ELECTRIC IMPULSE GENERATION
Electrical impulses are the result of the movement of charged particles, called ions, across cell membranes. Since the movement of ions across cell membranes is governed by various ion channels, ion channels are integral to the generation and conduction of electrical impulses. Electrical impulses in physiologic systems generally follow a similar pattern, referred to as an action potential.
The initial phase of an action potential, termed depolarization, is characterized by an increase in the electrical potential across a cell membrane. This sharp positive deflection is typically the result of the opening of sodium or calcium ion channels, which allows the inward flow of positively charged ions into the cell. A second phase, referred to as the plateau phase, can exist in certain cell types and results as the inward flow of positively charged ions into the cell is counterbalanced by the outward flow of positively charged ions out of the cell. In the third phase of the action potential, termed repolarization, the ion channels responsible for current flow into the cell close, while those ion channels that allow the flow of positively charged ions out of the cell remain open. This results in a decrease in the electrical potential of the cell, repolarizing the cell membrane back to the baseline level.
Action potentials are the means by which electrical signals are generated and propagated along excitable tissue, such as in the nerves, heart and skeletal muscle. Many diseases are characterized by abnormalities in the electrical activity of these organ systems. For example, epilepsy is a disorder in which there is abnormal electrical activity in certain regions of the brain, leading to seizures. Similarly, cardiac arrhythmias such as atrial fibrillation are caused by abnormal electrical activity in certain regions of the heart. The ion channels which control such electrical activity therefore offer unique therapeutic opportunities for targeted, effective treatments.
For more information on how ion channels generate signals, check out Blackwell Publishers' NEUROBIOLOGY - Molecules, Cells and Systems.
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