Fifty-nine rubromotoneuronal (RM) cells were identified in two rhesus monkeys on the basis of their postspike facilitation (PSpF) of rectified electromyographic (EMG) activity. These cells were studied in relation to a step tracking task requiring wrist movements between fixed target zones in flexion and extension. Movement away from a 0 position was opposed by spring-like loads (auxotonic). Additionally, nine cells were evaluated using an isometric task. Neuronal discharge could be divided into three basic components: background discharge in the absence of movement, phasic modulation during movement, and tonic modulation during sustained holding against external loads. 2. Four basic patterns of RM cell activity were observed in relation to ramp-and-hold wrist movements: phasic-tonic (44%), pure phasic (22%), pure tonic (2%), and unmodulated (24%). The discharge of unmodulated cells did not covary with movement parameters but, as with other RM cells, background discharge did increase in association with task performance. 3. The phasic discharge of RM cells led to the onset of target muscle EMG activity by an average of 89 +/- 82 ms (mean +/- SD, n = 104) in extensors and 88 +/- 74 ms (n = 30) in flexors. Target muscles are defined as ones showing PSpF of EMG activity. It was found that 94% of extensor and 87% of flexor RM cells discharged before or synchronous with the onset of target muscle EMG activity. 4. Thirty-one RM cells (53%) showed a tonic increase in cell activity during the static hold phase of the task. Twenty-three of these were tested for relations to static torque. Fifteen extension related cells and one flexion cell had significant, positive regression slopes for the relation between tonic discharge rate and static torque. The mean rate-torque slope for extension related cells was 160 Hz/Nm and 103 Hz/Nm for flexion related cells. These mean slopes are about one-third those of corticomotoneuronal (CM) cells. 5. Cell discharge rate was correlated with velocity and rate of change of torque (dT/dt) for 32 RM cells with a phasic component of discharge during movement. The peak increase in phasic discharge above tonic firing rate (PDI, peak dynamic index) was significantly correlated only with velocity in eight cells and only with dT/dt in five cells. The phasic discharge of four additional cells was correlated with both velocity and dT/dt, but for three of these cells, the correlation was stronger for velocity. The mean slope for the relation between velocity and PDI was 0.31 Hz.deg-1.s-1.(ABSTRACT TRUNCATED AT 400 WORDS)
