On the control of myotomal motoneurones during “fictive swimming” in the lamprey spinal cord in vitro

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Abstract
Intracellular recordings were made from myotomal motoneurons during fictive swimming in the in vitro preparation of the lamprey spinal cord, while monitoring the efferent burst activity in the ventral roots. The pattern of rhythmic activity in the motoneurons is described, as well as how synaptic inputs from the premotoneuronal level exert their control of motoneuron activity. All motoneurons investigated displayed rhythmic, symmetric oscillations of their membrane potential during fictive swimming. The period of depolarization occurred in phase with the burst discharge in the ventral root containing the motoneuron axon. Of the cells, .apprx. 1/3 fired bursts of action potentials during the depolarized phase, while the remaining motoneurons exhibited subthreshold oscillations. Intracellular injection of Cl- reversed the sign of the hyperpolarized phase, demonstrating phasic active inhibition of the motoneurons during rhythmicity. The depolarized phase was unaffected after Cl- injection, showing that the motoneurons also received phasic active excitation. Pre-triggered averaging of the motoneuron recording (using the ventral root spikes from other motoneurons for triggering), revealed that some degree of synchronous excitation of several motoneurons occurred, suggesting common excitation from the same premotor-interneurons. Evidently, the rhythmic oscillations of membrane potential in lamprey myotomal motoneurones during fictive locomotion depend on phasic excitation alternating with phasic active inhibition. The premotoneuronal mechanism responsible for this control may consist of reciprocally organized groups of excitatory and inhibitory interneurons.