ACCOMMODATION IN MOTONEURONS AS MODIFIED BY CIRCUMSTANTIAL CONDITIONS

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Abstract
Single spinal motoneurons of the cat were stimulated with linearly increasing currents flowing through an intracellular micropipette, and the changeability of threshold-latency curve depending upon circumstantial conditions was studied in one and the same motoneuron. In a motoneuron, the threshold of which was relatively constant in wide range of the current inclination under slight anesthetization, an additional administration of Nembutal increased the threshold to slowly rising current but hardly changed the rheobase and the resting membrane potential, so that the threshold-latency relation was depicted by an upwards convex curve. Slight stretch of the innervating and/or synergistic muscle of a motoneuron lowered the threshold to slowly rising current without appreciable change of the rheobase and the resting membrane potential. Consequently, this procedure produced a threshold-latency curve of a constant level. A similar but weaker effect was often elicited by stretch of the antagonistic muscle. Higher amplified recording of the motoneuronal membrane potential revealed that such slight stretch of synergists or antagonists produced relatively pure EPSP''s or IPSP''s of less than 1-2 mV, converging irregularly and asynchronously upon the motoneuron. A small rectangular current step superimposed upon linearly increasing current, applied hundreds of milliseconds after the onset of the latter current, had a marked triggering effect upon spike initiation of the motoneuron; and therefore, fired it with a lower current intensity than that of linearly increasing current alone. Influences of anesthetics and background synaptic bombardments upon the accommodative attitude of motoneurons were discussed in relation to the triggering effect.

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