A Four Group Classification of the Rat Superior Collicular Cells Responding to Optic Nerve Stimulation

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Abstract
With lightly anesthetized rats, activities of single cells responding to single optic nerve (ON) shocks were recorded from the superficial layers of the superior colliculus, i.e., St. [stratum] zonale (SZ), St. griseum superficiale (SGS) and St. opticum (SO). According to response latencies and recording depths, 4 classes of cells were identified: I, II, III, and IV cells. Class I cells, recorded in the SZ or the upper half of SGS, were innervated by slowly conducting ON fibers of velocities slower than 4 m/s. Class II cells, recorded from the middle part of the SGS, were innervated by ON fibers of intermediate velocity (4-8 m/s). Class III cells, recorded from the lower half of SGS or the upper part of SO, were innervated by fast conducting ON fibers (faster than 8 m/s). Class IV cells were recorded from the depth similar to the class III but they were innervated by slowly conducting fibers (4-1.7 m/s). Class I cells revealed a long-lasting inhibition after initial excitation, being a sharp contrast with cells of classes II and III which were almost free from inhibition. Inhibition exerted upon class IV cells were of intermediate strength. Possible anatomical correlates of the 4 classes of cells and hypothetical modes of their ON innervation were discussed.