The ionic basis of cell excitation
Excitable cells – nerve cells and the various types of muscle cells – have a prominent role in the physiological processes that are targeted by drug therapy. We will therefore spend some time looking at how electrical cell excitation works.
The fundamental prerequisite of excitability is the presence of a membrane potential. A membrane potential is present in apparently all living cells. In non-excitable cells, the ori-entation of the membrane potential is always such that the cell interior is electrically negative against the outside. This orientation also prevails in excitable cells that are not cur-rently excited, i.e. currently are at their resting potential. One fundamental function of this negative-inside mem-brane potential in all cells consists in powering active trans-port, usually in the form of sodium cotransport.
Membrane potentials also exist across membranes within cells. An important example is the potential across the inner mitochondrial membrane, which is the major driving force of ATP synthesis. However, since the intracellular potentials don't have a prominent role in cell excitation and pharmacology, the following discussion will focus on the potentials that occur at the cytoplasmic membrane.
In excitable cells, the orientation of the membrane poten-tial is reversed for a brief period of time during excitation. This transient reversal is called the action potential. Its du-ration may vary from ~1 millisecond to several hundred milliseconds, depending on the cell type. An action poten-tial is typically triggered locally on a small patch of the cell membrane. However, from there, it will rapidly spread all over the entire cell membrane, rapidly altering the function-al state of the cell. Moreover, excitation of one cell often triggers excitation of neighbouring cells by means of elec-trical or chemical coupling. The physiological significance of electrical cell excitation thus is that it provides the most rapid means of signal transduction and communication within and between cells.
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