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 connections from ligamentous structures in cervical facet joints and the fusimotor system of dorsal neck muscles. In seven cats, anaesthetized with alpha -chloralose, bradykinin (BK) of concentrations between 12 and 50 mug was injected into the facet joint between C1 and C2. Recordings were made from single muscle spindle afferents (MSA) originating in contralateral trapezius and splenius muscles (TrSp). Fusimotor induced changes in the sensitivity of the muscle spindle afferents were assessed by recording the responses to sinusoidal stretches of the TrSp muscles. The mean rate of discharge and the depth of modulation of a fitted sine were taken as quantitative estimates of the response. A total of 25 MSAs were recorded, and 21 of these showed clear-cut alterations in their responses to the sinusoidal stretches following Bk. injections into contralateral facet joint. The majority of the responding afferents (13/21) showed changes in their responses indicating an increased activity of static fusimotoneurones, although responses of dynamic and mixed static and dynamic nature were also seen. Local anaesthetics applied to the intraarticular receptors abolished the effects. Injection (i.v,) of a general anaesthetic (pentobarbital) abolished the effects.. The results show that there exist reflex connections between receptors in cervical facet joints and fusimotoneurones of dorsal neck muscles, and this might be of importance in the pathophysiology behind whiplash associated disorders (WAD). (C) 2001 International Association for the Study of Pain. Published by Elsevier Science B.V. All rights reserved.