Influences on the fusimotor-muscle spindle system from chemosensitive nerve endings in cervical facet joints in the cat: possible implications for whiplash induced disorders

The aim of the present study was to establish if there exists reflex connec tions from ligamentous structures in cervical facet joints and the fusimoto r system of dorsal neck muscles. In seven cats, anaesthetized with alpha -c hloralose, bradykinin (BK) of concentrations between 12 and 50 mug was inje cted into the facet joint between C1 and C2. Recordings were made from sing le muscle spindle afferents (MSA) originating in contralateral trapezius an d splenius muscles (TrSp). Fusimotor induced changes in the sensitivity of the muscle spindle afferents were assessed by recording the responses to si nusoidal stretches of the TrSp muscles. The mean rate of discharge and the depth of modulation of a fitted sine were taken as quantitative estimates o f the response. A total of 25 MSAs were recorded, and 21 of these showed cl ear-cut alterations in their responses to the sinusoidal stretches followin g Bk. injections into contralateral facet joint. The majority of the respon ding afferents (13/21) showed changes in their responses indicating an incr eased activity of static fusimotoneurones, although responses of dynamic an d mixed static and dynamic nature were also seen. Local anaesthetics applie d to the intraarticular receptors abolished the effects. Injection (i.v,) o f a general anaesthetic (pentobarbital) abolished the effects.. The results show that there exist reflex connections between receptors in cervical fac et joints and fusimotoneurones of dorsal neck muscles, and this might be of importance in the pathophysiology behind whiplash associated disorders (WA D). (C) 2001 International Association for the Study of Pain. Published by Elsevier Science B.V. All rights reserved.