Coordination of locomotor activity in the lamprey: role of descending drive to oscillators along the spinal cord.

In the lamprey and most fish, locomotion is characterized by caudally propagating body undulations that result from a rostrocaudal phase lag for ipsilateral burst activity. One of the mechanisms that might contribute to rostrocaudal phase lags is a gradient of oscillator burst frequencies along the spinal cord that presumably are produced, in part, by descending drive from the brain. The purpose of the present study was to test whether a gradient of oscillator frequencies does exist along the lamprey spinal cord. First, during brain-initiated locomotor activity in in vitro brain/spinal cord preparations, the cycle times (=1/frequency) of locomotor activity generated by the functionally isolated rostral spinal cord (activity blocked in middle and caudal cord) were significantly shorter than control cycle times when the entire spinal cord was generating locomotor activity. Second, the cycle times of locomotor activity generated by the functionally isolated caudal cord (activity blocked in rostral and middle cord) were significantly longer than control cycle times for activity generated by the entire spinal cord. Thus, no one region of the spinal cord appears to dictate the overall cycle times of locomotor activity generated by the entire spinal cord, although overall cycle times tended to be closest to those of the isolated rostral spinal cord. Finally, although short- and long-distance coupling as well as oscillator frequency gradients probably contribute to rostrocaudal phase lags of spinal locomotor activity, the asymmetrical nature of short-distance coupling, in which the descending component dominates, appears to be the main factor.