The classical spinal flexor reflex pattern of ipsilateral flexion and contralateral extension is not a rigidly fixed spinal reflex pattern, but can be modulated from supraspinal structures or by changing peripheral conditions. Here we show how acute muscle pain promotes a transient bilateral flexion reflex response to group III/IV muscle afferents in the feline spinal cord.                                                                                                                                                          Selective activation of group III/IV afferents of the gastrocnemius-soleus (GS) muscle by local intra-arterial injections of KCl  has been used to study bilateral reflex transmission under four different conditions of the left GS muscle: (a) controls without any inflammatory conditions (n=6); (b) with strong eccentric stretch of the muscle (n=4); (c) “acute” myositis induced by carrageenan (1-2% in Ringer’s solution, 5 ml i.m., n=6); (d), sub-acute (“chronic”) myositis induced by i.m. infiltration of GS with complete Freund’s Adjuvant (CFA) 9-12 days before terminal experiments (under full anaesthesia, see below, n=4). All terminal experiments cats were done under initial 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 repeatedly approved by ethic commissions of the Medical Faculty of the University of Göttingen and of the District Council Brunswick), artificially ventilated, spinalised at C1 and paralysed with pancuronium. Reflex transmission was investigated by bilateral monosynaptic reflex testing of the posterior biceps-semitendinosus (PBST) and the GS muscles. Although Group III/IV stimulation evoked the flexion reflex ipsilaterally under all conditions, the contralateral responses did not follow a general extension reflex pattern. In control experiments only 9% of tests revealed a contralateral inhibition of PBSt flexor motoneurones and fewer  than half induced a small contralateral facilitation of the extensor GS. Following eccentric GS contraction for up to about 1hour or following acute carrageenan ”myositis” up to about 2 hours an unidirectional facilitation of PBST reflex effects from the pre- 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, less distinct vice versa, predominance of crossed facilitation of PBSt but no crossed facilitation of GS, instead, in about 50% of tests crossed inhibition of GS was observed. Thus, in “chronic” muscle pain the crossed extension reflex functioning as a bilateral reflex adjustment to stabilise posture is replaced by a bilateral flexion reflex pattern possibly as an adaptive postural mechanism for minimising pain and perhaps to promote healing.

References

Kniffki KD et al. (1981). J Physiol 319,543-554.