Principles of Cellular Electrophysiology
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biology
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published 26/11/2007
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Summary
Resting Membrane Potential In nerve cells, potassium ions (K+) are at higher concentration inside the membrane than outside whereas the opposite is true for sodium (Na+), calcium (Ca2+), and chloride (Cl–) ions (Fig. 1.9-1). The bulk solutions on either side of the membrane are electrically neutral, with most of the intracellular negative charge being contributed by large organic anions (acids and proteins). The differential distribution of ions across neuronal membranes results in part from the action of membrane pumps that use energy from adenosine triphosphate (ATP) to drive ions against a concentration gradient into or out of the cell. The best characterized pump is the Na+-K+ adenosine triphosphatase (ATPase) that transports 3 Na+ out of and 2 K+ into the cell during each cycle. Because an unequal amount of charge is moved during each cycle, the pump is electrogenic and produces an electrochemical potential across the membrane that makes the inside of the membrane negative with respect to the outside. Na+-K+ ATPase activity is a major contributor to brain energy utilization, with as much as 40 percent of brain oxygen consumption resulting from pump activity required to reestablish ionic homeostasis following action potential firing and synaptic transmission. The cardiac glycosides digoxin (Lanoxin) and ouabain are effective inhibitors of Na+-K+ ATPase in the heart and improve myocardial contractility by depolarizing cardiac myocytes and increasing intracellular Ca2+.
Table of Contents
- At rest, neuronal membranes are permeable to K+ and Cl– and to a lesser extent to Na+, partly because of the flow of ions through nongated leakage channels.
- For each ion in solution there is a specific membrane potential at which the opposing forces of the electrical gradient and concentration gradient are balanced.
- Because ions do not directly penetrate the lipid membrane but rather flow through ion channels, the ion channels can be thought of as variable resistors.
- Active Membrane Properties: Action Potentials Changes in membrane potential have important effects on excitability because certain ion channels are activated (gated) by voltage changes.
- Many axons are encased in myelin sheaths that allow them to send action potentials over longer distances.
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