EFFECTS OF HORIZONTAL CELL NETWORK ARCHITECTURE ON SIGNAL SPREAD IN THE TURTLE OUTER RETINA - EXPERIMENTS AND SIMULATIONS

In the Pseudemys turtle retina five functionally distinct, electricall y coupled networks of horizontal cells distribute signals in the outer plexiform layer, These networks differ significantly in their archite cture, as determined by intracellular labeling with Neurobiotin after physiological recording and identification, The density of H1 horizont al cells is highest, ranging around 1800 cells/mm(2) at approximately 2.3 mm eccentricity. H1 horizontal cell somata are connected via 6-10 thin, short dendrites, The H1 horizontal cell axon terminal network is composed of thick axon terminals, forming a three-dimensional, sheath -like structure, Networks of coupled H2 and H3 horizontal cells have c ell densities of around 210 cells/mm(2) and 350 cells/mm(2), respectiv ely, at the same eccentricity of 2.3 mm, Cell bodies are connected wit h 6-12 long, thin dendrites, Here we report for the first time H4 hori zontal cell networks, Cell density is approximately 970 cells/mm(2) at 2 mm eccentricity, and cell bodies are connected with 6-10 thin, shor t dendrites, General properties of passive voltage spread were compare d for three of these horizontal cell networks using NeuronC, Realistic network architectures were obtained by digitizing the intracellularly labeled networks, respectively, One network obtained from coupled H1 horizontal cell bodies, one from coupled H1 horizontal cell axon termi nals, and one from H2 horizontal cells were simulated, These three rea listic networks were compared with an artificial, electrically coupled regular triangular network, Passive signal spread in these networks s trongly depended on the exact network architecture using otherwise ide ntical parameters, Changes in coupling strength affected signal spread in these networks differently, As in the experimental situation, chan ges in synaptic conductance influenced signal spread, Some principal e ffects of extensively coupled horizontal cells on photoreceptor signal processing were simulated with one type of photoreceptor connected by telodendria, synapsing onto an underlying triangular network and rece iving feedback synapses, Under certain conditions, spatial information is coded in single photoreceptors, This was also the case in the expe rimental situation, In the simulation, spatial filter adjustment for o ptimal spatial coding in photoreceptors can be achieved by changing co upling strength in the horizontal cell network. Copyright (C) 1996 Els evier Science Ltd.