Investigation into non-mono synaptic corticospinal excitation of macaque upper limb single motor units

There has been considerable recent debate as to relative importance, in the primate, of propriospinal transmission of corticospinal excitation to uppe r limb motoneurons. Previous studies in the anesthetized macaque monkey sug gested that, compared with the cat, the transmission of such excitation via a system of C-3-C-4 propriospinal neurons may be relatively weak. However, it is possible that in the anesthetized preparation, propriospinal transmi ssion of cortical inputs to motoneurons may be depressed. To address this i ssue, the current study investigated the responses of single motor units (S MUs) to corticospinal inputs in either awake (n = 1) or lightly sedated (n = 3) macaque monkeys. Recordings in the awake state were made during perfor mance of a precision grip task. The responses of spontaneously discharging SMUs to electrical stimulation of the pyramidal tract (PT) via chronically implanted electrodes were examined for evidence of non-monosynaptic, presum ed propriospinal, effects. Single PT stimuli (up to 250 muA; duration, 0.2 ms, 2 Hz) were delivered during steady discharge of the SMU (10-30 imp/s). SMUs were recorded from muscles acting on the thumb (adductor pollicis and abductor pollicis brevis, n = 18), wrist (extensor carpi radialis, n = 29) and elbow (biceps, n = 9). In all SMUs, the poststimulus time histograms to PT stimulation consisted of a single peak at a fixed latency and with a br ief duration [0.74 +/- 0.25 (SD) ms, n = 56], consistent with the responses being mediated by monosynaptic action of cortico-motoneuronal (CM) impulse s. Later peaks, indicating non-monosynaptic action, were not present even w hen the probability of the initial peak response was low and when there was no evidence for suppression of ongoing SMU activity following this peak (n = 20 SMUs). Even when repetitive (double-pulse) PT stimuli were used to fa cilitate transmission through oligosynaptic linkages, no later peaks were o bserved (16 SMUs). In some thumb muscle SMUs (n = 8), responses to PT stimu lation were compared with those evoked by transcranial magnetic stimulation , using a figure-eight coil held over the motor cortex. Responses varied ac cording the orientation of the coil: in the latero-medial position, single peak responses similar to those from the PT were obtained; their latencies confirmed direct excitation of CM cells, and there were no later peaks. In the posterio-anterior orientation, responses had longer latencies and consi sted of two to three subpeaks. At least under the conditions that we have t ested, the results provide no positive evidence for transmission of cortica l excitation to upper limb motoneurons by non-monosynaptic pathways in the macaque monkey.