AUTOGENETIC MODULATION OF EXCITABILITY OF SINGLE VENTRAL HORN CELLS

Abstract

The monosynaptic responses of single ventral horn cells to shocks to the gastrocnemius nerve have been used (de-efferented cats) to measure the effect of a change of background by stretch (gastrocnemius muscle) on the level of excitability of these cells. Since single ventral horn cells deliver all-or-none discharges, a method had to be developed in order to measure excitability in terms of the probability of discharging in 10 trials (for the test shock in each selected position during stretch). With this prepn. it became possible to study facilitation and inhibition in active neurones as well as in neurones from the subliminal fringe as activated by stretch. It was shown that autogenetic inhibition cannot be identified with motoneurone refractoriness or subnormality, but is a genuine process that gives profound and lasting inhibition without the motoneurone having fired a single spike. This genuine inhibition was easily produced by stretch at high initial tension of the muscle. The effect of the discharge of a spike from the ventral horn cell on its capacity to fire a 2d spike was studied. This was found to be a function of its level of excitability, as detd. by stretch and initial tension of the muscle to which it belonged. If this level was low, a long pause, from 50 to 100 msec. or more, was necessary before the motoneurone responded to the test shock; if high, it responded to the test shock already after 2.5 msec. Concepts such as relative refractoriness and subnormality cannot be used to explain "inhibitions" without further specifications, because their quantitative significance is wholly dependent on the level of excitability of the neurone tested. The neurone is a strategic point for shifting the level of excitability, and its integrative behavior is intimately tied up with this property.