Trigeminal motoneurons of the guinea pig brain stem slice preparation were studied using intracellular recording techniques. The voltage response to a 100-ms constant-current pulse was studied and a population of cells was found which did not exhibit sag or overshoot of their voltage response to a pulse of hyperpolarizing current of < 1 nA but did exhibit both phenomena when a current pulse of > 1 nA was used. The sag and overshoot observed with large-current pulses were reduced or blocked when 4 mM CsCl was added to the bathing solution. This observation supports the hypothesis that these phenomena were due to the voltage- and time-dependent activation of the Q-current. The method of peeling exponentials was then used to correct raw voltage data from cells in which the Q-current was present. The mean membrane time constant was within 1% and the mean input resistance was within 2% of the means for these parameters when measured in these same cells under conditions in which the Q-current was absent. We conclude from these experiments that the method of peeling exponentials is valid for obtaining estimates of the membrane time constant and input resistance from cells that exhibit sag and overshoot due to voltage- and time-dependent changes in the magnitude of the Q-current.
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