The present article describes the various patterns of movement evoked in the limbs and neck by microstimulation (33-ms trains, 330 Hz, 0.2-ms pulses at ≤35 μA) of the medullary reticular formation (MRF) of seven chronically implanted, unanesthetized, intact cats. Altogether 878 loci were stimulated in 83 penetrations. However, as stimulation in the more lateral regions of the MRF was less effective, the results are based on stimulation in 592 loci made in 56 penetrations at distances of between 0.5 and 2.5 mm lateral to the midline. Of these 592 loci, movement of one or more parts of the body was evoked from a total of 539 (91%) sites. Most of these movements were compound in nature, involving movement of one or more limbs as well as the head. Discrete movements were observed only with respect to the head; limb movements were always accompanied by head movement. In addition, hindlimb movements were always accompanied by forelimb movements, although the inverse was generally not true. The most common effects of the stimulation were as follows: a turning of the head to the ipsilateral side (79% of stimulated sites); flexion of the ipsilateral elbow (41%); and extension of the contralateral elbow (45%). Effects in the hindlimbs were more variable and less frequent, with the majority of the effective loci causing flexion of the ipsilateral knee (9%) together with extension of the contralateral knee (8%). In total, including both flexion and extension, 18% of the stimulated sites caused movement of the ipsilateral hindlimb and 11% of the contralateral hindlimb. Although movements of the head were obtained from the whole extent of the brain stem, movements of the forelimbs showed a dorsoventral organization with flexion of the ipsilateral elbow being evoked from the more dorsal regions of the brain stem, whereas contralateral elbow extension was evoked more frequently from the ventral regions. There was a large area of overlap from which movements of both limbs could be obtained simultaneously. Movements of the hindlimbs were more frequently evoked from central and ventral areas of the brain stem and from the most rostral aspect of the explored region. In examining the combinations of movements evoked by the MRF stimulation, it was found that the most commonly evoked pattern was movement of the head to the stimulated side together with flexion of the ipsilateral forelimb and extension of the contralateral forelimb (26.5% of sites). Of movements including the hindlimbs, the most common patterns involved movement of one or both of the forelimbs together with movement of the ipsilateral hindlimb. Movements of both ipsilateral limbs (fore- and hindlimb) were evoked from regions lying dorsally to those that evoked movements of the contralateral forelimb and ipsilateral hindlimb. Movements of both forelimbs together with the ipsilateral hindlimb were also most commonly evoked from the more ventral and rostral regions of the brain stem, as were movements in all four limbs. These results suggest that there is a certain organization within the brain stem such that ipsilateral forelimb movements are preferentially evoked by stimulation to the dorsal MRF, whereas contralateral forelimb movements and movements of the hindlimbs are evoked from more ventral and rostral regions. However, given the large degree of overlap between regions evoking movements in different limbs, it is probable that the MRF is organized to control coordinated patterns of movement rather than discrete movements of individual limbs. It is suggested that these coordinated movements would play an important role in regulating the response of the organism to voluntary movements, as well as to perturbations evoked by vestibular or somatic stimulation. The type and pattern of movement evoked would be determined by the specificity of the area of termination of different afferent systems within the brain stem.
