Modulation of the spinal excitability by muscle metabosensitive afferent fibers

The aim of this study was to identify the effect of chemical activation of muscle metabosensitive afferent fibers from groups III and IV on Hoffmann (H-) reflex modulation in the vastus medialis muscle. The experiment was conducted in rats and was divided into two experiments. The first experiment consisted of recording the metabosensitive afferent activity from femoral nerve in rats in response to KCl intraarterial injections in nontreated adults and adults treated neonatally with capsaicin. Thus, the dose-response curve was determined. The second experiment consisted of eliciting the H- and M-waves before and after KCl injection in nontreated adult animals and those treated neonatally with capsaicin. Thus, the H(max)/M(max) ratio was measured. Results indicated that, 1) in nontreated animals, afferent fibers peak discharge was found after 10 mM KCl injection; 2) no significant increase in afferent discharge rate was found in capsaicin-treated animal after KCl injections, confirming that capsaicin is an excitotoxic agent that had destroyed the thin metabosensitive nerve fibers; 3) in nontreated animals, H(max)/M(max) ratio was significantly attenuated after a 10 mM KCl injection activating metabosensitive afferent fibers; and 4) in capsaicin-treated animals, no significant change in H(max)/M(max) ratio was observed after the KCl injection. These results reinforce the hypothesis that the spinal reflex response was influenced by metabosensitive muscle fibers and provide direct evidence that activation of these fibers could partially explain the reported decrease in H-reflex when metabolites are released in muscle.