The Action Potential
Action Potentials are big changes in membrane potential. Remember in a resting cell, there are more positive ions outside the cell and fewer positive ions inside the cell. During an action potential, this situation is reversed. More positive ions are found inside the cell than outside the cell during an action potential. The inside of the cell actually becomes positive with respect to the outside. The membrane is said to depolarize. The membrane then repolarizes and returns to normal after a short period of hyperpolarization. Action potentials work according to the all-or-none principle. This means either an action potential happens or it doesn't happen, there is no in between. It's sort of like being pregnant. One is either pregnant or not pregnant, there is no in between. Or think about firing a gun, it either fires or it doesn't fire. It is not possible to barely shoot a bullet from the barrel of a gun. It comes out with complete force or not at all. Like firing a gun, action potentials have a threshold. To fire a gun, the trigger must be pulled. Triggers on some guns can be pulled back a little bit without the gun firing. However, once the trigger is pulled to a certain point, the hammer falls and the bullet fires. With action potentials, there is a threshold level that must be reached before the action potential will fire.
This means the magnitude of the stimulation on the cell must be large enough to create and action potential. If the stimulus strength is not at or above the threshold level, an action potential will not be fired. The threshold level for many neurons is -55mV. This means that the membrane must depolarize to -55mV or an action potential will not be fired. A stimulus may be strong enough to cause the membrane to depolarize to the threshold level or it may not be strong enough. In order for an action potential to fire, threshold must be reached.
Figure 4. Graph of Action Potential. Plotting voltage measured across the cell membrane against time, the action potential begins with depolarization, followed by repolarization, which goes past the resting potential into hyperpolarization, and finally the membrane returns to rest.
The above figure shows how the membrane potential might change over time. The membrane was stimulated two times, but not to the threshold level. Therefore, an action potential was not fired. However, the third time the membrane was stimulated; the membrane did reach its threshold level. The result was an action potential. The action potential has three phases; the depolarization phase, the repolarization phase, and the after-hyperpolarization phase. As you can see, the action potential is a rapid change in membrane potential. In a matter of milliseconds, the membrane potential went from -70mV up to +30mV. The inside of the cell actually became more positive than the outside of the cell.
The threshold level may at first seem like some magical level at which the neuron fires an action potential. However, there is nothing magical too it at all. The threshold level is simply the point at which a very important positive feedback loop kicks in. Remember, positive feedback loops lead to the extreme. The cell experiences a change in one direction and responds by promoting a change in the same direction. Once the threshold level is reached (enough sodium ions were allowed in from the ligand-gated channels opening that the membrane potential gets up to -55mV), special voltage-gated sodium gates in the cell membrane open up. When these gates open, sodium ions stream into the cell along their electrical and diffusion gradients (because there was less inside than outside the cell). As sodium ions move into the cell, the inside of the cell becomes more positive (it depolarizes). This depolarization of the membrane causes even more sodium gates to open. When these additional sodium gates open, more sodium ions move into the cell. As more sodium ions move into the cell, the membrane depolarized even more. This additional depolarization causes even more sodium gates to open which causes more sodium ions to enter the cell which causes the cell membrane to depolarize even more... .... The net effect is the membrane becomes very permeable to sodium ions very quickly and sodium ions move into the cell along their diffusion and electrical gradients. This explains the depolarization phase.
Once the membrane reaches +30 mV, the voltage-gated sodium gates close and special voltage-gated potassium gates open. Sodium ions can no longer move into the cell and the sodium-potassium pump is continually pumping them out. The opening of the voltage-gated potassium channels causes the membrane to become even more permeable to potassium ions. As a result, more potassium ions leave the cell along their diffusion gradient (and electrical gradient at first). This movement of potassium ions out of the cell and the action of the sodium-potassium pump causes the cell membrane to repolarize.
The special potassium gates actually stay open for a few milliseconds after the membrane potential returns to resting. This causes the potassium ions to continue to diffuse out of the cell. The movement of potassium ions out of the cell results in the membrane potential becoming more negative than the resting membrane potential, which is called the after-hyperpolarization phase.
During the after-hyperpolarization phase, the cell is not responsive to stimuli of threshold strength. Because the membrane is hyperpolarized, it would require a stimulus of greater that threshold strength to fire another action potential. This is called the relative refractory period. During the relative refractory period, the cell is unresponsive to threshold strength stimuli. However, it will respond to suprathreshold strength stimuli.
There is also an absolute refractory period. During the absolute refractory period, the cell is completely unresponsive to successive stimuli, no matter how strong. The absolute refractory period corresponds with the depolarization and repolarization phases of the action potential. This is simply common sense; the cell can not fire two action potentials at once any more than a rifle with a single barrel can fire two bullets at exactly the same time.
The action potential is created at some point on the neuron's membrane. This action potential at this local area of the membrane triggers action potentials in the adjacent areas of the membrane. By this mechanism, the neurons entire membrane depolarizes and repolarizes. This process spreads across the membrane within milliseconds, even in a neuron with axons or dendrites over a meter long. Action potentials involve the entire cell. The entire cell depolarizes to +30mV and repolarizes to -70mV. Also, remember that action potentials are all-or-none, so once threshold is reached there is no stopping the signal from reaching the axon terminals.
