The Extent And Role Of Multisegmental Coupling In The Lamprey Spinal Locomotor Pattern Generator. William L. Miller and Karen A. Sigvardt. Center for Neuroscience and Department of Neurology, University of California, Davis CA 95616.
APStracts 6:0478N, 1999.
Timing of oscillatory activity along the longitudinal body axis is critical for locomotion in the lamprey and other elongated animals. In the lamprey spinal locomotor central pattern generator (CPG), intersegmental coordination is thought to arise from the pattern of extensive connections made by propriospinal interneurons. However, the mechanisms responsible for intersegmental coordination remain unknown, in large part because of the difficulty in obtaining quantitative information on these multisegmental fibers. We performed system-level experiments on isolated 50-segment preparations of spinal cord of adult silver lampreys, Ichthyomyzon unicuspis, to determine the dependence of CPG performance on multisegmental coupling. Coupling was manipulated through use of an experiment chamber with movable partitions, which allowed separate application of solution to rostral, middle and caudal regions of the spinal cord preparation. During control trials, fictive locomotion, induced by bath application of D-glutamate in all three regions, was recorded extracellularly from ventral roots. Local synaptic activity in a variable number of middle segments was subsequently blocked with a low Ca++, high Mn++ saline solution in the middle compartment, while conduction in axons spanning the middle segments was unaffected. Spectral analysis was employed to assess the effects of blocking propriospinal coupling on intersegmental phase lag, rhythm frequency, correlation and variability. Significant correlation and a stable phase lag between the rostral and caudal regions of the spinal cord preparation were maintained during block of as many as 16 and sometimes 20 intervening segments. However, the mean value of this rostro-caudal phase decreased with increasing number of blocked segments from the control value of approximately 1% per segment. By contrast, phase lags within the rostral and caudal end regions remained unaffected. The cycle frequency in the rostral and caudal regions decreased with the number of blocked middle segments, and tended to diverge when a large number of middle segments was blocked. The variability in cycle frequency and intersegmental phase both increased with increasing number of blocked segments. In addition, a number of differences were noted in the properties of the motor output of the rostral and caudal regions of the spinal cord. The results indicate that the maximal functional length of propriospinal coupling fibers is at least 16-20 segments in I. unicuspis, while intersegmental phase lags are controlled at a local level and are not dependent on extended multisegmental coupling. Other possible roles for multisegmental coupling are discussed.
Received 1 March 1999; accepted in final form 14 September 1999.
APS Manuscript Number J181-9.
Article publication pending Journal of Neurophysiology.
ISSN 1080-4757 Copyright 1999 The American Physiological Society.
Published in APStracts on 21 December 1999