The present study has examined the detailed organization of the medullary reticular formation (MRF) as revealed by microstimulation (33-ms trains of 0.2-ms duration pulses at 330 Hz and 35 μA or less) in the intact, chronically implanted, unanesthetized cat. Stimulus-locked electromyographic (EMG) responses were recorded from flexors and extensors of each of the four limbs, as well as bilaterally from muscles of the neck and back, during stimulation of the same 592 loci that formed the basis of the preceding article. The thresholds of the responses were different for each group of muscles, with, on the average, the neck muscles being activated at the lowest range of currents, 13.8-16.5 μA; forelimb muscles at 16.9-17.9 μA; back muscles at 25.4-25.7 μA; and hindlimb muscles at 21.1-25.7 μA. Whereas stimulation within the MRF evoked movement of the head only to the stimulated side (preceding article), analysis of the EMG responses showed there was frequently bilateral activation of the neck muscles. Similarly, even though stimulation produced predominantly ipsilateral elbow flexion and contralateral elbow extension, most loci caused cocontraction of antagonistic muscles at these joints. Cocontraction was also frequently observed for the hindlimbs. Reciprocal activation of antagonistic muscles was less frequent but was observed in the ipsilateral forelimb as well as in both hindlimbs; it was never observed in the contralateral forelimb. Although excitatory responses were observed from widespread regions for all of the muscles under study, those regions of the MRF that evoked the strongest responses in each muscle showed a large degree of segregation. Muscles of the ipsilateral forelimb were most strongly activated from the rostrodorsal MRF, whereas muscles of the contralateral forelimb were most strongly effected by stimulation caudoventrally. Muscles of the hindlimbs were more strongly activated from the rostral brain stem, although with some exceptions. Responses in axial muscles were evoked from widespread regions of the brain stem but were concentrated further caudally than were the limb muscles. Excitatory responses were much more prevalent than inhibitory responses and were evoked from all regions of the MRF, including the most caudal and ventral areas. The shortest latency responses in each track were, on the average, as follows: 6.6-8.8 ms for the neck; 11.2-13.4 ms for the forelimbs; 13.8-14.2 ms for the back; and 15.9-17.2 ms for the hindlimbs. Inhibitory responses were also evoked from widely distributed regions, which were intermingled with those loci evoking excitatory responses. There was no evidence for any separation into discrete excitatory and inhibitory regions. These results suggest that the MRF is topographically organized, but without the clear segregation normally associated with this concept. Rather, there is a large overlap in the representation such that stimulation of a small group of neurons in any one region will evoke patterned responses involving activation of muscles of more than one limb, with the exact components of that pattern varying according to the specific region activated. Such an organization is what one might predict as being necessary for the production of the ordered patterns of muscle activity that are required for postural adjustments of axial and limb musculature in response to planned or reflexly produced movement.
