RESPONSES OF MOTONEURONS OF DIFFERENT SIZES TO GRADED STIMULATION OF SUPRASPINAL CENTERS OF THE BRAIN

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Abstract
Trains of electrical stimuli were applied to motor regions in the brainstem, cerebellum, basal ganglia and cerebral cortex of the cat and the resulting discharges of motor neurons were recorded from fine filaments of ventral roots containing 2-5 responsive axons. In general, regardless of the site of stimulation, the neurons whose axons yielded the smallest impulses were discharged at the lowest threshold and the other units were recruited in order of increasing size. By comparing each responsive unit with all of the others in the same filament, a total of 396 pairs of motor neurons were collected. In 86% of the comparisons the smaller unit had the lower threshold; in 4% the 2 thresholds were indistinguishable and in 10% the larger unit had the lower threshold. The percentages did not vary significantly with the site of stimulation. Conclusions: the size of a motoneurons is the chief determinant of its excitability and the order of recruitment regardless of the source of excitation and the neural circuits which transmit it.