Mechanical properties of a slow muscle in the cockroach.

The mechanical properties of the metacoxal muscle, 177d, in the cockroach, Periplaneta americana, was investigated. The muscle exhibited a mean resting tension of 2.6 +/- 1.3g SD. Neurally evoked tension summed with the resting tension and the relaxation phase of the evoked tension varied from less than 1 s to several minutes. This residual tension varied not only in duration but also in amplitude. Stimulation of inhibitory axons increased the rate of relaxation and thereby abolished the residual tension. However, inhibitory stimulation never reduced the resting tension. Stimulation of the main leg nerve at several times the threshold of the inhibitory axons could evoke residual tension. Recording of synaptic potentials from the two histochemically different fiber types (dorsal and ventral groups) revealed large hyperpolarizations in the ventral fibers and decreased duration and amplitude of excitatory potentials in the dorsal fibers. These results suggest that there are a variety of ways in which tension can be evoked, maintained, and controlled in these muscles.