The charge across a membrane is a potential. The "resting" potential of a cell carries a -70 mV charge inside relative to outside the cell. One can talk about the potential across the membrane of a cell at any time by calling it the membrane potential. One can talk about a specific dynamic change in the charge across the membrane of a cell, one that occurs either totally or not at all, as an action potential. You will also see that cells that receive synaptic information turn that into a postsynaptic potential toward the end of this unit. Let's explore the action potential here.
What is the action potential?
Keep in mind that the action potential runs down the axon, but doesn't typically occur anywhere else on the neuron. So just consider the axon here. We'll consider the remainder of the neuron in the postsynaptic potential section of this unit.
You saw what happened when the voltage-gated channels opened in the "gated channels" web page. The Na+ V-gated channel opened first, then it inactivated while the delayed rectifier opened. During the open times of these channels, ions are flowing across the membrane. Ions are charged, so if charges flow across the membrane, the membrane potential across the axon will change.
How does it change?
1. When sodium ions flood into the axon, the inside will get more positive (right? sodium ions are positively charged, so when they enter, they bring a postiveness with them).
2. When potassium ions leave the axon... after a delay due to their slow voltage-gated channel, the delayed rectifier... they restore the normal resting potential.
These quick changes in the potential of the axon are the action potential.
What does the action potential look like?
Here's my drawing of an action potential. Let's slowly walk through this figure.
The Y axis is voltage, and the X axis is time. You already know how to measure time. But how do we measure voltage? You just have to put an electrode into the axon and record it. You know those battery gauges that come with your batteries nowadays? The gauges tell you if your battery is fully charged. They measure voltage, too. It's a simple task to measure voltage, you just have to have one electrode in the cell and one outside the cell (like you have to press both battery contacts against the gauge).
Before the action potential occurs, the cell is resting, at -70 mV. But then, "some electrical event occurs." This electrical event, however it happened, is enough to cause the sodium voltage-gated channels to open. Once they open, sodium rushes in. That makes the inside of the axon more positive.
The axon becomes more and more positive, typically reaching +40 mV at the peak of the action potential. Meanwhile, sodium voltage-gated channels begin to inactivate (you saw that they inactivate like an automatic shut off in the gated channels web page). At the same time, the delayed rectifier finally begins to open.
Once the delayed rectifier opens, potassium floods out, restoring the membrane potential back to its resting value. You'll notice that it tends to overshoot this value (dipping below -70 mV) and then come back to it. That's because it takes being at a negative potential (near resting) to turn off the delayed rectifiers; so it overshoots resting potential as the delayed rectifiers are closing.
Finally, since the sodium voltage-gated channels have all been inactivated, they have to get de-inactivated. De-inactivation of these channels makes them openable again. Until they are de-inactivated, they remain inactive and unopenable. The only thing that causes de-inactivation to occur is the restoration of the membrane potential back down to a negative value near resting potential.
The action potential is actually a dynamic event
The picture above is a static picture... nothing is happening in it. But, remember, the X axis above is time. So these events are happening sequentially in time. If you were recording the voltage with an electrode, you would see these things happening over time. Let's see what that would look like with this animation.
Keep in mind that each of the events indicated by the arrows represents a change in the gated channels. They are either opening or closing or inactivated or de-inactivating.
With these changes in the gated channels come changes in ionic conductance. Sodium conductance causes the rise in the action potential, while a potassium conductance causes a restoration of the potential.
Some important terms for understanding the action potential
These three terms, depolarization, repolarization, and hyperpolarization are terms that are used to describe any change in membrane potential, not just the action potential. You will see these terms again in the postsynaptic potential web page.
Depolarization is any time the membrane potential goes to a more positive value than resting potential, while hyperpolarization is any time the membrane potential dips to a more negative value than resting potential.
An action potential is all-or-none
One "some electrical event occurs," the voltage-gated channels are triggered to open. If we compare this to a toll booth again, let's say you need to pass through a toll booth that requires 50¢. If you put in 30¢, it won't open. If you put in 40¢, it won't open. Even 45¢ won't open it. If you put in 75¢, it would open... but you only have to put in 50¢ to make it open. 50¢ is like the threshold for opening that toll booth gate.
All the voltage-gated channels (both for sodium and for potassium) open if a threshold change in the electrical potential of the axon occurs. If there's enough of an electrical stimulus from that electrical event, whatever it is, the voltage-gated channels will open and the entire action potential will occur. If there's not enough of an electrical stimulus, nothing will happen. If there's a really, overly-large electrical stimulus, an action potential will occur just like when an electrical stimulus occurs that is just at threshold. There's no such thing as a big and a little action potential. An action potential is an action potential, and it either occurs or it doesn't. All you have to do to make it occur is to trigger the voltage-gated channels.
Work through the action potential with the A.D.A.M. CD:
The action potential section of the Marieb A.D.A.M. CD is very good... especially through the first 13 pages of it. Run through it and work through the quiz, as well. Please note that on pages 14 and 15 of this CD, she discusses the refractory period. All this is is the time during which another action potential cannot fire. Basically, if your channels are closed or inactivated, how can you fire an action potential? You can't. That's all. Don't spend a lot of time on that. OK?
How does the action potential run down the axon?
That is the subject for the next page, entitled saltatory conduction.
© 2011 STCC Foundation Press