Bending the lamprey spinal cord causes a slowly-decaying increase in the frequency of fictive swimming

It is well known that rhythmic lateral bending of the isolated lamprey spin al cord/notochord can entrain the central pattern generator (CPG) for locom otion. During entrainment. the CPG's frequency is equal to the bending freq uency. We report here that bending can also have a slowly-decaying excitato ry effect on the CPG's frequency. Experiments were performed in which the c audal end of a 30-50 segment piece of spinal cord/notochord was repeatedly rhythmically bent for 0.5-12 cycles. A slowly-decaying excitation was said to be present if after the termination of bending the CPG's frequency was s ignificantly greater than baseline acid decayed back to baseline with a tim e constant of one or more cycles. In 14 of 16 animals, a slowly-decaying ex citation could be evoked by bending. In five of the nine animals tested, th is slowly-decaying excitation could be evoked with bending frequencies both faster and slower than the baseline frequency. Depending on the animal, th e slowly-decaying excitation could be elicited by as little as one-half cyc le of bending and by bending amplitudes as small as 6-21 degrees peak defle ction. We interpret these data as evidence of a second effect of bending di stinct from the phase-dependent effect that produces entrainment. (C) 2001 Published by Elsevier Science B.V.