Characteristics of Optic Nerve Innervation in the Rat Superior Colliculus as Revealed by Field Potential Analysis

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Abstract
With lightly anesthetized rats, field responses to single optic nerve (ON) and optic chiasm shocks were studied in superficial layers of the superior colliculus, i.e., St. [stratum] zonale (SZ), St. griseum superficiale (SGS), and St. opticum (SO). Based upon characteristics in the depth profile of the field response within the superficial layers, 3 zones of N3, N2, and N1 were identified from the SZ in this order. The narrow N2 zone was localized around the middle part of the SGS and less distinct than the other 2 zones. The N3 zone was characterized by a N3 wave and the N1 zone with N1 and P3 waves. N3 and P3 waves were activated by slowly conducting ON fibers (5.0 .+-. 1.4 and 4.2 .+-. 0.6 m/s, respectively) while the N1 wave was by fastest conducting fibers (16.3 .+-. 4.1 m/s). According to recent Golgi studies, hypothetical mechanisms underlying the 3 waves were proposed: the N3 wave was ascribed to the activities of small vertical fusiform cells in the SZ or the upper half of SGS, the N1 wave to those of narrow field vertical cells which predominate in the lower half of SGS, and the P3 wave to those of wide field vertical cells whose somata were in the lower half of SGS or in the SO. The N3, N1 and P3 waves differed from each other in recovery function tested with double ON shocks and in the topographical analysis of amplitudes through a whole extent of the colliculus. The 3 systems represented by the 3 waves were suggested to play different roles in visual information processing within the superficial strata.