Determination of afferent fibers mediating oligosynaptic group I input to cat medial gastrocnemius motoneurons

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Abstract
Electrical stimulation of the quadriceps (QUAD), medial tibial (MTIB) and flexor digitorum and hallucis longus (FDHL) muscle nerves was used to evoke oligosynaptic group I postsynaptic potentials (PSP) in medial gastrocnemius (MG) motoneurons. An attempt to specify the types of afferent fibers which mediate that oligosynaptic activity (FDHL to MG only) was performed. Simultaneous electrical activation of both Ia- and Ib-fibers produced distinct oligosynaptic PSP in MG motoneurons, but these were likely due primarily to Ib-afferent activity, since selective activation of Ia-afferents (by stretch) rarely produced oligosynaptic PSP in the same motoneurons. There was, evidence for some Ia contribution to these oligosynaptic PSP. This is consistent with the demonstration that Ia- and Ib-afferent fibers converge onto common interneurons and that selective activation of Ia-fibers can produce PSP similar to those evoked by concurrent stimulation of Ia- and Ib-fibers. It is argued that the oligosynaptic PSP evoked in MG motoneurons by submaximal group I stimulation of the FDHL, MTIB or QUAD muscle nerves can be ascribed predominantly to the activation of Ib-afferent fibers, with only minimal Ia and probably no group II contribution.

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