The Structure Responsible for Action Potential Spikes in the Cat's Retina

IT is well known from the work of Granit1 that isolated action potential spikes may be recorded from the eyes of many animals by placing a micro-electrode in contact with suitable spots on the retina through the opened bulb. The cat is particularly suitable. The importance of this work makes it especially desirable to define as precisely as possible the actual retinal structures responsible for the spikes recorded, for such questions as the following come to mind in attempting to interpret and evaluate the full significance of Granit's results. How is it possible to get such good insulation in a packed structure like the retina using a 25-µ electrode? Why are large isolated spikes only found in some spots and not elsewhere? Is it because some typical structure happens to be exceptionally favourably placed for recording, or is the structure a comparatively rare one? Or are the spikes due to the synchronous firing of a patch of retinal cells, and if so, is this normal or pathological?