Restoration of retino-tectal connections in frogs during regeneration of the optic nerve

Abstract

At various times after unilateral division of the optic nerve in the frogRana temporaria L. evoked potentials in response to electrical stimulation of the optic nerve were investigated in a segment distal to the site of operation, spike activity was recorded from endings of regenerating and intertectal axons when stimuli of different shapes were placed in the field of vision, and the distribution of axonal bulbs of growth by depth in the tectum mesencephal was studied electron-microscopically. During regeneration of the axons the responses of the retinal ganglionic cells to visual stimuli retained most of their individual features. Myelinated axons of the retinal ganglionic cells regenerate first (starting on the 21st day after operation). Myelination of these fibers lags significantly behind their growth and is complete more than 100 days after the operation. Unmyelinated axons of the retinal ganglionic cells grow up toward the tectum mesencephali after myelinated axons (80 or more days after the operation). Axonal bulbs of growth in the initial periods after the operation are located close to the pial surface and the level of spread of the myelinated axons of the retinal ganglionic cells differs significantly from their normal level of localization. Intertectal connections persist after division of the nerve and are activated by visual stimuli during regeneration of the axons of the retinal ganglionic cells. Connections were found mainly between intertectal fibers terminating superficially and retinal ganglionic cells belonging to class 1 and 2 detectors. Axons of the retinal ganglionic cells grow up toward the caudal region of the tectum mesencephali later than toward the rostral region.