Influences on the gamma-muscle spindle system from muscle afferents stimulated by KCl and lactic acid.

It is known that accumulation of contraction metabolites in muscles stimulates group III and IV afferents and induces excitation of gamma-efferents to the homonymous muscle. The aim of the present study was to investigate whether increased concentrations of lactic acid and KCl in one muscle may influence the activity in primary and secondary muscle spindle afferents (MSAs) from the chemically affected muscle and from surrounding muscles. The experiments were made on 7 cats anesthetized with alpha-chloralose. Recordings were made simultaneously from 2-8 single MSAs from the triceps surae (GS) and the posterior biceps and semitendinosus muscles (PBSt). The mean rate of firing and the depth of modulation of MSA responses to sinusoidal stretching of the receptor-bearing muscles were determined. Responses of 27 primary MSAs (11 from PBSt and 16 from GS) were recorded. On 24 of these clear-cut alterations in sinusoidal response were evoked by injection of 1 ml of KCl (50-600 mM) or 1 ml of lactic acid (20-200 mM) into the artery supply of the GS muscle. Also, all secondary MSAs recorded (4 from PBSt and 1 from GS) showed sizable effects to increased intramuscular concentrations of KCl and/or lactic acid. On both primary and secondary MSAs, from GS as well as from PBSt muscles, nearly all effects observed were compatible with activation of static fusimotor neurons. Effects on MSAs were completely abolished when the ipsilateral L7-L6 ventral roots were cut and when the GS nerve was anesthetized. Intravenous injections of KCl and lactic acid, as well as arterial injections of 0.9% NaCl, were ineffective in changing the MSA responses.(ABSTRACT TRUNCATED AT 250 WORDS)