On the mechanisms of a phase-dependent reflex occurring during locomotion in dogfish.

Abstract

In intact and spinal swimming fish, a shortlasting tail fin stimulation will elicit a reflex response on one side or the other of the body, depending on when in the swimming cycle the stimulus occurs. The present study investigates the mechanisms of this phase dependence by utilizing curarized spinal dogfish. Tail fin stimuli were delivered during “fictive locomotion”, i.e. while spontaneous coordinated motor activity could be recorded from the ventral roots. To compare the reflex effects in the presence and absence of phasic peripheral feedback signals, rhythmical bending movements could also be experimentally imposed on the paralyzed body. The following findings were made: (1) Stimulation of the tip of the tail fin, either electrical or by light touch, evoked brisk short latency responses resembling those seen in the actively swimming fish. It is concluded that a brief stimulation of touch receptors is sufficient to elicit the tail fin reflex. (2) As in the actively swimming fish, reflex responses were modulated in phase with the ongoing locomotor activity. (3) Movement-related peripheral feedback did not influence the phase-dependent modulation of the tail fin reflex, which is thus due to a central mechanism within the spinal cord. (4) Strong tail fin stimuli could cause profound disturbances of the locomotor cycle, including omission of bursts and rhythm resetting. (5) Such rhythm-disrupting effects were cancelled during passive body movements. This strongly stabilizing influence from phasic peripheral signals suggests a potent feedback mechanism from the swimming movement. In the event of an external disturbance, such a mechanism would provide instantaneous modification of the neural output, thereby ensuring a continuous, stable swimming rhythm.