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The forelimb motor cortex was stimulated via chronically implanted microelectrodes whilst electromyographic (e.m.g.) responses were recorded from muscles in the contralateral forelimb in cats walking steadily at 0.5 m/s. The stimuli were brief trains of 0.2 ms pulses (11 pulses at 330 Hz), intensity 5-20 microA and e.m.g.s were recorded from the following muscles: biceps brachii, brachialis, long and lateral heads of triceps brachii, latissimus dorsi, cleidobrachialis, extensor digitorum communis, palmaris longus and flexor and extensor carpi ulnaris. During locomotion, stimulation at 20 microA readily elicited brief, short-latency changes in the normal locomotor patterns of activity in all muscles studied. The changes included production of e.m.g. at times in the step cycle when the muscles are normally inactive and brief augmentations or diminution of the normal locomotor e.m.g.s. Individual electrodes usually influenced several muscles, and muscles acting antagonistically about the same joint were sometimes co-contracted. The first effect on locomotor flexor muscles (i.e. muscles active in relation to the swing phase of the step cycle) was almost always excitatory and such effects were often phase-dependent, usually occurring when the muscle was normally active or about to become active. Extensor muscles were excited from some cortical loci but inhibited from others (inhibitions were necessarily detectable only when the muscles exhibited locomotor-related e.m.g.s). Some micro-electrodes elicited excitation during swing (when the extensors are inactive) but elicited inhibition during stance. In several muscles the latencies of the excitatory e.m.g. changes could be as short as 6 ms measured from the first pulse in the stimulus train. In flexors, but not in extensors, latencies fluctuated according to the timing of the stimuli relative to the step cycle. Reduction in stimulus intensity reduced the amplitude of the e.m.g. changes, the number of muscles influenced and often increased the latency. However, both excitations and inhibitions were sometimes evident at 5 microA and thresholds for excitatory responses were, over-all, substantially lower than in the resting animal. Longer trains of stimuli were capable of resetting the step cycle. Response thresholds were greatly increased after pyramidectomy. These findings support the view that the natural bursts of impulses discharged by pyramidal tract neurones during steady locomotion are likely to contribute to regulating forelimb muscle activity on a step-by-step basis.
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