Rhythmic motor activity and phase dependent reflex transmission in the spinal tortoise (Testudo graeca, T. hermanni)

Abstract

1. In the tortoise the capability of the spinal cord of generating rhythmic motor activity and of modulating reflex transmission depending on the motor cycle was investigated. 2. In the intact animal co-ordinated locomotion was only observed if the feet had ground contact. Without ground contact only rhythmic struggling movements occurred. After spinalization some peripheral input was needed to initiate and sustain struggling movements in the air; the pattern of the movements was changed but not the frequency. After paralyzation the capability of generating a rhythmic activity was distinctly depressed in the spinal tortoise. The frequency of a rhythmic activity which could be induced in such a preparation by peripheral stimulation was very low, even after premedication with nialamide and DOPA. 3. In the spinal paralyzed preparation during rhythmic motor activity a modulation of the membrane potential of motoneurones occurred with phases of depolarization and hyperpolarization. The latter at least partly were due to synaptic inhibition. 4. In the spinal paralyzed preparation the transmission in excitatory reflex pathways from peripheral flexor reflex afferents (FRA) to motoneurones was phasically modulated during rhythmic motor activity in the way that the transmission was facilitated during the active phase of a motoneurone pool and inhibited during the reciprocal phase. In the inhibitory FRA pathways partly a particular kind of modulation of the transmission during the different phases was observed. 5. The results indicate that the rhythmic motor activity in the spinal paralyzed tortoise which largely matched the activity found in cats, resembles in some aspects locomotor activity and therefore by analogy with findings in cats and turtles may be denoted as ‘fictive locomotion’.