[The mode of excitation of cat motoneurons]

1. Effects of afferent nerve stimulation on the waveform of the IS-spike were studied intracellularly in 101 motoneurons of cats which were either anesthetized with sodium pentobarbitone or decerebrated at the intercollicular level. 2. In 78 motoneurons repetitive or double shock stimulation of afferent nerves (the L 7 or S 1 dorsal root, biceps-semitendinosus nerve and tibial nerve) could produce 2-4 kinds of IS-spikes, as distinguished by differences in waveform. In 34 motoneurons 2-3 small potentials were superimposed on the smallest IS-spike with progressively longer latencies to form a larger IS-spike. These potentials were thought as dendritic in origin because only the dendrites are known to be capable of exhibiting multiple spikes and in 14 motoneurons dendritic spikes were actually shown to exist. 3. In 13 motoneurons following a repetitive stimulation of a dorsal root there occurred an increase in excitability of the membrane generating the IS-spike (the IS-membrane) in the absence of any appreciable changes in the resting membrane potential. This would indicate that the IS-membrane is subject to plastic changes or the IS-spike itself also originates in dendrites which are known to exhibit such changes.