The retinal origins of the optic nerve conduction latency groups in the brush-tailed possum, Trichosurus vulpecula.

Abstract

AbstractThe antidromic compound action potential recorded near the centre of the possum optic disc, following stimulation of the optic chiasm, has four negative peaks corresponding to the optic nerve conduction latency groups t1, t2, t3 and t4. The t2 peak is reduced in the compound action potential recorded from the nasal margin of the optic disc but is prominent on the opposite side at the superior temporal margin. The t4 peak may be absent in this vicinity.The frequency distributions of the diameters of intraretinal ganglion cell axons at the nasal margin and at the superior temporal margin of the optic disc are bimodal. Neither of these size distributions can be directly correlated with the multipeaked antidromic compound action potentials recorded at these same sites. However, by considering the frequency distribution difference between these sites, it is possible to infer (i) that the optic nerve t2 group may be represented intraretinally on the temporal side by a population of medium sized axons with a modal diameter of 0.8 μm and (ii) that the t4 group may be represented intraretinally on the nasal side by a population of small axons with modal diameter 0.3 μm.In addition, a correlation of axon sizes with previously reported regional variations in the sizes of ganglion cell somas suggests that the optic nerve t4 conduction latency group may arise from small ganglion cells in the nasal streak and nasal peripheral retina and that the t2 group may arise from medium sized cells in the temporal area centralis.