Dr. Stokes et Rk. Josephson, CONTRACTILE PROPERTIES OF A HIGH-FREQUENCY MUSCLE FROM A CRUSTACEAN .2. CONTRACTION KINETICS, Journal of Experimental Biology, 187, 1994, pp. 275-293
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.