Locomotor modulation of disynaptic EPSPs from the mesencephalic locomotor region in cat motoneurons

When low-frequency tetanization of the mesencephalic locomotor region (MLR) produce fictive locomotion in unanesthetized, decerebrate cats, each MLR s timulus produces a distinctive cord dorsum potential (CDP) and oligosynapti c excitatory postsynaptic potentials (EPSPs) in many lumbosacral motoneuron s. The average segmental latency from the initial CDP wave [mean delay from stimulus: 4.3 +/- 0.9 (SD) ms] to the onset of detectable MLR EPSPs was 1. 6 +/- 0.4 ms, suggesting a disynaptic segmental connection. In gastrocnemiu s/soleus, flexor hallucis longus, flexor digitorum longus, tibialis anterio r, and posterior biceps-semitendinosus motoneurons (35/38 cells), MLR EPSPs either appeared or were enhanced during the phase of fictive stepping in w hich the target motoneurons were depolarized and the motor pool was active (the ON phase), with parallel changes between EPSP amplitudes and membrane depolarization. In contrast, MLR stimulation produced small (1/10) or no EP SPs in extensor digitorum longus (EDL) motoneurons, with no ON phase enhanc ement (4/10) or oligosynaptic inhibitory postsynaptic potentials during the ON phase (5/10). Eight of 10 flexor digitorum longus (FDL) cells exhibited membrane depolarization in the early flexion phase of fictive stepping, an d five of these showed parallel enhancement of disynaptic MLR EPSPs during early flexion. Three cases were studied when the FDL motor pool exhibited e xclusively extensor phase firing. In these cases, the disynaptic MLR EPSPs were enhanced only during the extensor phase, accompanied by membrane depol arizations. We conclude that the last-order interneurons that produce disyn aptic MLR EPSPs may well participate in producing the depolarizing locomoto r drive potentials (LDPs) found in hindlimb motoneurons during fictive loco motion. However, the absence of linkage between MLR EPSP enhancement and LD P depolarizations in EDL motoneurons suggests that other types of excitator y interneurons also must be involved at least in some motor pools. We compa red these patterns with the modulation of disynaptic EPSPs produced in FDL cells by stimulation of the medial longitudinal fasciculus (MLF). In all se ven FDL motoneurons tested, disynaptic MLF EPSPs appeared only during the e xtension phase, regardless of when the FDL motoneurons were active. The fac t that the modulation patterns of MLR and MLF disynaptic EPSPs is different in FDL motoneurons indicates that the two pathways do not converge on comm on last-order interneurons to that motor pool.