FUNCTIONAL SIGNIFICANCE OF CELL SIZE IN SPINAL MOTONEURONS

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Abstract
In decerebrate cats the order of recruitment of individual motor neurons discharging reflexly in response to stretch of the triceps muscle was compared with the amplitude of the impulses recorded from their axons in ventral root filaments. In 142 1/2 out of 165 pairs of units the smaller neuron, i.e., the one whose axon yielded the smaller impulses, had the lower threshold to stretch and the larger cell had the higher threshold. When stretch was released units "dropped out" of the discharge in the reverse order of their recruitment (225 out of 236 pairs). Units which responded tonically were, in general, smaller than those which responded phasically. A tentative explanation of the relation between size and excitability is advanced. "Fractionation", "discharge zone," "subliminal fringe," "facilitation" and "occlusion" are redefined briefly in terms of the sizes and excitability of the cells in a pool. It is pointed out that cell size determines the overall frequency of discharge or "usage" of a motor neuron, and some implications of this are discussed.

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