Figure 7. Model for ephaptic feedback.

(A) Schematic drawing of the cone synaptic terminal. HC, horizontal cell; BC, bipolar cell; I_Ca, cone Ca²⁺-current; V_ext, potential in the synaptic cleft. (B) The feedback model formulated by Fahrenfort et al. [6] was used to evaluate the effect of changes in voltage dependence of horizontal cell hemichannels. Here we will give a short description of the main features, for a full description see Fahrenfort et al. [6]. The model consists of a simple resistive network. The model was used to evaluate the relation between the horizontal cell membrane potential (V_HC) and the change in extracellular potential (V_ext) with different numbers of hemichannels in the horizontal cell membrane. V_ext is the potential in the synaptic cleft, V_HC is the horizontal cell membrane potential, g_hemi is the hemichannel conductance, g_{Glu, tip} is the glutamate conductance at the tips of the horizontal cell dendrites, g_Glu,neuropil is the glutamate conductance of the horizontal cell dendrites in the neuropil, g_K, is the potassium conductance of horizontal cells and E_K is the equilibrium potential for potassium. [Glu] is the Glutamate-concentration in the synaptic cleft. I_Ca is the cone Ca²⁺-current; g_ext is the conductance of the extracellular space in the synaptic complex.