Tonic and phasic depression of Ia EPSPs during fictive locomotion

Abstract

Intracellular recordings from hindlimb $\alpha$-motoneurons were used to examine the effects of fictive locomotion, evoked by stimulation of the midbrain locomotor region (MLR), on group Ia monosynaptic excitatory postsynaptic potentials (EPSPs). The EPSPs were evoked from homonymous or close synergist afferents by electrical stimulation of peripheral nerves at group I strength ($\le$2T). The intracellular recordings were used to examine differences in EPSP amplitude between the locomotor and non-locomotor state as well as the modulation of EPSP amplitude between the flexor and extensor phases of locomotion. The present study is the first to investigate the change of composite group Ia monosynaptic EPSP amplitude between the locomotor and non-locomotor states. In order to investigate group I synaptic activity in the vicinity of the $\alpha$-motoneurons, extracellular field potential (EFP) amplitudes recorded in the ventral horn were examined before, during and after locomotion. Taken toge her, the decrease in monosynaptic EPSP amplitude, the poor correlation between motoneuron membrane resistance and EPSP decreases, and the decreases in group I EFPs during locomotion are consistent with the hypothesis that during fictive locomotion there is a specific regulation of synaptic transmission from group Ia afferents to motoneurons. (Abstract shortened by UMI.)