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The Journal of Neurophysiology Vol. 81 No. 5 May 1999, pp. 2037-2045
Copyright ©1999 by the American Physiological Society
Department of Biology, Marquette University, Milwaukee, Wisconsin
Buchanan, James T.
Commissural Interneurons in Rhythm Generation and Intersegmental
Coupling in the Lamprey Spinal Cord. J. Neurophysiol. 81: 2037-2045, 1999.
Commissural interneurons in rhythm generation and intersegmental
coupling in the lamprey spinal cord. To test the necessity of
spinal commissural interneurons in the generation of the swim rhythm in
lamprey, longitudinal midline cuts of the isolated spinal cord
preparation were made. Fictive swimming was then induced by bath
perfusion with an excitatory amino acid while recording ventral root
activity. When the spinal cord preparation was cut completely along the
midline into two lateral hemicords, the rhythmic activity of fictive
swimming was lost, usually replaced with continuous ventral root
spiking. The loss of the fictive swim rhythm was not due to nonspecific
damage produced by the cut because rhythmic activity was present in
split regions of spinal cord when the split region was still attached
to intact cord. The quality of this persistent rhythmic activity,
quantified with an autocorrelation method, declined with the distance
of the split spinal segment from the remaining intact spinal cord. The
deterioration of the rhythm was characterized by a lengthening of burst
durations and a shortening of the interburst silent phases. This
pattern of deterioration suggests a loss of rhythmic inhibitory inputs.
The same pattern of rhythm deterioration was seen in preparations with
the rostral end of the spinal cord cut compared with those with the
caudal end cut. The results of this study indicate that commissural
interneurons are necessary for the generation of the swimming rhythm in
the lamprey spinal cord, and the characteristic loss of the silent
interburst phases of the swimming rhythm is consistent with a loss of
inhibitory commissural interneurons. The results also suggest that both
descending and ascending commissural interneurons are important in the
generation of the swimming rhythm. The swim rhythm that persists in the
split cord while still attached to an intact portion of spinal cord is
thus imposed by interneurons projecting from the intact region of cord
into the split region. These projections are functionally short because
rhythmic activity was lost within approximately five spinal segments
from the intact region of spinal cord.
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