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Department of Physiology, University of Göteborg, Sweden.

1. Changes in the excitability of the intraspinal terminals of fibres in the posterior knee joint and interosseous nerves were used as a measure of primary afferent depolarization (PAD) which is associated with presynaptic inhibition of transmission from afferent fibres. These were estimated from changes in the intensity of electrical stimuli required to activate the fibres in 50% of trials. In order to avoid the inclusion of group I muscle afferents which contaminate the joint and interosseal nerves, the analysis was restricted to fibres conducting at less than 75 m s-1 and/or displaying patterns of PAD which differed from those of group Ia and Ib muscle afferents in lower lumbar segments of anaesthetized cats. PAD was evoked by electrical stimulation of ipsilateral hindlimb nerves. 2. PAD of fibres in the posterior knee joint nerve was induced from group I (Ia and Ib) and group II muscle afferents and cutaneous afferents but not by stimulation of the joint or the interosseous nerves. The most effective stimuli were those applied to the superficial peroneal, sural, quadriceps and posterior biceps and semitendinosus nerves. 3. PAD of fibres in the interosseous nerve was also induced by stimulation of group I (Ia and Ib) and group II muscle afferents and cutaneous afferents and, in addition, by stimulation of joint and interosseous nerves. The most effective stimuli were those applied to the superficial peroneal, quadriceps, flexor digitorum longus and posterior biceps and semitendinosus nerves. 4. Individual fibres of the joint and the interosseous nerves were depolarized by only some of the conditioning stimuli. Even the most effective stimuli did not produce PAD in all of the fibres tested. Individual fibres of the joint and the interosseous nerves were depolarized by diverse combinations of afferents of different functional types and of different peripheral nerves. The differences in the sources of PAD were not associated with the conduction velocities and hence are unlikely to be related to differences in the receptor origin of the tested fibres. The diversity in the sources of PAD of individual fibres is interpreted as providing a high degree of differentiation in the control of transmission from receptors in joints and interosseal membranes.