Control of forelimb muscle activity by populations of corticomotoneuronal and rubromotoneuronal cells.

We review and synthesize evidence on the activity of corticomotoneuronal (CM) and rubromotoneuronal (RM) cells and single motor units in forearm muscles in monkeys performing alternating wrist movements. The CM and RM cells were identified by post-spike facilitation of rectified forelimb EMG activity. RM cells facilitated more muscles per cell (mean: 3.0 of 6 synergist muscles) than CM cells (2.4/6). Both groups had "reciprocal" cells which also suppressed antagonists of their facilitated target muscles. Unlike CM cells, some RM cells cofacilitated flexor and extensor muscles (5.8 or 12 muscles). During performance of a standard ramp-and-hold force tracking task the firing patterns of CM and RM cells, as well as single motor units, fell into distinct response types. Each population had phasic-tonic and tonic cells. Unique to the CM population were cells whose discharge increased during the static hold period; unique to RM cells were bidirectionally responsive and unmodulated neurons. Many motor units showed decrementing discharge. To estimate the ensemble activities of these populations the response histograms of different cells were summed (with force ramps aligned) in proportion to the relative frequency of each cell type. The population response histogram of CM cells was phasic-tonic, consistent with the predominant response type. The population response of RM cells was also phasic-tonic, but showed a shallower phasic modulation relative to discharge that was sustained during both directions of movement. The population histogram of motor units of a muscle was proportional to the average of rectified multiunit EMG, and typically exhibited decrementing activity during the static hold. The effects of excitatory postsynaptic potentials (EPSPs) on firing probability of motoneurons previously documented in intracellular studies are combined with the mean firing rates in the population histograms and the known amplitudes of CM-EPSPs and RM-EPSPs to infer the relative contributions of the supraspinal cells to tonic discharge of active motoneurons. This analysis suggests that for intermediate levels of force, the CM cells would increment motoneuron discharge by about 9 impulses/second (i.p.s.) and RM cells by about 2.4 i.p.s. The analysis also reveals differences in the population activity of CM and RM cells compared to their target motoneurons, which may be due to other input cells and to recruitment properties of motoneurons.