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 examined the extent to which primary afferent depolarization (PAD) might affect invasion of action potentials in intraspinal axonal and/or terminal branches of single muscle afferents. To this end, one stimulating micropipette was placed 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 strengths producing action potentials, increased the intraspinal threshold of the other branch when applied at short conditioning testing stimulus intervals (