In 37 cats anesthetized with alpha-chloralose recordings were made from single-afferent units of the medial articular nerve (MAN) of the right knee joint. First the mechanosensitivity of such units was characterized while the joint was in normal condition. Thereafter, keeping the afferents under continuous observation, an experimental arthritis was induced by injecting kaolin and carrageenan into the joint cavity. The effects of the developing arthritis including the time course of the changes were studied on low- and high-threshold units and on afferents that had no mechanosensitivity in the normal joint. 2. The arthritis increased the mechanosensitivity in the majority of the low-threshold units, i.e., in units that responded already in the normal joint to movements in the working range. Enhanced responses to movements were found for 12 of 16 thick myelinated group II, 10 of 10 fine myelinated group III, and 1 of 3 unmyelinated group IV afferents. The augmentation of reactions developed in most cases within the first hour after the injection of the inflammatory compounds, sometimes starting immediately after the injection. A further rise of the mechanosensitivity was observed within the following 2-4 h. In most group III units enhanced responses for movements were accompanied by an induction or increase of resting discharges. In 1 group II and 1 group IV unit spontaneous activity developed in the absence of any change of movement-sensitivity. 3. The inflammation led to enhanced mechanosensitivity in high-threshold afferents, i.e., in units that responded in the normal joint only to noxious movements exceeding the working range of the knee. One group II, 10 of 12 group III, and 5 of 10 group IV units of this type became responsive to movements in the working range during development of arthritis, in most cases within the second to third hour after induction of inflammation with a further increase later on. In a high proportion of these units resting activity was induced too. Few high-threshold units developed spontaneous discharges but no responses to movements in the working range. The time course for development of resting activity was similar to that for lowering of the mechanical threshold. 4. The experimental arthritis induced afferent activity in 1 of 2 group III and 10 of 14 group IV units that in the normal joint were unresponsive to local mechanical stimulation and to innocuous/noxious movements (but responsive to a bolus of a KCl-solution applied intraarterially close to the joint).(ABSTRACT TRUNCATED AT 400 WORDS)
