Axonal guidance of developing optic nerves in the frog. II. electrophysiological studies of the projection from transplanted eye primordia.

When a primordial eye was transplanted to the ear position in Rana pipiens embryos, the optic nerve from the ectopic eye penetrated the medulla and invariably established a tract in the dorsolateral white matter of the ipsilateral spinal cord. In response to visual stimulation of the transplanted eye, extracellular recordings with metal microelectrodes were conducted with the spinal cords of post-metamorphic animals. Visual activity in the spinal cord could only be recorded in those experimental animals in which the transplanted optic nerve succeeded in penetrating the medulla. This activity was frequently encountered in the gray matter of the cord well below the dorsolateral position of the transplanted optic tract. The discharge characteristics and adaptation properties of the visual activity were often similar to that of optic nerve fibers from normal eyes suggesting that axons or their collaterals branch off from the transplanted optic tract and arborize within the spinal cord. However, occassionally stimulation of the transplanted eye evoked activity with adaptation and/or response characteristics unlike that of normal optic nerve fibers. Visual activity in the spincal cords of our experimental animals could be driven by moving small dark objects within circumscribed regions in the visual field of the transplanted eye. However we were unable to find any evidene of a systematic mapping of the transplanted retina within these abnormally penetrated spinal cords.