Aims: 

The contribution of muscle nociceptive afferents to pathological muscle tone remains uncertain. The aim here is to provide an overview of the effects on motor activity of activating such afferents in different ways and time courses.

Methods: 

All terminal experiments were made on anaemically decapitated, high spinal cats using different modes of stimulation of thin, to a great extent nociceptive muscle afferents and the monosynaptic testing of reflex transmission.

Results: 

Short lasting activation of group III and IV muscle afferents by electrical stimulation of muscle nerves, or by intra-arterial injection of KCL into the gastrocnemius-soleus (GS) muscle, evoked an FRA (flexor reflex afferents) pattern of flexor excitation and extensor inhibition. Longer-lasting activation of nociceptive afferents for several hours was achieved by infiltration of the muscle with 1% carrageenan, causing an acute myositis with significant facilitation of both flexors and extensors. A more enduring activation of nociceptors, lasting 9–12 days, was produced by infiltrating GS with complete Freund's adjuvant, but this caused no significant facilitation of monosynaptic reflexes in either flexors or extensors. However, the chemical activation of group III and IV muscle afferents now induced bilateral flexor excitation, which sometimes was accompanied by bilateral inhibition of extensors.

Conclusion: 

The pattern of activation of nociceptive afferents, in particular the time course of stimulation, importantly influences the outcome of motor activity; this indicates the need for caution when one tries to explain symptoms of chronic pain by observations obtained from acute nociceptive stimulation.