Microstimulation of the medullary reticular formation during fictive locomotion.

1. The present study was designed to determine the effects of microstimulation of the medullary reticular formation (MRF) on the locomotor activity of the cat in the absence of phasic afferent feedback from the limbs. To this end, both short (33 ms) and long (200 ms) trains of stimuli (trains of 0.2-ms pulses at 330 Hz, 35 microA) were applied at 43 loci in the MRF (P:6-12 mm; L:0.5-1.5 mm), and in 3 loci in the medial longitudinal fasciculus (P7.5, L < 0.5 mm) during fictive locomotion in the decerebrate and paralyzed cat. The locomotor pattern was monitored by recording the activity of representative flexor and extensor muscle nerves from each of the four limbs. 2. Short trains of stimuli evoked transient excitatory and/or inhibitory responses in extensor and flexor nerves of each limb that were incorporated into the locomotor pattern. In the majority of sites, excitatory responses were obtained in the motor nerves to both flexor and extensor muscles of the fore- and hindlimbs. The exception to this rule was the ipsilateral triceps, in which the predominant response was inhibitory. The amplitude of these responses was dependent on the time of the locomotor cycle at which the stimulus was delivered, and it was always maximum during the period of activity of the respective nerve. 3. The shortest latency response in the nerves to different muscles of the forelimb averaged between 5.6 and 7.3 ms; for the hindlimbs the values were between 6.9 and 9.3 ms. 4. Changing the depth at which the stimulation was applied in any one trajectory usually produced changes only in the amplitude of the evoked responses but occasionally also caused a change in the sign of these responses, especially in the most ventral regions of the MRF. 5. At 72% of the loci (31/43), short trains of stimulation also changed the duration of the activity in the recorded nerves. These changes were often (20/31 loci) sufficiently strong to alter the duration of the overall locomotor cycle. If one considers only the largest changes produced at each locus, stimulation during the period of ipsilateral extensor activity produced an average reduction in the ipsilateral locomotor cycle duration of 12.8 +/- 8.8% (mean +/- SD), whereas stimulation when the ipsilateral flexor nerve was active produced an average increase in locomotor cycle duration of 27.1 +/- 20.8%. 6. Long trains of stimuli produced similar but larger effects than the shorter trains and always reset the locomotor rhythm.(ABSTRACT TRUNCATED AT 400 WORDS)