Fig. 1.

Diagram of coupling between group I and group II muscle afferents and ventral spinocerebellar tract (VSCT) neurons via interneurons in reflex pathways from group I and group II muscle afferents investigated in this study. A: diagram of putative connections. X and Y denote interneurons that might mediate inhibitory actions of group I and/or group II afferents on VSCT neurons. X represents interneurons in “private” pathways between these afferents and VSCT neurons and Y represents interneurons acting on both VSCT neurons and hindlimb alpha-motoneurons (MNs). The dotted circle represents motor nuclei within which the intraspinal stimuli were applied and within which axon collaterals of both afferent fibers and interneurons were stimulated. The findings in this study provide evidence for coupling via interneurons Y, albeit without excluding contribution of interneurons X. Stimulation sites in the cerebellum and at the level of the contralateral lateral funiculus (co Th) are indicated by double arrows. Peripheral afferents were stimulated to evoke disynaptic postsynaptic potentials (PSPs) in VSCT neurons, whereas stimulation in motor nuclei aimed at exciting terminals of interneurons synapsing with motoneurons to examine whether monosynaptic PSPs could be evoked in this way in VSCT neurons. B and C: location of stimulation sites within the cerebellum from which the investigated neurons were antidromically activated in 5 experiments.