388 THE BIOLOGY OF MARINE ANIMALS 



is struck at the myoneural junctions between antagonistic transmitter 

 effects (53, 122, 127). 



Crustacean muscles are capable of two kinds of contraction, one a fast 

 synchronous action, the other a slow tonic contraction which develops 

 gradually and smoothly. In many muscles the fibres receive branches of 

 two or more motor axons, individual stimulation of which evokes fast or 

 slow contractions (fast and slow nerve fibres) (Fig. 9.12). A single impulse 

 in the fast fibre sometimes causes a twitch contraction (e.g. closer muscle of 

 Stenopus claw); more often several (closer muscle of Homarus claw) or 

 many impulses in the fast fibre are necessary to produce a twitch (Cancer, 

 etc.). Even in homonomous muscles of the same animal there may be 

 clearly marked differences in neuromuscular regulation. Thus, the closer 

 muscles of legs 2-5 of the lobster give a twitch contraction to a single 

 pulse in the fast fibre, the cutter claw requires two impulses, and the crusher 

 claw three. Normally many impulses are needed in the slow fibre to cause 

 contraction — a slow smooth tetanus. Regulation of the rate and strength 

 of contraction depends upon the effect of successive impulses on the 

 neuromuscular junction. 



Peripheral facilitation in crustacean muscles involves summation of 

 graded local reactions which become intensified with each additional nerve 

 impulse. In the absence of a propagated muscle impulse there is a progres- 

 sive growth of local electrical responses (e.p.p's) in the vicinity of the nerve 

 endings. With intracellular recording, end-plate potentials are found to be 

 distributed over the whole length of the muscle fibre. These non-propagated 

 potentials are accompanied by local contractions, controlled continuously 

 in rate and strength by the number and frequency of motor impulses. 

 Normally the muscular contraction is built up by non-propagated res- 

 ponses. But by stimulation of the fast axons at a sufficiently high frequency, 

 propagated action potentials are produced, associated with vigorous 

 twitches of whole muscle fibres. These spike potentials appear to have as 

 their consequence the elimination of inequalities in the strength of res- 

 ponse along the length of the muscle fibre. Thus, the fast motor response in 

 crustacean muscle may be local or propagated, depending on the rate of 

 nervous stimulation (38, 39, 126). 



Inhibitory fibres run together with the excitatory fibres to the muscles 

 (Figs. 9.12, 9.13). The inhibitory impulse is capable of interrupting trans- 

 mission of motor activity at two separate stages, i.e. between the motor 

 impulse and production of the e.p.p. (a-action), and between the e.p.p. 

 and local contraction of the muscle (/^-action). In both actions the in- 

 hibitory influence is restricted to the vicinity of the motor-nerve endings. 

 The functional significance of the inhibitory fibres is not well understood. 

 It is noteworthy that each of the inhibitory fibres usually supplies many 

 different muscles, and hence has a widespread effect. However, there are a 

 few inhibitors with restricted distribution. Such may allow differentiation 

 of function in certain muscles that have a common motor axon, e.g. opener 

 and stretcher muscles of the decapod limb (62, 123, 125). 



