Spatial Distribution of Cortical Output Zones Affecting Combinations of Forelimb Muscles in the Monkey

We investigated the dimensions of output zones affecting specific combinations of forelimb muscles in the precentral “motor” cortex of macaque monkeys. Single-pulse intracortical microstimulation (S-ICMS) was used to evoke subthreshold effects in multiple wrist and finger muscles. S-ICMS consisted of 15 Hz stimulus trains at low intensity (5 μA) to avoid spread of effects due to temporal summation and to activate circumscribed foci of cortical cells in the bank of the precentral gyrus.

To detect subthreshold effects on active muscles, stimuli were delivered during wrist movements against elastic loads, and stimulus-triggered averages of rectified electromyographic (EMG) activity of 12 identified forelimb muscles were compiled. These averages detected statistical increases and decreases in averaged rectified EMG activity termed poststimulus facilitation and poststimulus suppression, respectively (7, 8). The “muscle profile” of a cortical output site was defined as the distribution and relative magnitude of these effects evoked in the recorded muscles. To document the spatial extent of a cortical output zone producing a particular “muscle profile,” we delivered S-ICMS at successive sites along electrode tracks in the precentral bank, tangential to and near layer V cells.

In some cases the electrodes encountered cortical cells whose post-spike effects on muscles were also documented by spike-triggered averages of EMG activity. Near cells that had post-spike effects, S-ICMS evoked a similar profile of effects on these muscles, over distances of several hundred microns from the location of the cells with post-spike output effects.

The locations of tracks were marked by electrolytic lesions at specific depths and were identified in subsequent histological reconstructions. The physiological effects evoked from different sites were correlated with the locations of Nissl-stained cortical cells and corticospinal cells labeled by horseradish peroxidase (HRP) transported from the cervical spinal cord.

The “muscle profiles” of poststimulus effects elicited by 5-10 μA S-ICMS applied at successive sites were the same for multiple neighboring sites extending over tangential distances of about 1 mm. S-ICMS applied at 300 to 500 μm intervals evoked significant poststimulus facilitation or suppression effects over a range of 650-1250 μm. Assuming an effective excitation radius of 65 μm for 5-μA current pulses, the lower limit of the mean tangential dimension of cortical output zones on the precentral bank (n = 17) was estimated to be 800 μm.

A specific muscle could be affected, either singly or in combination with synergist muscles, from aggregate regions of output sites extending about 5 x 4 mm mediolateral x dorsoventral. These regions were comparable in size to output areas containing all the corticomotoneuronal (CM) cells that produced postspike facilitation in that muscle.

The cortical entry points of tracks with CM cells producing postspike facilitation in specific muscles were reconstructed for three monkeys. In two monkeys, the flexor and extensor CM cells were thoroughly intermingled; in the third, the flexor CM cells were preferentially located medially and showed less overlap with the region containing extensor CM cells.

These results indicate that each motor cortex site represents a different combination of muscles. The effects evoked from cortical sites separated by several hundred microns invariably involved different profiles of muscle activity. The muscle fields of remote CM cells were rarely identical, while the fields of neighboring CM cells were often similar. Given the number of unrecorded muscles, we conclude that primate motor cortex is a mosaic of output sites representing forelimb muscles in different combinations.