Influences on the γ-muscle spindle system from muscle afferents stimulated by increased intramuscular concentrations of bradykinin and 5-HT

Abstract

There is evidence that static muscular contractions induce a release of bradykinin (BK) in the working muscle, and that increased concentration of BK and 5-HT in a muscle increases the discharge rate of a subpopulation of group III and group IV muscular afferents. It is also known that activity in group III and IV muscle afferents may activate γ-motoneurones to both homonymous and heteronymous muscles. The aim of the present study was to investigate whether increased concentration of BK and 5-HT in one muscle may influence the activity in primary and secondary muscle spindle afferents (MSAs) from the chemically affected muscle and from surrounding, muscles, via fusimotor reflexes. The experiments were made on six cats anaesthetised with α-chloralose. The triceps surae (GS) and the posterior biceps and semitendinosus (PBSt) muscles were subjected to sinusoidal stretches. Simultaneous recordings of 2–11 MSAs from these muscles were made and the mean rate of firing and the modulation for each MSA were determined. Responses of 47 MSAs (26 PBSt and 21 GS) were recorded. The responsiveness of the MSAs to injections of BK (9–100 mg/ml, 0.5–1.0 ml) and 5-HT (25–150 mg/ml, 0.5–1.0 ml) was 89% and 83%, respectively, for injections into the arterial supply of the ipsilateral GS muscle, and 84% and 40% respectively for injections to the contralateral GS muscle. Of 10 secondary MSAs, only one was unresponsive to BK injections, while several MSAs responded to both ipsilateral and contralateral BK injections. BK injections resulted in dynamic, static and mixed responses in primary MSAs, while 5-HT injections only gave rise to static responses. The duration of the effects was usually 1–3 min. However, on some occasions the elevations in MSA activity persisted for up to 20 min. Cutting of the nerve to the injected muscle always abolished the effects of the injections, and control injections of the solution in which the BK and 5-HT were dissolved were ineffective in changing the MSA responses. Intravenous injections occasionally induced effects on the MSAs, but such effects were significantly smaller than those caused by close arterial muscle injections. Thus, increased concentration of BK and 5-HT may excite primary and secondary MSAs from homonymous as well as heteronymous muscles, including contralateral muscles, most probably via fusimotor reflexes evoked by activity in chemosensitive muscle afferents.