There is a population of sodium channels throughout the cell membrane. In this mind map a series of four paired ovals along the lower left side of the cell represents four different conformations of a single sodium channel. At the cells resting membrane potential of -85 mV most sodium channels are in the first conformation (#1 paired ovals).
Sodium channels have two gates:
In the first conformation (#1 paired ovals) the slow inactivation gate is open but the fast activation gate is closed. This prevents the influx of sodium ions along their gradient. However, when cations (e.g., calcium (Ca++) and potassium (K+)) diffuse into the cell from adjacent cells via gap junctions ( cylinder), this cells' inner membrane surface becomes more positive. The result of this influx of positive ions is seen in section A of the action potential graph; it is called slow depolarization.
When the voltage reaches -70 mV (threshold potential) the sodium channels begin a series of conformational changes represented by dark arrows. The fast activation gate retracts instantly allowing the rapid influx of sodium ions along their electrochemical gradient (block arrow). This is shown in the second conformation. The result of this influx of sodium ions is seen as section B in the action potential graph; it is called rapid depolarization.
After 2 mSec the slow inactivation gate has blocked the channel by bending over the opening. This is illustrated as the third conformation. The rapid depolarization peaks out at +20mV. The channel will remain like this until other ion fluctuations occur to make the membrane potential more negative than threshold potential (-70mV). When this does occur the channel will convert to the fourth conformation which involves the repositioning of the fast activation gate across the channel. Only then can the slow inactivation gate re-open and complete the cycle by returning the channel to its first conformation.
Last updated: 7/16/2005