Effects of PAD on conduction of action potentials within segmental and ascending branches of single muscle afferents in the cat spinal cord

In anesthetized and paralyzed cats under artificial respiration, we examine d the extent to which primary afferent depolarization (PAD) might affect in vasion of action potentials in intraspinal axonal and/or terminal branches of single muscle afferents. To this end, one stimulating micropipette was p laced at the L6 spinal level within the intermediate or motor nucleus, and another one at the L3 level, in or close to Clarke's column. Antidromically conducted responses produced in single muscle afferents by stimulation at these two spinal levels were recorded from fine lateral gastrocnemius nerve filaments. In all fibers examined, stimulation of one branch, with strengt hs producing action potentials, increased the intraspinal threshold of the other branch when applied at short conditioning testing stimulus intervals (<1.5-2.0 ms), because of the refractoriness produced by the action potenti als invading the tested branch. Similar increases in the intraspinal thresh old were found in branches showing tonic PAD and also during the PAD evoked by stimulation of group I afferent fibers in muscle nerves. It is conclude d that during tonic or evoked PAD, axonal branches in the dorsal columns an d myelinated terminals of muscle afferents ending deep in the L6 and L3 seg mental levels continue to be invaded by action potentials. These findings s trengthen the view that presynaptic inhibition of muscle afferents produced by activation of GABAergic mechanisms is more likely to result from change s in the synaptic effectiveness of the afferent terminals than from conduct ion failure because of PAD.