EFFECTOR MECHANISMS 393 



by faradic stimulation of the muscle. Furthermore, d.c. contraction of this 

 muscle is abolished by weak a.c. stimulation of the pedal ganglion. 

 Relaxation of the adductor muscles of Pecten and Mytilus has been ob- 

 tained by reflex sensory excitation. This could be due to central inhibition, 

 but the same effect is produced by electrical stimulation of the peripheral 

 end of a cut nerve tract supplying the slow adductor muscle of Pecten. 

 The excitatory system is cholinergic, and acetylcholine produces muscle 

 depolarization and tonic contraction. 5-Hydroxytryptamine causes prompt 

 relaxation of tonic contractions produced by acetylcholine and electrical 

 stimulation, without producing any changes in membrane potential; it 

 does not appear to rank as a natural inhibitory substance. Inhibitory fibres 

 probably act on local ganglion cells, reducing their activity (3, 18, 55a, 

 55b, 81, 82, 119, 124, 126a). 



Other Groups. Smooth muscles of other animals sometimes show two 

 kinds of contractions, quick and slow responses, with and without propaga- 

 tion of action potentials. Body-wall muscle of holothurians {Thy one) gives 

 propagated spike potentials, easily fatigued. This is a non-facilitating 

 muscle served by successive branches of the radial nerve in an overlapping 

 pattern (102). Pharyngeal retractors show two kinds of response, namely 

 fast and slow contractions, corresponding to which fast and slow potentials 

 have been recorded. The fast response is soon fatigued by repetitive 

 stimulation and shows no increment above fusion frequency. The slow 

 response, on the contrary, is facilitated to some extent under direct 

 stimulation (Thyone). With indirect stimulation via the radial nerve, 

 marked facilitation of the pharyngeal retractor muscle is obtained (Cucu- 

 marid). This facilitation is believed to take place between internuncial 

 neurones of the motor complex (98, 99, 103). 



Golfingia retractor muscles likewise give quick phasic and slow tonic 

 contractions. Associated with these responses are all-or-nothing fast 

 spikes, fatiguing on repetitive stimulation, and graded slow potentials, 

 which facilitate on repetitive stimulation (Fig. 9.16). Muscle fibres are 

 discrete and morphologically uniform; transmission of excitation through- 

 out the muscles is by way of parallel nerve fibres. The fast and slow muscle 

 potentials are preceded by nerve potentials, seemingly in large and small 

 nerve fibres, respectively. On present evidence, one group of nerves 

 elicits the fast contraction, another group the slow contraction. This leaves 

 unresolved the problem whether the two kinds of contractions in Gol- 

 fingia muscle (as in holothurian muscle) are produced in one and the 

 same muscle fibre. If this is the case, a situation may exist resembling 

 crustacean neuromuscular control, whereby two kinds of excitatory fibres 

 evoke fast propagated and slow non-propagated action potentials, res- 

 pectively (103, 104, 124, 125, 126a). 



Biochemical Observations. The shortening of muscle is dependent 

 upon the behaviour of a particular fibrous protein, actomyosin. Composed 

 of two fractions, actin and myosin, linked together within the muscle 

 fibre, actomyosin accounts for about half the dry weight of muscle. Myo- 



M.A. — 13* 



