Figure 3. The individual synapse can determine the VE position or its polarity at a distinct target along the cable.

(A) Synaptic input initiates, simultaneously, axial currents along the cytosol (I_i) and along the ER lumen (I_ER). The ratio (I) between these currents at the synapse (I_{ER (x = 0)} and I_{i (x = 0)}, respectively), modulates the position of the VE. These currents may be modified by active transition of charges (i _mER; Eq. G1.4, G1.5) across the ER membrane, at the synapse, during synaptic activation (see text for details). The traces represent steady-state nVE introduced by 3 different synaptic signals with identical V _{mP(x = 0)} (i.e. potential across plasma membrane at the synapse), identical cable properties, but different I ratios (blue, red and black traces represent simulations of synaptic activation with positive, negative and zero I, respectively). (B) VE-peak position is presented as a function of I. Its position is represented as percent of the distance between the synapse and VE-peak when I = 0. Inset: The amplitude of nVE-peak as a function of I. Y-axis describes the percent change in the peak level of nVE, from the default (I = 0) level. (C) The effect of I on V _mE at several fixed distances from the synapse. Each trace represents the nVE level as a function of I at a fixed position (0.2, 0.6, 1, 2 space constants from the synapse; red, black blue and blue, respectively). V _mE amplitude at each position is described as percent of V _mP amplitude (EPSP) at that specific distance from the synapse. Inset: Triangles depict the sampling position of traces with corresponding color. Note that at each target, VE amplitude can reach 100% of EPSP level or drop below zero. (D) Steady-state calcium level is plotted as function of distance from a point source of calcium (1 pA). The calculations assumed basal Ca²⁺ level of 70 nM and an endogenous mobile buffer of kD = 50 µM and concentration of 0.5 mM (see text for details). The region with a significant calcium elevation was assumed to be where Ca²⁺ level raised above twice the basal level (dashed line; 12 µm). With a typical dendritic spine density [35],[36] the estimated extent of Ca-signal spread along the dendritic shaft, is predicted to cover a region occupied by ∼20 dendritic spines.