Are receptor-activated ciliary motor responses mediated through voltage or current?

Abstract

THE locomotion of ciliate protozoa is effected by cilia which modify the direction and frequency of their cyclic beating in response to sensory input from the environment^1,2. In Paramecium and Stylonychia mechanical stimulation of the anterior end of the cell induces a Ca²⁺-dependent depolarising receptor potential which triggers a regenerative Ca²⁺ response^3,4. Ca²⁺ accumulating in the ciliary space activates the axonemal motor system⁵ so that the normal (posterior) direction of the ciliary power stroke is reversed and the frequency is augmented⁶. Posterior mechanical stimulation of the cell elicits a K⁺-dependent hyperpolarising receptor potential followed by augmentation of ciliary frequency without reversal^4,7, a mechanism which is less well understood⁸. Because shifts in voltage are associated with loops of electric currents across the membrane, we have examined the crucial signals transmitted from the mechanoreceptive regions to the cilia. We report here that these signals are potentials of either polarity. This is the first report of intracellularly recorded receptor currents.