Gravikinesis in Paramecium: Theory and isolation of a physiological response to the natural gravity vector

1.	We have investigated a physiological component of the gravitaxis of Paramecium using established mechanisms of ciliate mechanosensitivity. The horizontal, up and down swimming rates of cells, and the sedimentation of immobilized specimens were determined. Weak DC voltage gradients were applied to predetermine the Paramecium swimming direction.
2.	An observed steady swimming rate is the vector sum of active propulsion (P), a possible gravity-dependent change in swimming rate (), and rate of sedimentation (S). We approximated P from horizontal swimming. S was measured after cell immobilization.
3.	Theory predicts that the difference between the down and up swimming rates, divided by two, equals the sum of S and . is supposed to be the arithmetic mean of two subcomponents, a and p, from gravistimulation of the anterior and posterior cell ends, respectively.
4.	A negative value of (0.038 mm/s) was isolated with a(0.070 mm/s) subtracting from downward swimming, and p(0.005 mm/s) adding to upward propulsion. The data agree with one out of three possible ways of gravisensory transduction: outward deformation of the mechanically sensitive lower soma membrane. We call the response a negative gravikinesis because both a and p antagonize sedimentation.