Functional classes of primate corticomotoneuronal cells and their relation to active force.

I. The activity of corticomotoneuronal (CM) cells, identified by clear postspike facilitation (PSF) of rectified EMG activity in spike-triggered averages, was recorded in precentral cortex of monkeys making two types of ramp-and-hold wrist responses. “Auxotonic” wrist movements against elastic loads required active torque proportional to wrist displacement, and isometric responses involved ramp-and-hold torque trajectories with no wrist displacement. 2. On the basis of their firing pattern during the ramp-and-hold responses, all CM cells (ut = 135) could be classified into four types: phasic-tonic (59%), tonic (28%), phasic-ramp (8%), or ramp (5%). All CM cells (as defined by PSF) were active during the static hold period; “tonic” cells discharged at a constant rate, while “ramp” cells showed steadily increasing discharge during the hold period. During the dynamic phase of the response, i.e., during the torque ramp, the “phasic” cells exhibited an additional peak of activity exceeding the final tonic level associated with the hold period. Single CM cells exhibited the same response pattern in association with both isometric and auxotonic responses; thus, the torque trajectory rather than displacement or velocity seems to be the primary determinant of the CM cell’s response pattern. Other precentral cells, which discharged phasically at onset of movement but exhibited no tonic discharge during the hold period, did not produce PSF. The four types of CM cells did not differ significantly in the size of their muscle field, their response to passive movements, or their location in the cortex. 3. From response averages we measured the onset time of CM cell activity relative to the onset of EMG activity in its facilitated target muscle(s). The phasic-tonic and phasic-ramp cells began firing significantly earlier with respect to their target muscles (mean onset latencies: -71 and -63 ms, respectively) than the tonic and the ramp cells (+5 and + 101 ms, respectively). Since the peak PSF occurred about 10 ms after the spike, the initial discharges in many CM cells would contribute to subthreshold facilitation of their target motoneurons. 4. To investigate the relation between firing rate of CM cells and active torque, monkeys were required to exert different levels of active torque for the same displacement. During the hold period the tonic activity of each cell was a linearly increasing function of static torque over all or much of the range examined. The ratetorque slope of CM cells-i.e., the increase in firing rate per increase in static torque in the linear range-was similar for auxotonic responses (constant displacement) and isometric responses (zero displacement). The mean rate-torque slope of all extension CM cells (4.8 impulses l s-l/lo5 dyn cm) was about twice that of flexion-related cells (2.5 impulses . s-l/ lo5 dyn l cm). This difference may reflect different degrees of cortical influence on wrist flexor and extensor muscles, although mechanical factors cannot be entirely excluded. 5. Over the range of torque studied, only a few CM cells had appreciable nonzero torque thresholds for tonic firing. Most