Sensory Mechanisms in Paramecium

ABSTRACT Small, brief mechanical stimuli were delivered with a microstylus to the surface of Paramecium caudatum bathed in solutions of 1 mM-CaCl2, 1 mM KC1 +1 mM Tris HC1, pH 7 2. Stimulation of the caudal end produced a graded hyperpolarizing receptor potential which reached a maximum within 50 msec and decayed more slowly. 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. 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. The peak of the receptor potential was shifted about + 50 mV per 10-fold increase in extracellular K+. C−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+. 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.