EFFECTS OF ELECTROTONUS ON THE ELECTRICAL ACTIVITIES OF SPINAL MOTONEURONS OF THE TOAD

Abstract

1. Using single intracellular microelectrodes in a bridge circuit, the effect of electrotonus on the activity of motoneurons was explored in excised spinal cords of toads.2. A linear relationship was found between the applied current and the changes of the membrane potential, usually within a range of ±2.5 × 10-9 A, which caused shifts of the membrane potential by ±6 to 8 mV. With stronger currents, the rectifying action of the membrane was usually observed.3. The overshoot of the spike antidromically evoked remained almost constant, either when the membrane was depolarized or hyperpolarized to a certain extent. Stronger such polarizations, however, brought about insufficient or over-compensation.4. The effects of electrotonus on the afterpotentials were studied with similar results to those known from other cells.5. The size of PSP's was reduced by depolarization and augmented within a certain range of hyperpolarization. Monosynaptic EPSP was ineffective in evoking motoneuron spikes in the normal state, but sometimes became effective when the membrane had been suitably depolarized.6. With the combined application of two long square pulses, the motoneurons were excited directly under various background polarizations. Depolarized motoneurons showed responses only with short latency and a rather constant threshold depolarization. On the contrary, hyperpolarized motoneurons responded even with long latencies, the spike arising from a level of depolarization which becomes markedly higher with an increase in the latent time. The accommodation of motoneurons is discussed in connection with these findings.