Depression of group Ia monosynaptic EPSPs in cat hindlimb motoneurones during fictive locomotion

1.The effects of fictive locomotion on monosynaptic EPSPs recorded in moton eurones and extracellular field potentials recorded in the ventral horn wer e examined during brainstem-evoked fictive locomotion in decerebrate cats. Composite homonymous and heteronymous EPSPs and field potentials were evoke d by group I intensity (less than or equal to 2 T) stimulation of ipsilater al hindlimb muscle nerves. Ninety-one of the 98 monosynaptic EPSPs were red uced in amplitude during locomotion (mean depression of the 91 was to 66% o f control values); seven increased in amplitude (to a mean of 121% of contr ol). Twenty-one of the 22 field potentials were depressed during locomotion (mean depression to 72 % of control). 2. All but 14 Ia EPSPs were smaller during both the flexion and extension p hases of locomotion than during control. In 35% of the cases there was < 5% difference between the amplitudes of the EPSPs evoked during the flexion a nd extension phases. In 27% of the cases EPSPs evokcd during flexion were l arger than those evoked during extension. The remaining 38% of EPSPs were l arger during extension. There nas no relation between either the magnitude of EPSP depression or the locomotor phase in which maximum EPSP depression occurred and whether an EPSP was recorded in a flexor or extensor motoneuro ne. 3. The mean recovery time of both EPSP and field potential amplitudes follo wing the end of about of locomotion was approximately 2 min (range, < 10 to > 300 s). 4. Motoneurone membrane resistance decreased during fictive locomotion (to a mean of 61% of control, n = 22). Because these decreases were only weakly correlated to EPSP depression (r(2) = 0.31) they art? unlikely to fully ac count fur this depression. 5. The depression of monosynaptic EPSPs and group I field potentials during locomotion is consistent with the hypothesis that during fictive locomotio n there is a tonic presynaptic regulation of synaptic transmission from gro up Ia afferents to motoneurones and interneurones. Such a reduction in neur otransmitter release would decrease group Ta monosynaptic reflex excitation during locomotion. This reduction may contribute to the tonic depression o f stretch reflexes occurring in the decerebrate cat during locomotion.