Initiation and sensory gating of 'fictive' swimming and withdrawal responses in an in vitro preparation of the lamprey spinal cord.

An in vitro preparation has been developed in which the caudal part of the lamprey spinal cord (resting on the notochord) is still innervating the tail fin. Mechanical stimulation of the tail fin elicits ventral root activity that would normally lead to an initial body flexure (tail fin withdrawal) followed by escape swimming. These patterns of ventral root activity are correlated with motor activity and movements elicited by a similar tail fin stimulus in intact and spinal lampreys. The ascending control systems activated by tail fin stimulation consists of long axons which project for at least 20 segments, and are not dependent on propagation through local circuits in the gray matter. Sensory input activated by passive bending of the notochord/spinal cord in the in vitro preparation gates the tail fin withdrawal motor activity, such that tail fin stimulation elicits ventral root bursts on the same side as the direction of bending. This new in vitro preparation survives for several days and will provide new opportunities to study the normal initiation and coordination of two different patterns of behavior, swimming and withdrawal.