SPONTANEOUS ALL-OR-NOTHING ACTION-POTENTIALS IN THE CILIATE BURSARIDIUM DIFFICILE

The electrical membrane properties and the swimming behaviour of the f reshwater ciliate Bursaridium difficile were studied by current clamp recordings and video analysis. The resting membrane potential was -45 +/- 6 mV (mean +/- SD, n = 80), and the input resistance and membrane capacitance were 109 +/- 42 megaohms (Mn) (n = 63) and 457 +/- 150 pic ofarads (pF) (n = 42), respectively. Based on an estimated surface are a of 6.8 x 10(-4) cm(2), the corresponding specific membrane resistanc e and capacitance are 7.3 x 10(4) Omega x cm(2) and 0.7 mu F/cm(2). Bu rsaridium difficile generates spontaneous, all-or-nothing action poten tials with a well-defined threshold in normal medium. The spontaneous firing frequency was 0.22 +/- 0.06 Hz (n = 80). The maximum rate of ri se of the action potentials was less than 1 V/s, and they displayed a prolonged plateau phase (0.5-1 s). The action potentials were abolishe d in nominal Ca2+-free solution and are thus Ca2+-spikes. The swimming pattern of Bursaridium in homogeneous surroundings is composed of for ward swimming periods interrupted by regular, short periods of backwar d swimming followed by a change in the forward swimming direction. The turning frequency corresponded to the spontaneous firing frequency, a nd only forward swimming was observed in nominal Ca2+-free solution. T he periods of backward swimming activity are thus linked to the sponta neous action potentials.