Contributions of the reticulospinal system to the postural adjustments occurring during voluntary gait modifications

To test the hypothesis that reticulospinal neurons (RSNs) are involved in t he formation of the dynamic postural adjustments that accompany visually tr iggered, voluntary modifications of limb trajectory during locomotion, we r ecorded the activity of 400 cells (183 RSNs; 217 unidentified reticular cel ls) in the pontomedullary reticular formation (PMRF) during a locomotor tas k in which intact cats were required to step over an obstacle attached to a moving treadmill belt. Approximately one half of the RSNs (97/183, 53%) sh owed significant changes in cell activity as the cat stepped over the obsta cle; most of these cells exhibited either single (26/97, 26.8%) or multiple (63/97, 65.0%) increases of activity. There was a range of discharge patte rns that varied in the number, timing, and sequencing of the bursts of modi fied activity, although individual bursts in different cells tended to occu r at similar phases of the gait cycle. Most modified cells, regardless of t he number of bursts of increased discharge, or of the discharge activity of the cell during unobstructed, control, locomotion, discharged during the p assage of the lead forelimb over the obstacle. Thus, 86.9% of the modified cells increased their discharge when the forelimb ipsilateral to the record ing site was the first to pass over the obstacle, and 72.2% when the contra lateral limb was the first. Approximately one quarter of the RSNs increased their discharge during the passage of each of the four limbs over the obst acle in both the lead (27.1%) and trail (27.9%) conditions. In general, in any one cell, the number and relative sequencing of the subsequent bursts ( with respect to the lead forelimb) was maintained during both lead and trai l conditions. Patterns of activity observed in unidentified cells were very similar to the RSN activity despite the diverse population of cells this u nidentified group may represent. We suggest that the increased discharge th at we observed in these reticular neurons reflects the integration of affer ent activity from several sources, including the motor cortex, and that thi s increased discharge signals the timing and the relative magnitude of the postural patterns that accompany the voluntary gait modification. However, based on the characteristics of the patterns of neuronal activity in these cells, we further suggest that while individual RSNs probably contribute to the selection of different patterns of postural activity, the ultimate exp ression of the postural response may be determined by the excitability of t he locomotor circuits within the spinal cord.