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The Journal of Neurophysiology Vol. 85 No. 2 February 2001, pp. 679-698
Copyright ©2001 by the American Physiological Society
1Department of Kinesiology, University of Waterloo, Waterloo, Ontario N2L 3G1; and 2Department of Physiology, University of Montréal, Montreal, Quebec H3C 3J7, Canada
Prentice, Stephen D. and
Trevor Drew.
Contributions of the Reticulospinal System to the Postural
Adjustments Occurring During Voluntary Gait Modifications. J. Neurophysiol. 85: 679-698, 2001. To test the
hypothesis that reticulospinal neurons (RSNs) are involved in the
formation of the dynamic postural adjustments that accompany visually
triggered, voluntary modifications of limb trajectory during
locomotion, we recorded the activity of 400 cells (183 RSNs; 217 unidentified reticular cells) in the pontomedullary reticular formation
(PMRF) during a locomotor task 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%) showed significant
changes in cell activity as the cat stepped over the obstacle; 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 patterns
that varied in the number, timing, and sequencing of the bursts of
modified activity, although individual bursts in different cells tended to occur at similar phases of the gait cycle. Most modified cells, regardless of the number of bursts of increased discharge, or of the
discharge activity of the cell during unobstructed, control, locomotion, discharged during the passage of the lead forelimb over the
obstacle. Thus, 86.9% of the modified cells increased their discharge
when the forelimb ipsilateral to the recording site was the first to
pass over the obstacle, and 72.2% when the contralateral limb was the
first. Approximately one quarter of the RSNs increased their discharge
during the passage of each of the four limbs over the obstacle 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 trail conditions. Patterns of activity observed in unidentified cells were
very similar to the RSN activity despite the diverse population of
cells this unidentified group may represent. We suggest that the
increased discharge that we observed in these reticular neurons reflects the integration of afferent activity from several sources, including the motor cortex, and that this 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
expression of the postural response may be determined by the
excitability of the locomotor circuits within the spinal cord.
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