Vorticella contracted (i.e. shrinkage of the cell body and coiling of the stalk) in response to being touched with a microneedle.
The threshold excursion of the microneedle required to evoke a contraction was smallest on the cell body. On the stalk, it was larger in regions farther from the cell body.
Hitting the stalk did not evoke a contraction if the stalk was mechanically clamped in a region between the site of the hit and the cell body.
Rapidly drawing a small portion of the cell body into a micropipette by suction evoked a contraction, whereas a similar stimulus applied to the stalk did not.
The threshold depression of the surface membrane of the cell body required to evoke a contraction was inversely proportional to the rate of depression.
Tilting the stalk of a specimen detached from its substratum evoked a contraction. The threshold degree of tilting was inversely proportional to the angular velocity of tilting.
Tilting the stalk is assumed to cause a localized depression of the surface membrane of the cell body around the stalk.
We concluded (1) that the cell body is mechanosensitive and is the site where contractions are initiated; (2) that hitting the stalk evokes a contraction because the hit exerts a mechanical effect on the cell body; and (3) that the rate of expansion of the membrane of the cell body is responsible for activation of a hypothetical mechanoreceptor mechanism which initiates a contraction.
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