The contribution of group III and IV muscle afferents from free nerve endings to pathological motor phenomena resulting in chronic muscle pain is still under debate. The hypothesis that muscular nociceptors support their own activation by an increase of muscle stiffness via the gamma-spindle-loop has been questioned since gamma-activity was inhibited during artificial myositis (Mense & Skeppar, 1991).
Therefore, we investigated the effect of an acute carrageenan-induced myositis of the gastrocnemius-soleus (GS) on monosynaptic reflexes of flexors (posterior biceps semitendinosus, peroneus) and extensors (GS, quadriceps, anterior biceps semimembranosus, tibialis) and on spinal motor reflex pathways from group III and group IV muscle afferents (activated by injection of KCl or bradykinin into the muscle artery of GS) without and with blocking all myelinated fibres in the GS nerve by TTX (100 µM). With approval of the local ethical committee the experiments were performed in anaemically decapitated high spinal, paralysed (pancuronium bromide) cats (initial anaesthesia before decapitation and paralysis: O2/N2O, 1:2, halothane initially 2.5 %, then increasingly replaced by ether, as required for full anaesthesia; for technical details, see Schomburg et al. 2001). Finally the animals were humanely killed.
In experiments without application of TTX, i.e. with intact conduction in group III and group IV afferents, the monosynaptic reflexes of flexors and extensor showed a distinct increase after infiltration of GS with carrageenan (1 %) and a slight increase of the facilitation of the flexors by chemically activated group III and IV afferents. The inhibition from these afferents to the extensors reacted less uniformly. The effects of carrageenan started within 1 h, reached their maximum after 1.5 h and generally returned to the control values after about 3.5-4 h.
After application of TTX, i.e. with a block of myelinated fibres, but intact conduction in group IV fibres (Schomburg & Steffens, 2002) the effects of carrageenan and their time course were similar to those without TTX block.
The results show that the input from acutely inflamed muscles may induce an increase of the reflex responsiveness of flexors and extensors, which is not mediated via the gamma-spindle-loop. Since the effects also occurred after a block of all myelinated fibres from the inflamed muscle by TTX, a distinct part of the effects can be assumed to be induced by group IV muscle afferents.
This work was supported by the Deutsche Forschungsgemeinschaft.