Mechanical stimulation of Nitella often produces responses resembling propagated negative variations but traveling faster and going past a killed spot. They appear to result from a mechanical disturbance traveling along the cell and stimulating each spot it touches (i.e. the stimulus itself travels). They are called mechanical variations to distinguish them from propagated negative variations.
A mechanical disturbance may cause an irreversible change (death wave), but in traveling along the cell it may lose intensity and then produce only a reversible response (mechanical variation) which may eventually change to a propagated negative variation.
The all or none law does not apply to incomplete mechanical variations, for the response varies with the strength of the stimulus.
