Sensory Mechanisms in Paramecium

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Abstract
1. Small, brief mechanical stimuli were delivered with a microstylus to the surface of Paramecium caudatum bathed in solutions of 1 mM-CaCl2, 1 mM KCl + 1 mM Tris HCl, pH 7.2. 2. Stimulation of the caudal end produced a graded hyperpolarizing receptor potential which reached a maximum within 50 msec and decayed more slowly. 3. The input conductance at the peak of the caudal receptor potential increased to a value of at least 6 times that of the resting membrane. 4. The potential diminished in amplitude when the membrane was hyperpolarized by injected d.c. current, and reversed sign with sufficient hyperpolarization. The reversal potential in a solution of 1 mM-CaCl2 + 4 mM-KCl was -37 mV, while the resting potential was -20 mV. 5. The peak of the receptor potential was shifted about + 50 mV per 10-fold increase in extracellular K+. Cl- and Ca2+ and other cations produced little or no shift in the potential peak of the response. It is concluded that mechanical stimulation of the caudal surface produces a local increase in conductance, predominantly to K+. 6. Extracellular tetraethylammonium converts the normally hyperpolarizing receptor potential to a depolarization similar to the potential produced in response to mechanical stimulation of the anterior surface. The TEA effect is antagonized by calcium ions.