Synaptic effects on recruitment gain: a mechanism of importance for the input-output relations of motoneurone pools?

The present theoretical analysis concerns quantitative aspects of the graduation of motoneuronal recruitment. It is pointed out that variations in the relative distribution of synaptic input to low- and high-threshold cells will serve to alter the ultimate magnitude of the threshold differences between them. These threshold differences determine the pool's 'recruitment gain', i.e. the ease with which a variation of excitatory drive to the pool may alter the number of active motoneurones. In this context, not only the command signal itself but also any steady synaptic 'background' activity is of importance, whereby the background effects on recruitment gain depend on the presence of differences in intra-pool distribution between these synapses and those of the command signal. Given the appropriate intra-pool distribution, even inhibitory background effects might serve to increase the recruitment gain. It is stressed that synaptic effects on recruitment gain should be taken into account when analyzing the input-output relations of motoneurone pools.