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HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY 



a.c. Stimulus the muscle relaxed comparatively 

 rapidly, but after d.c. stimulation would do so very 

 much more slowly. In this slowly relaxing state the 

 muscle would support considerable tensions for 

 long periods and seemed to be less susceptible to 

 fatigue. In one example, when a d.c. stimulus was 

 applied to the muscle for 14 sec. of every minute, a 

 continuous gradually increasing contraction was ob- 

 served during the experimental period (15 min.). 

 When, however, a.c. was used, also for 14 sec. per 

 min., each burst of stimulation gave rise to a discrete 

 contraction, the strength of which decreased during 

 successive minutes. It was also found that the main- 

 tained tension following a d.c. shock could be abol- 

 ished by a.c. stimulation. That is, following this a.c- 

 induced contraction, the rate of relaxation was rapid 

 even though the muscle had been in the slowly- 

 relaxing state immediately previously. These effects 

 have since been confirmed by a number of authors 

 (see below). In a recent paper, however, Hoyle & 

 Lowy (37) report that while they commonly found 

 similar results, they often did not. In some case both 

 types of stimuli gave rise to contractions of equal dura- 

 tion and sometimes a.c. evoked more prolonged 

 shortening than did d.c. Nevertheless, since the two 

 types of stimulation are not likely to occur naturally, 

 interest lies mainly in their use as experimental tools, 

 and the dififerent responses, although not invariable, 

 might still reflect two types of mechanisms within the 

 muscle. 



Fletcher (24) confirmed VVinton's observations as 

 part of a general study of the ABRM in which he also 

 recorded the muscle action potentials. In most experi- 

 ments he found no muscle potentials (after the initial 

 one) during a d.c. -induced tonus. This observation 

 laid the ground for the 'catch-mechanism' hypothesis, 

 van Nieuwenhoven (77) also confirmed Winton's 

 main findings but was able, as well, to duplicate some 

 of these effects with indirect stimulation. Relatively 

 strong faradic stimulation of the pedal ganglion, from 

 which the muscle appeared to receive its innervation, 

 gave rise to the familiar prolonged contraction. Subse- 

 quent ganglionic stimulation of lower intensity abol- 

 ished the remaining tension. Thus he suggested that 

 one set of nerves from the ganglion would set the 

 "catch-mechanism" while another would reverse this 

 action. Twarog (71) has recently made some pharma- 

 cological observations which she interprets in terms of 

 this same scheme. She found, for example, that rela- 

 tively small concentrations of acetylcholine (ACh) 

 sufficed to give rise to prolonged contractions of 

 the ABRM which were accompanied by steady 



depolarization. Washing out the ACh restored the 

 resting potential of the muscle without, however, 

 reducing its tension. This maintained contraction was 

 then found to be abolished by the addition of very 

 small concentrations of 5-hydroxytryptamine (5-HT). 

 Then subsequent additions of ACh gave rise only to 

 transient shortening (which could, nevertheless, be 

 of larger amplitude than that following the initial 

 application of ACh). Twarog has also demonstrated 

 the presence of ACh, a choline esterase and 5-HT in 

 this muscle. She suggests that ACh is the chemical 

 mediator evoking contraction (and setting the 

 'catch-mechanism') and that 5-HT would be the 

 transmitter responsible for active relaxation. That 

 these substances are released during activity has not 

 been shown. Hoyle & Lowy (37) have confirmed this 

 inhibiting or relaxing effect of 5-HT on this muscle 

 but conclude that it is not the natural transmitter. 

 In their experiments the contractions were evoked iiy 

 electrical stimulation rather than by the application 

 of ACh. Before considering further their observations 

 on 5-HT, it will be convenient to describe another 

 effect which they obtained. If the muscle was in a 

 tonic state, subthreshold stimulation would often bring 

 about relaxation. It is likely that in their experiments 

 excitation was effected by way of the nerve and this 

 observation is therefore very similar to that made by 

 van Nieuwenhoven (see above) with 'weak' shocks 

 applied to the ganglion. These authors, however, 

 refer to the phenomenon as inhibition. Returning 

 now to the effects of 5-HT, they found that once this 

 drug had been added to the bath the prolonged, 

 tonic contractions could no longer be evoked by 

 d.c. stimulation, although the phasic contractions 

 were still readily obtainable. The effect still per- 

 sisted, however, after several hours, despite periodic 

 washing with sea water. Inasmuch as the inhibition 

 (or relaxation) following subthreshold nerve stimu- 

 lation was rapidly reversible, they suggested that 5-HT 

 could not be the normal mediator of this effect. 



Lowy (46, 47, 48) and Hoyle & Lowy (37) have 

 made a number of other observations, all of which 

 they interpret in terms of the tetanus hypothesis. For 

 example, they have almost always been able, by using 

 large amplifications, to record small irregular elec- 

 trical activity, presumably muscle potentials, through- 

 out the duration of a prolonged contraction in dis- 

 agreement with Fletcher's findings. The potentials 

 were localized and different patterns of activity were 

 recorded simultaneously from different regions of the 

 muscle. These potentials were considerably smaller 

 than those found by other authors in this muscle (23, 



