The characteristics of the self-sustained oscillation in the plasmodial strand of Physarum polycephalum have been investigated in one steady and two transient conditions. An isolated Physarum strand changes its length periodically when it is suspended. In the behaviour of the self-sustained oscillation under the conditions, we provide the first demonstration that the changes in the periods of the oscillation can be ascribed to effects on the shortening phase, while the lengthening periods are almost unaffected. This result means that the asymmetric self-sustained oscillation of the Physarum strand is composed of an active contracting process, presumably due to actin filaments and myosin-like molecules in the strand, and a passive lengthening process which is merely an extension of the strand under a load.
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