CENTRAL SPROUTING AND FUNCTIONAL PLASTICITY OF REGENERATED PRIMARY AFFERENTS

A combination of neuroanatomical and electrophysiological techniques w as used to study the effects of peripheral axotomy and regeneration of primary afferents on their central projections in the spinal cord. In dividual regenerated afferent fibers were impaled with HRP-filled elec trodes in the dorsal columns of alpha-chloralose-anesthetized cats and activated by current pulses delivered via the intracellular electrode . The resulting cord dorsum potentials (CDPs) were recorded at four ro strocaudal locations and HRP was iontophoretically injected into the f iber. Central distributions of boutons and CDPs were compared with per ipheral receptor type to determine the accuracy of peripheral regenera tion and the effects of central-peripheral mismatches. Reconstruction of the central projections of 13 individual afferents for which the ad equate stimulus and CDPs had been recorded revealed many abnormalities . For example, unlike controls, four group I and II afferents with cen tral projections typical of proprioceptors (concentrated in laminae V, VI, and VII) innervating either cutaneous or noncutaneous targets evo ked measurable CDPs. Three other group II or A beta afferents innervat ing low-threshold mechanoreceptors with central terminations confined to the dorsal horn exhibited extensive collateralization in laminae I and II in addition to large numbers of terminals in laminae Ill-IV. Th ese fibers activated central networks whose adaptation behavior was id entical to those evoked by high-threshold mechanoreceptive afferents i n controls. These results suggest that primary afferents and their cen tral connections are capable of significant modifications following ax otomy and regeneration. In addition, the anatomical studies indicate s ome reorganization in the laminar distribution of boutons as well as i n bouton size.