Electrotonic Signals along Intracellular Membranes May Interconnect Dendritic Spines and Nucleus
Figure 2
The CIC system predicts a Virtual-Electrode along the ER membrane under realistic parameters.
Steady-state description of the CIC-model prediction under the parameters described in Table 2. (A) The potential along the cytosol (blue; Vi) and the potential along the ER lumen (red; VER) decay at different rates along distance (x[λ]) given in electrotonic units. As a result, the ER transmembrane potential (green; VmE, given by the difference between VER and Vi) generates a qualitatively unique pattern (inset). Namely, VmE is negative along ∼0.65 space-constants from the synapse, where it crosses the zero line, becomes positive and then decays to zero VmE. We name the positive segment (where VmE>0) that follows a negative segment (where 0<VmE), ‘Virtual-Electrode’ (VE; Dashed area, see text for details). Note that practically VmP = Vi (solid red line), since we assume that Ve→0. (B) Comparison between the prediction of the CIC model (solid line) and the prediction of the conventional cable model (dashed line) for the transmembrane potential along the plasma membrane (VmP). Both traces were normalized to VmP at the synapse (VmP(x = 0)). Bottom: Subtracting the prediction of the conventional cable model from the prediction of the CIC model (green line). Note that the maximal difference between the two predictions lies below 2% of VmP(x = 0). (C) The spatial pattern of the peak potential across the ER membrane (VE-peak) is compared to the EPSP amplitude at the same distance from the synapse (nVE; green) and shows that at the peak, the VE reaches ∼41% of the EPSP. A second way to compare the VmE to the EPSP is presenting VmE as a fraction of VmP at each point along the cable. (black line) This representation of VmE shows that at positions beyond the peak of the VE, the VmE amplitude reaches values greater than the VmP amplitude. (D) Parameter dependency of VE amplitude and pattern: VmE was recalculated after increasing (blue) or decreasing (red) N or E (the non-conductive cross-section or the ER cross-section, respectively) by 15% from the default parameters (gray) provided in Table 2. VmE is presented in relation to EPSP as nVE representation and as a fraction of VmP at each point along the cable (solid lines and dashed lines, respectively).