Proceedings of The Physiological Society

Physiology 2014 (London, UK) (2014) Proc Physiol Soc 31, PCB067

Poster Communications

Chronic muscle pain facilitates bilateral flexion reflex pattern in the feline spinal cord

E. D. Schomburg1, H. Steffens1, P. Dibaj2, T. Sears3

1. Institute of Physiology, University of G

The acute onset of limb pain entrains bilateral reflex adjustments to stabilise posture, characteristically, ipsilateral flexion and contralateral extension. However, ‘chronic' muscle pain is associated with different motor patterns dependent, or not, on sustained nociceptive inputs and or adaptations e.g. ‘central sensitisation'. Here, using selective activation of Group III/IV muscle afferents by i.m. KCL injections into the gastrocnemius-soleus (GS) muscle, we have studied bilateral reflex transmission under a), acute myositis induced by carrageenan (i.m.); b), chronic myositis induced by i.m. infiltration of GS with complete Freund's adjuvant (CFA) 9-12 days before the terminal experiment. For these cats were placed initially under halothane-nitrous oxide anaesthesia (O2/N2O, 1:2; halothane first at 2.5%, followed by ether) then anaemically decapitated as described by Kniffki et al. (1981, technique also repeatedly approved by the animal protection commissions of the Medical Faculty of the University of Göttingen and of the District Council Brunswick, last "licence for the performance of experiments in living animals" no. 509.42502/01-23.03), artificially ventilated, spinalised at C1 and paralysed with pancuronium. Reflex transmission was investigated by monosynaptic reflex testing. Under both acute and chronic myositis, although Group III/IV stimulation evoked the flexion reflex ipsilaterally, the contralateral responses did not follow a general extension reflex pattern. In the control experiments only 9% of tests with chemically activated group III and IV muscle afferents were accompanied by the typical contralateral inhibition of posterior biceps semitendinosus (PBSt) flexor motoneurones, while less than half induced a small contralateral facilitation of the extensor GS. Initially, following induction of an acute myositis a unidirectional facilitation of reflex effects from the pre-treated to the un-treated contralateral side occurred. In ‘chronic' myositis about 90% of tests revealed a bidirectional i.e. from the pre-treated to the un-treated side and vice versa, predominance of crossed facilitation of PBSt but no longer any crossed facilitation of GS, instead, in about 50% of tests inhibiton of transmission to GS was observed. Thus in chronic ‘myositis' the crossed extension reflex is replaced by a bilateral flexion reflex pattern. And in good correspondence, the increased number of lumbar spinal interneurons displaying c-fos proto-oncogene expression largely reflected the described reflex effects, further indicating that the ‘chronic' pain state is accompanied by long-term modifications in spinal reflex transmission mediating adaptive postural changes minimising pain and perhaps to promote healing.

Where applicable, experiments conform with Society ethical requirements