Ion channels and cell excitability

Summary form only given. Ion channels are protein pores (enzymes) that pass through the lipid bilayer and provide a high-dielectric, high-mobility pathway for the passage of ions. Biological channels are also allosteric; their mean conductance can be modified by the addition of free energy from a number of sources. This modulation generally occurs through changes in the probability of a channel being open rather than by changes in the open-channel conductance. Three primary inputs provide free energy for channel gating: the membrane electric field, the membrane tension, and ligand binding. Within a single cell, ion channels are coupled by the membrane potential, the entry of permeant ions, and other biochemical processes within the cell. Changes in ion flux can also affect cell volume which, in turn, can change cell volume and activate mechanosensory ion channels. Even in a single cell, the integrated activity of many types of channels, which are coupled by nonlinear processes, over a wide range of time scales and different space scales, often in nonhomogeneous patterns of density, can lead to remarkably complex behavior