Recruitment order among motoneurons from different motor nuclei

Recruitment order among motoneurons from different motor nuclei. J. Neuroph ysiol. 81: 2485-2492, 1999. The principles by which motoneurons (MNs) inner vating different multiple muscles are organized into activity are not known . Here we test the hypothesis that coactivated MNs belonging to different m uscles in the decerebrate cat are recruited in accordance with the size pri nciple, i.e., that MNs with slow conduction velocity (CV) are recruited bef ore MNs with higher CV. We studied MN recruitment in two muscle pairs, the lateral gastrocnemius (LG) and medial gastrocnemius (MG) muscles, and the M G and posterior biceps femoris (PBF) muscles because these pairs are coacti vated reliably in stretch and cutaneous reflexes, respectively. For 29/34 M G-LG pairs of MNs, the MN with lower CV was recruited first either in all t rials (548/548 trials for 22 pairs) or in most trials (225/246 trials for 7 pairs), whether the MG or the LG MN in a pair was recruited first. Intertr ial variability in the force thresholds of MG and LG MNs recruited by stret ch was relatively low (coefficient of variation = 18% on average). Finally, punctate stimulation of the skin over the heel recruited 4/4 pairs of MG-L G MNs in order by CV. By all of these measures, recruitment order is as con sistent among MNs from these two ankle muscles as it is for MNs supplying t he MG muscle alone. For MG-PBF pairings, the MN with lower CV was recruited first in the majority of trials for 13/24 pairs and in reverse order for 9 /24 pairs. The recruitment sequence of coactive MNs supplying the MG and PB F muscles was, therefore, random with respect to axonal conduction velocity and not organized as predicted by the size principle. Taken together, thes e findings demonstrate for the first time, that the size principle can exte nd beyond the boundaries of a single muscle but does not coordinate all coa ctive muscles in a limb.