Vestibulospinal and reticulospinal neuronal activity during locomotion in the intact cat. II. Walking on an inclined plane

The experiments described in this report were designed to determine the con tribution of vestibulospinal neurons (VSNs) in Deiters' nucleus and of reti culospinal neurons (RSNs) in the medullary reticular formation to the modif ications of the walking pattern that are associated with locomotion on an i nclined plane. Neuronal discharge patterns were recorded from 44 VSNs and 6 3 RSNs in cats trained to walk on a treadmill whose orientation was varied from +20 degrees (uphill) to -10 degrees (downhill), referred to as pitch t ilt, and from 20 degrees roll tilt left to 20 degrees roll tilt right. Duri ng uphill locomotion, a majority of VSNs (25/44) and rhythmically active RS Ns (24/39) showed an increase in peak discharge frequency, above that obser ved during locomotion on a level surface. VSNs, unlike some of the RSNs, ex hibited no major deviations from the overall pattern of the activity record ed during level walking. The relative increase in discharge frequency of th e RSNs (on average, 31.8%) was slightly more than twice that observed in th e VSNs (on average, 14.4%), although the average absolute change in dischar ge frequency was similar (18.2 Hz in VSNs and 21.6 Hz in RSNs). Changes in discharge frequency during roll tilt were generally more modest and were mo re variable, than those observed during uphill locomotion as were the relat ive changes in the different limb muscle electromyograms that we recorded. In general, discharge frequency in VSNs was more frequently increased when the treadmill was rolled to the right (ear down contralateral to the record ing site) than when it was rolled to the left. Most VSNs that showed signif icant linear relationships with treadmill orientation in the roll plane inc reased their activity during right roll and decreased activity during left roll. Discharge activity in phasically modulated RSNs was also modified by roll tilt of the treadmill. Modulation of activity in RSNs that discharged twice in each step cycle was frequently reciprocal in that one burst of act ivity would increase during left roll and the other during right roll. The overall results indicate that each system contributes to the changes in pos tural tone that are required to adapt the gait for modification on an incli ned surface. The characteristics of the discharge activity of the VSNs sugg est a role primarily in the overall control of the level of electromyograph ic activity, while the characteristics of the RSNs suggest an additional ro le in determining the relative level of different muscles, particularly whe n the pattern is asymmetric.