A three-group classification of rat retinal ganglion cells: histological and physiological studies

In a whole mount preparation of the hooded rat retina, stained with methylene blue, the ganglion cell density was counted over the retina. The density ranged from 6200-6500/sq.mm to 1000-1500/sq.mm. The area with a density higher than 6000/sq.mm was called the central area and was centered at about 1 mm (20 degrees) away from the center of the optic disc, in the upper temporal quadrant. Based upon soma diameter histograms made for several locations with different eccentricities, ganglion cells were classified into three groups; large (L), medium-sized (M) and small (S). Relative frequencies of the three groups, on average, were 5, 28 and 67%, respectively. The absolute densities of S- and M-cells increase sharply towards the central area whereas the L-cells show a relatively flat distribution over the retina. The three-group classification of the retinal ganglion cells was supported by measuring their axonal conduction velocities. Single unit recordings were made from the retina and the lateral geniculate nucleus. In each case, two sites were chosen in the optic pathway for electrical stimulation: the optic chiasm and the superior coliculus in the retinal study, and the distal end of the optic nerve and the optic chiasm in the geniculate study. From differences of response latency to the two stimulating sites, conduction velocities of the optic nerve fibers were estimated. The average velocities for the L-, M- and S-axons determined by retinal recordings were 16.8, 11.4 and 6.3 m/sec and those determined by geniculate recordings 18.6, 12.0 and 5.2 m/sec.