PRIMARY AFFERENT DEPOLARIZATION OF MYELINATED FIBERS IN THE JOINT ANDINTEROSSEOUS NERVES OF THE CAT

1. Changes in the excitability of the intraspinal terminals of fibres in the posterior knee joint and interosseous nerves were used as a mea sure of primary afferent depolarization (PAD) which is associated with presynaptic inhibition of transmission from afferent fibres. These we re estimated from changes in the intensity of electrical stimuli requi red to activate the fibres in 50 % of trials. In order to avoid the in clusion of group I muscle afferents which contaminate the joint and in terosseal nerves, the analysis was restricted to fibres conducting at less than 75 m s-1 and/or displaying patterns of PAD which differed fr om those of group I a and I b muscle afferents in lower lumbar segment s of anaesthetized cats. PAD was evoked by electrical stimulation of i psilateral hindlimb nerves. 2. PAD of fibres in the posterior knee joi nt nerve was induced from group I (I a and I b) and group II muscle af ferents and cutaneous afferents but not by stimulation of the joint or the interosseous nerves. The most effective stimuli were those applie d 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 (I a and I b) and group II muscle afferents and cutaneous afferents and, in addition, by stimulat ion of joint and interosseous nerves. The most effective stimuli were those applied to the superficial peroneal, quadriceps, flexor digitoru m longus and posterior biceps and semitendinosus nerves. 4. Individual fibres of the joint and the interosseous nerves were depolarized by o nly 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 comb inations of afferents of different functional types and of different p eripheral nerves. The differences in the sources of PAD were not assoc iated with the conduction velocities and hence are unlikely to be rela ted to differences in the receptor origin of the tested fibres. The di versity in the sources of PAD of individual fibres is interpreted as p roviding a high degree of differentiation in the control of transmissi on from receptors in joints and interosseal membranes.