Factors which decide the excitability of a spinal motoneuron in the cat.

Single motoneurons and axons in the spinal cord of the cat anesthetized with Nembutal, Ravonal and ether were stimulated directly by linearly increasing and steady state currents applied through a microelectrode. Factors were studied which concern to the excitability of the motoneurons in due consideration of the role of miniature potential changes.1. Motoneurons were classified into two types according to the difference of the threshold-latency curve. In light anesthesia, however, almost all motoneurons was found to be the tonic type. This suggests that tonic and phasic classification could not apply to the motoneurons and to the muscles corresponding each other.2. A great number of motoneurons showed drifting of threshold in a wide range without sizable changes in the membrane potential. The value of threshold current intensity distributed in an area that was enclosed by three lines, which designated the IS and the SD threshold current intensity, and the minimal gradient of the initial segment.3. Evidence was given that spikes fired from the top of the spontaneous mini. EPSPs of 2 mV height superimposed upon the gentle slope of the currents and never fired in the absence of the miniature potentials even though the depolarization reached more than the rheobase.4. A current simulated of miniature potentials which was superimposed on a linearly increasing current converted a phasic type motoneuron into a tonic type with approximately a half of the former threshold current.5. The motoneuron that had not fired with simple stretch of the gastrocnemius nor with trapezoid currents, discharged by simultaneous application of the stretch and currents accompanying enhanced miniature potentials without sizable changes in the membrane potential.6. In light anesthesia, the mini. EPSPs increased in size by accompanying the local responses and triggered the spikes, but not increased in deep anesthesia. The rebound from the mini. IPSPs also seemed to concern to this triggering action.7. Administration of ether brought about a change in accommodation of motoneurons and disclosed minimal gradient requirement without changing the membrane potential, the spike height and the threshold current intensity for firing.8. The miniature potentials enhanced strikingly by administration of succynil choline chloride were found to show the same action in triggering of the spikes as the miniature potentials produced by natural stimuli.9. The rheobase of the motoneurons was of the order of 10-9 A in light anesthesia and 10-8 A in deep anesthesia. In such deep anesthesia, there was no change in the accommodation of the initial segment, but remarkable changes were observed in the accommodation of the soma. The recovery process from the anesthesia started in the order from the initial segment to the soma.10. The axon in the spinal cord of cats showed the minimal current gradient requirement to linearly increasing currents, but the accommodation of the axon did not show a serious change in deep anesthesia, in spite of that the threshold current intensity to a rectangular current was increased from the order of 10-9 A to 10-8 A.