Retrograde effects on synaptic transmission at the Ia/motoneuron connection

The fidelity of impulse propagation through the complex axonal tree en rout e to the various target cells of that fiber is an important question in neu robiology. Anatomists can trace pathways, but if impulses-fail to propagate down to the terminals to release transmitter onto the target cell, there i s a significant 'disconnect' between anatomy and physiology. These issues h ave been studied at length in the spinal cord of the cat where it has prove n possible to examine the connections made by afferent fibers on motoneuron s under different stimulus conditions. EPSP amplitude varies systematically during high frequency stimulation of the afferents according to the identi ty of the target motoneuron. This variation is a function of the state of t he motoneuron's relation to its peripheral target. It changes after motoneu ron axotomy and recovers with reinnervation of the periphery. Neurotrophins delivered to the axotomized motor axons fail to induce recovery. Chronic s timulation of the motor nerve alters muscle properties with coordinated cha nges in properties of the synapses on motoneurons innervating the stimulate d muscle. We cannot yet definitively establish the mechanisms determining s ynaptic behavior during high frequency stimulation. However, the retrograde : regulation of these properties suggests that it is an important variable and thus is worthy of intensive further study. (C) 1999 Editions scientifiq ues et medicales Elsevier SAS.