REDUCTION OF PRESYNAPTIC ACTION-POTENTIALS BY PAD - MODEL AND EXPERIMENTAL-STUDY

A compartmental model of myelinated nerve fiber was used to show that primary afferent depolarization (PAD), as elicited by axo-axonic synap ses, reduces the amplitude of propagating action potentials primarily by interfering with ionic current responsible for the spike regenerati on. This reduction adds to the effect of the synaptic shunt, increases with the PAD amplitude, and occurs at significant distances from the synaptic zone. PAD transiently enhances the sodium current activation, which partly accounts for the PAD-induced fiber hyperexcitability, an d enhances sodium inactivation on a slower time course, thus reducing the amplitude of action potentials. In vivo, intraaxonal recordings fr om the intraspinal portion of group I afferent fibers were carried out to verify that depolarizations reduced the amplitude of propagating a ction potentials as predicted by the model. This article suggests PAD might play a major role in presynaptic inhibition.