CONTRACTILE PROPERTIES OF A HIGH-FREQUENCY MUSCLE FROM A CRUSTACEAN .2. CONTRACTION KINETICS

1.The flagella (small appendages on the maxillipeds) of the crab Carci nus maenas beat regularly when active at about 10Hz (15 degrees C). Th e beat of a flagellum is due to contraction of a single small muscle, the flagellum abductor (FA). The optimal stimulus frequency for tetani c contraction of the FA was about 200 Hz. When the muscle was stimulat ed at 10 Hz with paired stimuli per cycle, the interstimulus interval that maximized peak force was 2-4 ms, which corresponded well to the i nterspike intervals within bursts recorded from motor axons during nor mal beating. 2. Contraction of the isolated FA showed pronounced neuro muscular facilitation and many stimuli were needed to activate the mus cle fully. The dependence on facilitation in isolated muscles appeared to be greater than that in vivo. It is suggested that neuromodulators in the blood of the crab enhance neuromuscular transmission and reduc e the dependency on facilitation in intact animals. 3. The FA had a na rrow length-tension curve. Tetanic tension became vanishingly small at muscle lengths less than about 90% of the maximum in vivo length. The maximum length change of the muscle during in vivo contraction was ab out 5%. 4. The maximum isometric force of the FA was low (about 6Ncm(- 2)) but its shortening velocity was high. Vm, the maximum shortening v elocity determined from isotonic shortening, was 4.0 muscle lengthss(- 1); Vo, the maximum shortening velocity from slack test measurements, was about 8 lengthss(-1). 5. The structure and physiology of the FA ar e compared with those of locust flight muscle, chosen because it too i s a muscle capable of long-duration, high-frequency performance.