Conduction of velocity groups in the optic nerve of the North American opossum (Didelphis virginiana): retinal origins and central projections.

The axonal conduction velocity groups in the optic nerve of the North American opossum were analyzed electrophysiologically and related to soma size groups of ganglion cells in terms of their retinal origin and laterality of projection. On the basis of analysis of field potentials and single unit responses recorded at the optic disc, three velocity groups were identified (d1, d2, and d3) and estimated to have average conduction velocities of 12, 8, and 5 meters/second. From recordings of the field potential around the perimeter of the optic disc, it was found that the d1 group was equally represented at all points around the disc, whereas the d2 group was largest in amplitude in superior temporal regions. Electrical stimulation of the optic tracts indicated that axons in the d1 group project either ipsilaterally or contralaterally, whereas the d2 group projects predominantly ipsilaterally, and the d3 group projects predominantly contralaterally. In order to relate these physiological data directly to soma size groups, horseradish peroxidase (HRP) was injected into one optic tract, and subsequently the retinae were processed for peroxidase reaction product in the ganglion cells. Labeled cells were seen in contralateral nasal, contralateral temporal, and ipsilateral temporal retina. Cells in all size classes were labeled in contralateral nasal retina. In contralateral temporal retina, labeled cells were either 10-17 micrometers diameter (small and medium) or 23-27 micrometer diameter (large), whereas in ipsilateral temporal retina, most labeled cells (94%) were 15-30 micrometer diameter (medium and large). The correspondence between these conduction velocity groups and the soma size groups described in the preceding paper (Rapaport et al., "81) is discussed.