Trophic Regulation of Nerve Sprouting

Abstract

The hypothesis that the density of endings in a given nerve field is regulated by the interaction of sprouting factors continually manufactured by the target tissue, with neutrilizing (antisprouting) agents carried to the nerve endings by neuronal transport, was strengthened by quantitative studies on the mechanosensory innervation of salamander skin. After fast axoplasmic transport was partially interrupted in 1 nerve by colchicine application, its terminal field was invaded by sprouting fibers from neighboring axons, even though its own endings might be unchanged in number, distribution and sensory threshold. The area of a given nerve field (the extent of target territory over which the nerve endings occur) is susceptible to a spatial control that is not located in the target itself, but relates to the coordinate system provided by the body. The stimulus that initiates regeneration in a cut nerve is different from that causing collateral spouting of intact nerves at a target tissue. Regenerating fibers seem to have central drive and can ignore the limitations imposed by the body space control of territory. When guided by degenerating nerve trunks, regenerating axons of a nerve will readily innervate alien territory whose border is not crossed by the collateral sprouts of intact fibers of the same nerve.