Cat superior colliculus: extracellular potentials related to W-cell synaptic actions.

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Abstract
The bulk of the crossed retinal projection to the cat''s superior colliculus is known to end just beneath the stratum zonale in the dorsalmost tier of the superficial gray layer. Extracellular microelectrode recordings in this region revealed the presence of large negative waves, 5-8 ms in duration, which were commonly preceded by small prepotentials. The potentials could be evoked by light stimulation of the contralateral retina and occurred at constant latency and in an all-or-none fashion following electrical stimulation of the optic pathway. Since the large negative potentials were observed only in a narrow zone just beneath the stratum zonale, they were named juxtazonal potentials (JZP). JZP and their prepotentials were not observed anywhere in the superficial gray layer in response to stimulation of the ipsilateral eye. Stimuli moving faster than 10.degree./s were ineffective in evoking JZP and their prepotentials, but stimuli moving less than 1.degree./s elicited showers of these potentials. Extensive chronic ablation of occipital cortex, including areas 17, 18 and 19, produced no detectable effects on JZP and prepotentials recorded ipsilateral to the ablated cortex. JZP followed their prepotentials by 0.3-0.5 ms. Optic tract stimuli of increasing intensity recruited JZP of different latencies which united to form a large negative field potential. The peak of this potential depended on optic tract fibers conducting at velocities characteristic of W-cell axons. Individual JZP and their prepotentials evoked by optic tract stimulation occurred at latencies typical of the direct W-cell pathway to the colliculus. JZP evoked by optic tract stimulation occurred at the same time that deeper cells were excited by the direct W-cell projection from the retina. These results, together with available ultrastructural evidence, support the hypothesis that the prepotential arises when an action potential invades the terminals of a single W-cell axon, producing synchronous activation of multiple excitatory contacts; the resulting extracellular currents generate the JZP.