Extent and role of multisegmental coupling in the lamprey spinal locomotorpattern generator

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 coordinati on is thought to arise from the pattern of extensive connections made by pr opriospinal interneurons. However, the mechanisms responsible for intersegm ental coordination remain unknown, in large part because of the difficulty in obtaining quantitative information on these multisegmental fibers. Syste m-level experiments were performed on isolated 50-segment preparations of s pinal cord of adult silver lampreys, Ichthyomyzon unicuspis, to determine t he dependence of CPG performance on multisegmental coupling. Coupling was m anipulated through use of an experiment chamber with movable partitions. wh ich allowed separate application of solution to rostral, middle, and caudal regions of the spinal cord preparation. During control trials, fictive loc omotion, induced by bath application of D-glutamate in all three regions, w as recorded extracellularly from ventral roots. Local synaptic activity in a variable number of middle segments was subsequently blocked with a low-Ca 2+, high-Mn2+ saline solution in the middle compartment, whereas conduction in axons spanning the middle segments was unaffected. Spectral analysis wa s used to assess the effects of blocking propriospinal coupling on interseg mental phase lag, rhythm frequency, correlation, and variability. Significa nt correlation and a stable phase lag between the rostral and caudal region s 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 rostrocaudal phase decreased with increasing number of blocked segments fro m the control value of approximately 1% per segment. By contrast, phase lag s within the rostral and caudal end regions remained unaffected. The cycle frequency in the rostral and caudal regions decreased with the number of bl ocked middle segments and tended to diverge when a large number of middle s egments was blocked. The variability in cycle frequency and intersegmental phase both increased with increasing number of blocked segments, in additio n, 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, whereas intersegmental phase lags are controlled at a local level and are not dependent on extended multisegment al coupling. Other possible roles for multisegmental coupling are discussed .