598 



Comparative Animal Physiology 



inhibition no contraction occurs, but facilitation may continue to be built 

 up as shown on release from inhibition. 



The inhibitory fiber alone causes no observable electrical response in the 

 muscle. When both slow excitor and inhibitor fibers are being stimulated 

 repetitively (slow excitor as in crayfish abductor) the inhibitor fiber may 

 cause diminution of both contraction and action potential if its impulses ar- 

 rive less than 10 msec, (maximum 2-4 msec.) before the excitor impulses 

 (supplemental or electrical inhibition); or the inhibitor may cause reduction 

 of contraction only, if its impulse precedes the excitor impulses by longer 

 times or arrives simultaneously (simple or mechanical inhibition).^^- 



The distinction between fast and slow responses is quantitative. The op- 

 timum frequency in the slow fiber is lower than in the fast. At low frequen- 

 cies (40-50/sec.) the difference in response between the two fibers is slight, 

 but at high frequencies (over 100/sec.) the fast contraction is greater. 22» The 

 difference between the fast and slow contraction of the same muscle de- 

 creases in the genus series: Camharus > Randallia > Blepharipoda ^Can- 

 cer, so that in Cancer there is little difference between the two except in 



A B 



Fig. 230. Inhibition of slow contraction (A) and of the fast contraction (B). Records 

 from top downward are myogram, motor stimulus, timer (seconds), and inhibitor 

 stimulus. Note greater inhibition of slow contraction, postinhibitory facilitation of fast 

 contraction (from v. Harreveld & Wiersma^"). 



latency. ^-''' ^^° The flexor of the carpopodite of Panulirus has five axons: 

 four exciters and one inhibitor. ^^"^ The amount of facilitation required to 

 elicit a contraction varies with the species. The ratio of optimum inhibitory 

 to excitatory frequencies is fairly constant for a given muscle system.^-^ For 

 example, in the slow bender of Pachygrapsiis three excitor impulses are sup- 

 pressed by one inhibitor, in the slow closer response of Camharus one excitor 

 requires three inhibitor impulses, whereas the fast closers of these species re- 

 spond to single excitor impulses and are not inhibited. In several instances 

 one herve axon, either excitor or inhibitor, serves two muscles. Often inner- 

 vation is not reciprocal, and it is difficult to understand the function of the 

 inhibitors.''^^ 



