Acceleration-sensitivity threshold of Physarum.
Free-living cells show distinct gravisensitivities and often use the gravity ('g') vector for their spatial orientation.  The rhythmic contractions of the ameboid Myxomycete (acellular slime mold) Physarum polycephalum are a sensitive parameter which can be modified by external stimuli.  Space experiments and ground-based 0 x g simulation studies established that the contraction period transiently decreases after a transition from 1 x g to 0 x g with a back-regulating process starting after 30 min.  For determination of the threshold of acceleration sensitivity, a slow-rotating centrifuge microscope (NIZEMI--Niedergeschwindigkeits-Zentrifugenmikroskop) was used, providing in space accelerations from 0 x g to 1.5 x g.  A stepwise acceleration increase revealed that the lowest acceleration level capable of inducing a response was 0.1 x g.  The response to the acceleration increase was an increase in contraction period, in contrast to a stimulus deprivation, which led to a period decrease.  The time schedule of the acceleration responses and back-regulating process seems to be fixed, suggesting that every acceleration being above the threshold can induce a complete response-regulation process.  The low acceleration-sensitivity threshold favors rather large and dense cell organelles as candidates for the gravity receptor in Physarum.
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DLR-Institute of Aerospace Medicine, Cologne, Germany.
Briegleb
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Wolke
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D000054:Acceleration; D000222:Adaptation, Physiological; D000818:Animals; D002498:Centrifugation / Q000295:instrumentation; D018466:Gravity Sensing; D010804:Physarum / Q000166:cytology / Q000502:physiology*; D012984:Software; D013026:Space Flight; D014893:Weightlessness