The effect of acute joint inflammation on flexion reflex excitability in the decerebrate, low-spinal cat

Experiments were performed to investigate whether acute knee joint inflammation, induced by intra-articular injection of carrageenan and kaolin, significantly influences the magnitude of the flexion withdrawal reflex of knee flexors in the decerebrate, low-spinal cat. It was observed that after injection of these inflammatory agents, reflex intensity increased with a time course that matched the development of the inflammatory process, as assessed by monitoring the intra-articular temperature. Control experiments involving intra-articular injection of saline showed neither a rise in intra-articular temperature, nor any change in reflex intensity. The normal pattern of modulation of reflex intensity with changing joint angle was abolished in the inflamed joint with the reflex being equally intense at all positions tested. Abolition of neural activity in the inflamed joint by intra-articular injection of lignocaine resulted in a marked decrease in the reflex back to control values, suggesting that there is no maintained increase in central excitability in the absence of joint afferent input. It was also observed that the spontaneous activity of knee flexor motor units was modulated during continuous passive rotation of the knee, with activity being greater in extension than in flexion. This pattern reversed when the knee was acutely inflamed, but could be returned to the control pattern by joint anaesthesia, although now the response was attenuated when compared to the control.