Major contribution of Aδ-fibres to increased reflex transmission in the feline spinal cord during acute muscle inflammation

The contribution of activated nociceptive muscle afferents to pathologically increased muscle tone remains obscure. The aim of the present study was to investigate whether an acute myositis of the gastrocnemius-soleus (GS) influences spinal reflex activity and to test whether Aδ-fibre or C-fibre were mainly responsible for any effects. In high spinal cats monosynaptic reflexes (MRs) of flexors and extensors and transmission in reflex pathways from group III and IV muscle afferents (activated by intra-arterial KCl injection) were investigated. After infiltration of GS with carrageenan there was a distinct increase in the MRs of flexors and extensors, coupled with facilitation of the flexors, induced by chemically activated group III and IV afferents. The inhibition evoked in extensors by these afferents was also mainly enhanced but less consistently. The reflex effects of carrageenan started within 1h and reached their maximum after about 1.5h. After blocking the input of all myelinated A-fibres, including Aδ-fibres, from the inflamed muscle by TTX, only a small facilitatory effect on MRs remained and the facilitation of excitatory transmission in the excitatory pathway to the flexor PBSt was abolished. The results show that the action of carrageenan-induced inflammation on spinal reflex function derives mainly from Aδ-fibres.