Effects on peroneal motoneurons of cutaneous afferents activated by mechanical or electrical stimulations

The postsynaptic potentials elicited in peroneal motoneurons by either mechanical stimulation of cutaneous areas innervated by the superficial peroneal nerve (SP) or repetitive electrical stimulation of SP were compared in anesthetized cats. After denervation of the foot sparing only the territory of SP terminal branches, reproducible mechanical stimulations were applied by pressure on the plantar surface of the toes via a plastic disk attached to a servo-length device, causing a mild compression of toes. This stimulus evoked small but consistent postsynaptic potentials in every peroneal motoneuron. Weak stimuli elicited only excitatory postsynaptic potentials (EPSPs), whereas increase in stimulation strength allowed distinction of three patterns of response. In about one half of the sample, mechanical stimulation or trains of 20/s electric pulses at strengths up to six times the threshold of the most excitable fibers in the nerve evoked only EPSPs. Responses to electrical stimulation appeared with 3-7 ms central latencies, suggesting oligosynaptic pathways. In another, smaller fraction of the sample, inhibitory postsynaptic potentials (IPSPs) appeared with an increase of stimulation strength, and the last fraction showed a mixed pattern of excitation and inhibition. In 24 of 32 motoneurons where electrical and mechanical effects could be compared, the responses were similar, and in 6 others, they changed from pure excitation on mechanical stimulation to mixed on electrical stimulation. With both kinds of stimulation, stronger stimulations were required to evoke inhibitory postsynaptic potentials (IPSPs), which appeared at longer central latencies than EPSPs, indicating longer interneuronal pathways. The similarity of responses to mechanical and electrical stimulation in a majority of peroneal motoneurons suggests that the effects of commonly used electrical stimulation are good predictors of the responses of peroneal motoneurons to natural skin stimulation. The different types of responses to cutaneous afferents from SP territory reflect a complex connectivity allowing modulations of cutaneous reflex responses in various postures and gaits.