The muscle specimen
was pinned on the silgard block in deep experimental chamber. Fresh crayfish
saline containing 0.1-1 microM TTX was continuously applied onto
TTX stops the spontaneous heart beat by blocking voltage-gated Na channel in the cardiac ganglion neurons (cardiac pacemaker).
In some experiments, saline containing TTX and various pericardial organ neurohormons (Oct, 5-HT, Dop, Proc., etc.) was applied to examine their modulatory effects onto the muscle fibers.
Simultaneous recordings of the membrane
potential and the isometric tension of the muscle were performed using
capilary microelectrodes and a strain guage mechano-electric
transducer. The muscle fiber was stimulated by injecting
square current pulses through another microelectrode.
Vm: membrane poetential.