Journal of Neurophysiology, Vol 71, Issue 2 611-622, Copyright © 1994 by APS
Corrective responses to loss of ground support during walking. II. Comparison of intact and chronic spinal cats
G. W. Hiebert, M. A. Gorassini, W. Jiang, A. Prochazka and K. G. Pearson
Department of Physiology, University of Alberta, Edmonton, Canada.
1. The preceding study described a corrective response in cats when one
hind leg steps into a hole. In this investigation we examine the extent to
which this behavior is organized at the spinal level by comparing the
responses elicited in intact and chronic spinal cats. 2. Adult cats were
trained to step bipedally with their hind legs on a treadmill. After
training, the responses to stepping into a hole cut in the treadmill belt
were monitored with a video recorder and by recording electromyograms from
muscles in both hind legs. The responses to stepping into the hole were
also recorded in chronic spinal cats that had recovered the ability to step
with their hind legs a few weeks after spinalization. 3. The behavioral
responses in the two groups of animals differed in two respects. First, the
latency of the onset of the flexion movement to remove the foot from the
hole was shorter in intact animals (70-150 ms in intact vs. 130-350 ms in
spinal animals). Second, the flexion movement in the intact animals was
stronger. The exaggerated flexion movement in intact animals lifted the paw
well clear of the hole and allowed support to be regained on the treadmill
belt. The weaker flexion movement in spinal animals was usually
insufficient to lift the paw completely from the hole. 4. Differences in
the motor patterns recorded from flexor muscles during the corrective
response in intact and spinal animals correspond with the differences in
the kinematics. First, the onset of flexor activity after the foot entered
the hole was delayed by approximately 100 ms in spinal animals relative to
intact animals. Second, in intact animals the magnitudes of flexor bursts
were increased relative to the flexor bursts associated with the swing
phase during stepping, whereas in spinal animals flexor bursts during the
corrective response resembled those occurring during swing. 5. Similarities
in the duration and the timing of bursts in different flexor muscles in
intact and spinal animals during the corrective response and during swing
indicated that the corrective response involves activation of the spinal
system that normally produces swing phase flexor activity. We conclude that
activation of this system is facilitated by input from supraspinal
structures during the corrective response in intact animals. 6. In all
intact animals and three of five spinal animals, support of the
hindquarters when the foot entered the hole was maintained by the
contralateral leg.(ABSTRACT TRUNCATED AT 400 WORDS)
