  Coordination of cytoplasmic flows on large scales in space and time are at
the root of many cellular processes, including growth, migration or division.
These flows are driven by organized contractions of the actomyosin cortex. In
order to elucidate the basic mechanisms at work in the self-organization of
contractile activity, we investigate the dynamic patterns of cortex contraction
in true slime mold \textit{Physarum polycephalum} confined in ring-shaped
chambers of controlled geometrical dimensions. We make an exhaustive inventory
of the different stable contractile patterns in the absence of migration and
growth. We show that the primary frequency of the oscillations is independent
of the ring perimeter, while the wavelength scales linearly with it. We discuss
the consistence of these results with the existing models, shedding light on
the possible feedback mechanisms leading to coordinated contractile activity.

Adaptation and Self-Organizing Systems;Biological Physics