Letters written by Bob Eisenberg (Physics Today, July 2013, page 10) and John Michael Williams (December 2013, page 9) raise the issue of how to model a cell in an electrostatic field.
Cytoplasm, the gel-like interior of a cell, is a good ionic conductor. When an external electric field is applied, the ions quickly redistribute themselves to produce a field that essentially cancels the applied field inside the cytoplasm. The resulting change in potential difference must then appear almost entirely across the membrane. The time scale for that process is on the order of a microsecond. Consequently, for applied fields with a frequency of less than about 1 MHz, the molecules in the cytoplasm are not directly affected by the external field.
Effects produced inside the cell must then be due to the action of the external field on the cell membrane. Three physical models have been proposed to describe that process: electrodiffusion of cell-surface receptors; opening of voltage-gated channels; and electromechanical torque acting on transmembrane, negatively charged glycoproteins connected to the cytoskeleton. Fields on the order of 100 V/m in the extracellular fluid applied for up to a few seconds have been shown to produce a wide variety of bioeffects. Fields on the order of 1 kV/m applied for about 15 minutes are required for electrodiffusion,1 and typical applied fields are not strong enough to open voltage-gated channels.2 In contrast, the electromechanical model avoids these problems and makes specific predictions that are confirmed experimentally.3 Methods for modeling the fields in complex cellular systems can be found in reference 3.
