FIBER TYPES CONTRIBUTING TO DORSAL-ROOT REFLEXES INDUCED BY JOINT INFLAMMATION IN CATS AND MONKEYS

1. Injection of kaolin and carrageenan into the knee joint of cats or monkeys resulted in an acute inflammation. Four hours after injection of the knee joint, efferent activity could be evoked in articular afferent fibers and in dorsal root filaments. We interpret this efferent activity to be dorsal root reflexes (DRRs). Under our experimental conditions, the DRRs were generally synchronized compound action potentials, although in some cases single-unit activity was also observed. 2. DRRs were not produced in animals with uninflamed knee joints and normal body temperatures. 3. Recordings from two different sites on cut dorsal root filaments ipsilateral to the inflamed knee joint allowed the determination of the conduction velocities of groups of afferent fibers carrying DRRs. The DRRs occurred in A beta-, A delta-, and C fibers. However, in these experiments the peripheral destination of the afferent fibers was unknown. 4. To prove that DRRs occurred in joint afferents, recordings were made from two different sites on the proximal stump of the medial articular nerve that innervated the inflamed knee. The DRRs were again found in all fiber types, i.e., group II, III, and IV (A beta, A delta, and C) articular afferent fibers. 5. Compound DRRs were recorded from the central end of a cut dorsal root filament after electrical stimulation at C fiber intensity of a dorsal root adjacent to the filament. This DRR activity was eliminated by extensive dorsal rhizotomies of the L(2)-S-1 roots. 6. The central delay of DRRs evoked by stimulation of a dorsal root occurred as early as 3 ms and that from the periphery occurred later, beginning at similar to 40 ms, indicating that the pathway within the spinal cord to produce the DRRs has few synapses, because most of the delay can be attributed to the time required for conduction in fine peripheral nerve fibers. 7. DRRs were evoked by flexion of the inflamed knee joint or by brisk mechanical stimulation of the hindlimb. The greatest responses occurred after stimulation of areas close to the knee joint, but the receptive field could extend onto the foot. 8. We conclude that the efferent activity in dorsal root filaments and articular nerves is DRRs that develop in response to peripheral input in the acute inflammatory state. Furthermore, the DRRs are dependent on continued peripheral input, because they were no longer evoked after extensive dorsal rhizotomies. 9. The DRRs occur not only in myelinated articular afferent fibers but also in unmyelinated fibers, and are likely to contribute to the peripheral inflammation by release of inflammatory substances in the knee joint. It is possible that some of the DRRs recorded from dorsal root filaments are in afferent fibers that supply tissues other than the knee joint, but this was not investigated.