NEUROMUSCULAR MECHANISMS 149 



lack of knowledge of the transfer from electrical to mechanical effects. In 

 this connection an observation on the spread of contraction in a living 

 muscle fiber is interesting. Matthaei and Tiegs (1955) photographed a 

 slowly spreading contraction wave in a slightly damaged spider muscle 

 fiber. The contraction originated from under an end-plate structure. It 

 first spread across the muscle fiber before it went in two directions away 

 from this region. 



Further clues with regard to this problem should be obtainable from the 

 effects of the inhibitory impulses. Fatt and Katz (1953c), studying es- 

 pecially the inhibition in the opener muscle of the hermit crab, have come 

 to a number of interesting conclusions. Depending on the magnitude of the 

 membrane potential they found that inhibitory impulses could have either 

 no electrical effect at all (which was the condition when the membrane 

 potential was "normal"), cause a hyperpolarization when the membrane 

 potential was low, or give a depolarization when it was higher than normal. 

 The time course of these inhibitory polarization effects was of the same 

 order, but slightly longer than that of the excitatory junctional potentials. 



Reductions of the excitatory junctional potential up to 90^ of its value 

 could be obtained when the inhibitory impulse preceded the excitatory one. 

 When an inhibitory stimulus was given during the course of an excitatory 

 junctional potential change, its decay time was speeded up. These results 

 certainly go far in explaining the inhibition of the junctional potentials by 

 the postulation of one inhibitor-receptor reaction, which changes the ion 

 permeability of the muscle-fiber membrane and competes with the action of 

 an excitatory transmitter. However, they offer no ready explanation for 

 mechanical inhibition which occurs without even a reduction of the facili- 

 tation of the junctional potentials ( Marmont and Wiersma, 1938 ; Wiersma 

 and Ellis, 1942). It is difficult to believe that the membrane could change 

 its electrical properties without at the same time influencing the facilita- 

 tion process. Hence it still seems likely that the main effect of inhibitory 

 stimulation is on a transmission process between the membrane changes 

 and the contractile process. 



It has been shown that spacing of the impulses in inhibitory stimulation 

 can have an effect similar to that of excitatory stimulation. Ripley and 

 Wiersma ( 1953) found that the same number of inhibitory impulses, when 

 given in pairs at a short interval, gave a more pronounced inhibition in the 

 opener muscle of the claw of the crayfish than when they were given all at 

 equal time intervals. 



That transmitters are involved seems quite certain. Concerning their 

 nature little is known as yet, which may well be due to the way in which the 

 nerve fibers end, sublemnally in the muscle substance. The endings may 

 thus be well protected from the direct influence of drugs. In accord with this 



