ABSTRACT
Bioelectricity has its origin in the voltage differences present between the inside and outside of cells. These potentials arise from the specialized properties of the cell membrane, which separates the intracellular from the extracellular volume. From the perspective of electrophysiology, cell membranes are not just containers separating intracellular from extracellular volumes, rather, membranes are the site of the electrically active elements — pumps, channels, and connexons joining cells — that create the voltages and currents that cause electrical events to occur. (In contrast, both intracellular and extracellular volumes serve largely as passive conductors of current, though differences in ionic concentrations between inside and outside are large and electrically significant.) Much of the membrane surface is made of a phospholipid bilayer, an
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electrically inert material [Byrne and Schultz, 1988]. Because the membrane is thin (about 75 Å), it has a high capacitance, about 1 uF/cm2. (Membrane capacitance is less in nerve, except at nodes, because most of the membrane is myelinated, which makes it much thicker.)
Electrically active membrane also includes a number of integral proteins of different kinds, with genetic origins that are increasingly well understood [Marban, 1999]. Integral proteins are compact but complex structures extending across the membrane. Each integral protein is composed of a large number of amino acids, often in a single long polypeptide chain, which folds into multiple domains. Each domain may span the membrane several times. The multiple crossings may build the functional structure, for example, a channel, through which electrically charged ions may flow. The structure of integral proteins is loosely analogous to that of a length of thread passing back and forth across a fabric to form a buttonhole. As a buttonhole allows movement of a button from one side of the fabric to the other, an integral protein may allow passage of ions from the exterior of a cell to the interior, or vice versa. In contrast to a buttonhole, an integral protein has active properties, for example, the ability to open or close. Excellent drawings of channel structure and function are given by Watson et al. [1992].
