 n. Both authors described tapering 

 of the nerve fibers towards their ends, but there was 

 no consistently observed structure which would 

 constitute a motor nerve ending [see, however, the 

 illustrations in Schafer (69)]. Bozler (11) also 

 described the large bipolar neurons, and observed 

 smaller ones, as well as numerous multipolar neurons 

 with branching axons. One might then wonder if the 

 different types of neurons form physiologically 

 distinct nerve nets which would underlie different 

 behavioral responses (i.e. types of contraction). This 

 question will be con.sidered next. 



Aside from the swimming movements described 

 above, localized and more prolonged contractions of 

 regions of the bell can also be observed (10, 13, 33). 

 Since it has been shown that the excitation underlying 

 the swimming movement is spread over the entire 

 bell by a single impulse traversing a nerve net (31), 

 the presence of local contractions does suggest the 

 existence of another nerve net in which the spread of 

 excitation is limited. Further, Romanes (63) showed 

 that a wave of excitation, distinct from the wave of 

 contraction, could cross the bell. In one demonstra- 

 tion of this, he removed seven of the eight marginal 

 ganglia (in which the e.xcitation for the rhythmical 

 swimming movements arise) and applied to some 

 point on the bell a stimulus too weak to evoke a 

 contraction wave (swimming movement) directly. 

 Then after some delay, a contraction wave would 

 spread out from the intact ganglion. Additional 

 examples suggesting the presence of more than one 

 nerve net can be found in Horridge (33). 



One more case, however, will be considered for 

 here there is good correlation between histological 

 and physiological observations. Horridge (34) has 

 studied the ephyra larva of Auirllia which shows two 

 types of contractions. There are a) the generalized. 



rapid, rhythmic swimming movements and A) pro- 

 longed contractions involving a variable fraction of the 

 animal and normally associated with feeding. Follow- 

 ing strong mechanical stimulation, the prolonged 

 contraction may involve the whole animal. In histo- 

 logical preparations, two types of nerve net can be 

 distinguished. The first is composed of bipolar neurons 

 which are confined almost entirely to the epithelium 

 overlying the radial and circular musculature. The 

 fibers of these neurons are highly oriented, running 

 in the same direction as the muscle fibers. The second 

 net consists mainly of multipolar cells, with some 

 bipolars, and is present throughout the entire epithe- 

 lium. The fibers of this net are not particularly 

 oriented except in the region around the mouth. Some 

 of the neurons of this diffuse net send fibers to the 

 surface of the epithelium and would appear to be 

 sensory cells. This observation alone suggests that the 

 diffuse net is associated with the local, prolonged 

 contractions since these latter are evoked by tactile 

 stimuli. Experiments designed to test this hypothesis 

 yielded affirmative results and also provided evidence 

 that the swimming movements are mediated by the 

 other net of bipolar oriented neurons. For example, 

 eight radial cuts were made through the disc so that 

 the band of circular muscle, with its overlying net of 

 bipolar cells, was sectioned into eight separate arcs. 

 The cuts were not continued all the way to the center 

 of the disc and the animal thus remained in one piece. 

 It was then found that each arm, with its arc of circu- 

 lar muscle, still produced the rhythmical swimming 

 movements but that the beat of each was independent 

 of all the others. A strong tactile stimulus, however, 

 could still produce a co-ordinated contraction of the 

 whole animal. 



The Mechanism oj Transmission ni Coelenterates 



Practically nothing is known of the actual mech- 

 anism by which nerve-net excitation crosses the 

 junction to the muscle fibers. Tissue extracts have 

 been made and tested on neuromuscular transmission 

 but without success (65, 66). Numerous drugs have 

 been tested on the responses ot the sphincter muscle 

 of intact Calliaclis. Acetylcholine, curare, nicotine, 

 epinephrine, histamine and a number of other drugs 

 are without apparent effect (64). Several drugs, 

 however, were effective in high concentrations and 

 after prolonged exposure. Tyramine, tryptamine and 

 933F, after immersion of the animal in solutions of 

 io~* gm per ml for one- and one-half to several hours, 

 brought about se\eral-fold increases in the muscular 



