Molecular Properties of Voltage-Sensitive Sodium Channels
Electrical excitability is one of the most important and characteristic properties of neurons. Most vertebrate cells, including neurons, maintain large ionic gradients across their surface membranes such that the intracellular fluid contains a high concentration of potassium ions and low concentrations of sodium ions and calcium ions relative to the extracellular fluid. These large ion gradients are maintained by the action of energy-dependent ion pumps specific for Na+ and K+, or for Ca2+. In addition, essentially all vertebrate cells maintain an internally negative membrane potential of the order of -60 mV, since their surface membranes are specifically permeable to K+, and this allows K+ to leak out of cells faster than Na+ and Ca2+ can leak in. Nerve cells are electrically excitable because of the presence, in their surface membranes, of voltage-sensitive ion channels that are selective for Na+, K+, or Ca2+. One class of Na+ channels and many classes of Ca2+ and K+ channels have been described in neurons. The channels open and close as a function of membrane voltage, allowing rapid movement of the appropriate ions down their concentration gradient so that ionic current passes into or out of the cell, depolarizing or hyperpolarizing the membrane.
KeywordsSodium Channel Scorpion Toxin Electrical Excitability Phosphatidylcholine Vesicle Sodium Channel Protein
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