Sensory receptors in ankle joint capsules of normal and arthritic rats

The responses of afferent fibers innervating the capsule of the ankle joint have been investigated in 34 normal and 19 arthritic anaesthetised rats. Afferent fiber diameter and conduction velocity were in the same range in normal and arthritic rats. All receptors examined were excited by mechanical stimulation of the joint capsule and the majority adapted slowly. A resting discharge was absent in normal rats in contrast to the arthritic animals where it was present in about 25% of the sample. The mechanical thresholds of the responses, measured using either von Frey hairs or the force transducer, ranged from 4.6 mN to 65 mN for 11 units recorded in normal and exceeded 80 mN for 11 others. For 30 units recorded in arthritic animals the thresholds ranged from 0.4 to 46 mN. When tested, pressure on the ankle or small degrees of flexion or extension produced a high rate of discharge in receptors of arthritics while similar stimuli were ineffective in normals. Repeated indentation at short intervals caused a progressive reduction in response to 10 to 15% of control values in normal and almost to extinction in arthritic rats. Recovery occurred within minutes in normals but was delayed in arthritics. These results suggest that the changes in responsiveness of somatosensory neurons and in behaviour, previously described in arthritic rats, can be partly accounted for in terms of the altered properties of the joint capsule receptors.