How the size of motoneurones determines their susceptibility to discharge.

The susceptibility of motoneurones to discharge by physiological inputs varies inversely with their size. This relationship has far-reaching consequences for the control of muscular tension, and also accounts for significant differences in the properties of muscle fibres and motor units. The probable basis for the relationship is that the amplitude of excitatory postsynaptic potentials (e.p.s.ps) is also correlated inversely with cell size, but no satisfactory explanation for this finding is known. We now describe experiments which indicate that afferent impulses invade the relatively simple ramifications of fibres going to small motoneurones more completely than the extensive arborisations going to large cells. This results in activation of a higher percentage of the synaptic endings on small cells, which produces larger e.p.s.ps in them and accounts for their greater susceptibility to discharge.