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The Journal of Neurophysiology Vol. 87 No. 3 March 2002, pp. 1542-1553
Copyright ©2002 by the American Physiological Society
1Center for Neuroscience, University of Alberta, Edmonton, Alberta T6G 2S2, Canada; and 2Department of Anatomy, University of Groningen, 9713 AV Groningen, The Netherlands
Yakovenko, Sergiy,
Vivian Mushahwar,
Veronique VanderHorst,
Gert Holstege, and
Arthur Prochazka.
Spatiotemporal Activation of Lumbosacral Motoneurons in the
Locomotor Step Cycle. J. Neurophysiol. 87: 1542-1553, 2002. The aim of this study was to produce a
dynamic model of the spatiotemporal activation of ensembles of alpha
motoneurons (MNs) in the cat lumbosacral spinal cord during the
locomotor step cycle. The coordinates of MNs of 27 hindlimb muscles of
the cat were digitized from transverse sections of spinal cord spanning
the entire lumbosacral enlargement from the caudal part of
L4 to the rostral part of
S1 segments. Outlines of the spinal cord gray matter were also digitized. Models of the spinal cord were generated from these digitized data and displayed on a computer screen as three-dimensional (3-D) images. We compiled a chart of
electromyographic (EMG) profiles of the same 27 muscles during the cat
step cycle from previous studies and used these to modulate the number
of active MNs in the 3-D images. The step cycle was divided into 100 equal intervals corresponding to about 7 ms each for gait of moderate
speed. For each of these 100 intervals, the level of EMG of each muscle
was used to scale the number of dots displayed randomly within the
volume of the corresponding MN pool in the digital model. One hundred
images of the spinal cord were thereby generated, and these could be
played in sequence as a continuous-loop movie representing rhythmical
stepping. A rostrocaudal oscillation of activity in hindlimb MN pools
emerged. This was confirmed by computing the locus of the center of
activation of the MNs in the 100 consecutive frames of the movie. The
caudal third of the lumbosacral enlargement showed intense MN activity
during the stance phase of locomotion. During the swing phase, the
focus of activation shifted abruptly to the rostral part of the
enlargement. At the stance-swing transition, a transient focus of
activity formed in the most caudal part of the lumbosacral enlargement. This was associated with activation of gracilis, posterior biceps, posterior semimembranosus, and semitendinosus muscles. These muscles move the foot back and up to clear the ground during locomotion, a role
that could be described as retraction. The spatiotemporal distribution
of neuronal activity in the spinal cord during normal locomotion with
descending control and sensory inputs intact has not been visualized
before. The model can be used in the future to characterize
spatiotemporal activity of spinal MNs in the absence of descending and
sensory inputs and to compare these to spatiotemporal patterns in
spinal MNs in normal locomotion.
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