Sensory properties of articular afferents following Mycoplasma arthritis in the chicken.

The physiological properties of joint capsule mechanical nociceptors of monoarthritic chickens (Gallus domesticus) were studied by recording the electrical activity from single C (Group IV) and A-delta (Group III) fibres dissected from the parafibular nerve. By injecting live Mycoplasma gallisepticum cultures into the ankle joint a typical mycoplasma arthritis was induced which was restricted to a single joint. During the early stage of the disease (7-21 days after infection) there was histopathological evidence of an acute synovitis and the fibres showed evidence of sensitisation. Sensitisation was observed in the significantly increased receptive field size, decreased response thresholds, increased response to joint movement both noxious and innocuous, but only in the C-fibres was there an increase in spontaneous activity. During the more chronic stage of the disease (49-56 days after infection) there was pathological evidence of prolonged synovitis but the sensory fibres responded normally to mechanical stimulation and joint movement. These changes in sensitivity of the joint capsule mechanical nociceptors provides peripheral neural evidence of possible pain experienced during the acute stage of the disease but not at the chronic stage when the disease might be in period of remission. The absence of any clear correlation between pathology and receptor activity demonstrates the difficulty of trying to predict nociceptive consequences in animals on the basis of histopathology.