Phase-dependent responses evoked in limb muscles by stimulation of medullary reticular formation during locomotion in thalamic cats.

Electromyographic and kinematic responses of all four limbs were studied when loci within the medullary reticular formation (MRF) were stimulated (30-ms train of 0.2-ms pulses at 300 Hz, strength 35 microA) during treadmill locomotion in spontaneously walking thalamic cats. Responses could be evoked in flexor or extensor muscles of any given limb by such stimulation, depending on the time during the step cycle at which the stimulus was delivered. Stimulation normally excited flexor muscles but could either excite or inhibit extensor muscles depending on the exact position of the electrode. Excitatory responses in extensor muscles were often followed by a short period of inhibition of activity. The responses in muscles of the opposing limbs of the same girdle were, in general, reciprocally organized. For instance, a stimulus delivered during the swing phase of the ipsilateral limb normally evoked excitatory responses both in flexor muscles of that limb and in extensor muscles of the contralateral limb. The same stimulus delivered during the stance phase of the ipsilateral limb evoked excitatory responses in ipsilateral extensor muscles and in contralateral flexor muscles. Responses were also observed at the same time in fore- and hindlimbs that were well organized with respect to the locomotor cycle. Seventy-five percent of all responses occurred within 8-20 ms of the onset of the stimulus train. Responses evoked in muscles of the opposing limbs of one girdle (e.g., a flexor of one limb and an extensor of the other) had similar latencies, suggesting that the responses were synchronously organized on both sides of the body rather than one being a consequence of the other. Although the majority of responses in a given muscle were elicited during its period of activity, responses could occasionally be evoked when there was no activity in that muscle or could be absent despite activity in the muscle. The short trains of stimuli were normally potent enough to affect the limb trajectory, which reflected changes in the onset or the offset of the activity of most muscles. Thus the stimuli effectively changed both the duration of the period of activity in these muscles and the overall step cycle. Longer trains of stimuli (200 ms) markedly amplified these changes to the point of completely resetting the locomotor rhythm.(ABSTRACT TRUNCATED AT 400 WORDS)