The vertebrate retina is much more complex than the Limulus retina, and is not understood as well. The receptive field of ganglion cells shows center-surround structure, but its mechanisms are very different than in Limulus. The neurons There are about 100 million photoreceptors. The two types are rods, which are specialized for night, and cones, which are specialized for day. Since the horizontal cells are connected by gap junctions, we can model them as a network of resistors.
V n Let R be the axial resistance between nodes.
Each node is connected to ground by a conductance G and a capacitance C . This is interesting, because it looks like the network equations that we derived for chemical synapses, even though these synapses are electrical. The lateral interactions are excitatory. We can make a continuum approximation. In two dimensions, the solution is a Bessel function.