Action potentials are special to me. They are special to me because action potentials are what got me into science in the first place.
Well, ok, they didn't really get me in to science. Little Sci had been a Biology major for about two years before she first really studied the action potential in depth, and she was doing research (in watershed ecology, of all things. How we do change.) But I wasn't a very enthusiastic Biology major. General Bio was made up of huge classes with tests where you spit back information, Chemistry gave me headaches, and all the math they made me take was REALLY not my thing. I liked Philosophy a whole lot more, but I was determined to have a "useful" major. And I had no idea what I was going to do with myself after college. Grad school had been my fixed idea for some time, to keep me out of the scary big world and in the college life I loved so much. But for what? And why? Eh.
So after all the general biology classes were over, we could enroll in higher levels classes. The classes were generally smaller and got progressively more specialized, and you had to take a certain number of classes in certain categoried to be considered a Biology Major (tm). I enrolled in Animal Physiology.
The class looked cool enough, but really I was in it because of the professor teaching it. I still keep in touch with this professor, he actually gave me a letter of recommendation for my grant that was one of the most touching things I've ever read about me (he gave me a copy, obviously, I don't peek). This guy is a stellar biology teacher, and is now head of the department. I still keep in contact with him (and I'm publishing that paper SOON! I swear! A few more weeks!) Some day, I want to be as good an instructor as this professor. He will be the first person to get a copy of my thesis, and I don't care if he ever reads it.
So I'm in Animal Physiology, and it's cool. I really liked the heart and circulation section, muscle tone was very cool as well. For the first time in my science life, I felt like I was learning something I was going to USE. Not necessarily in the lab, but in real life. And then we got into the nervous system and started looking at action potentials. The professor was hopping around in front of the board, drawing away (yes, I'm old, we still drew on whiteboards in those days). And I drew along (I've always taken copious notes, and they are beautiful to behold. I have to write stuff down, and this was in the age before everyone had a laptop. Everyone who missed class found reasons to be my friend.) And it was so...elegant. It was beautiful, imagining the molecules flowing in and out of these tiny holes in the membrane. When I thought of it in slow motion, it was like a long, slow wave to me, sliding past.
The time for the final (I think it was on the final) came around. I spent 14 hours straight in the library studying Animal Physiology (well, ok, there were coffee and candy breaks). And I actually remember that time with extreme fondness. Yeah, I'm a total geek (and I'm sure you know that by now), but everything I was studying was SO COOL. And every few hours I would sit back and feel my mind twist at the thought of all the tiny things I was studying happening over and over and in millions of units. The amount of stuff it takes to make my hands TYPE this sentance absolutely boggles my mind. The "wow" feeling I got in the library that day, looking at my own hands writing, is probably the closest I have ever been to religion (and also I was rather sleep deprived). Action potentials were where it all came together. And it is to them that I owe my current career path.
And so now, after the excess of geekery, we're actually going to TALK about them. Keep in mind this will be VERY basic. And you know, when I pulled out my old Animal Physiology text to look at the section to go back to the basics, I found the section on action potential is still marked with a post-it. The post-it has many exclamation points.
Oh, and a note about my textbook. There are better books out there (who doesn't love "The Molecular Biology of the Cell"? Totally classic. Ah, the memories on RNA transcription...), but I challenge a Biology text to have a more amusing cover. It's a LOL Book cover. Observe:
That Pronghorn is all like "LOL WHAT?!" It still makes me laugh. And the book also compares an action potential to flushing a toilet. It doesn't get much better.
Action potentials are changes in electrical gradients of a cell membrane. We all know that chemicals are released to signal from one cell to another, but action potentials are necessary to send a signal from one end of a cell to the other. I'm sure you know that cells are surrounded by membranes. These membranes aren't all smooth and uniform, instead they are positively lumpy with all the receptors, channels, and transporters necessary to allow a cell to maintain homeostasis and allow it to do things like send action potentials.
Cell membranes are also maintained at a certain electrical potential. This electrical potential is caused by the balance of ions on both sides of the membrane. Not all ions are going to affect cell membrane potential, only the ones that can cross the membrane via channels can do so. The ones that are the biggest players in the action potential are the sodium ion (Na+), and the potassium ion (K+).
(ARGH! I was crafting this beautiful post, and I was almost done, and then my internet shut down for no reason! I spent TWO HOURS on that thing! I'm pissed. What I have tried to recreate of it is below.)
Ok, so an action potential can be broken down into five major phases, the initiation and rising phase, the peak, the falling phase, the undershoot, and the refractory period. I'm going to go through each one briefly, and hang in there, because one occurs on top of two others. Don't worry, I'm going to try to find relevant pictures.
Initiation and the rising phase
To start with, the cell membrane is pretty permeable to K+ (or at least, much more permeable than it is to other ions). So the cell membrane is holding at a K+ potential close to its favorite, around -70 mV. When an action potential is started, a few Na+ ion channels open in the membrane, and Na+ comes in. Most of the time, this little bit of Na+ isn't enough to start an action potential and is overwhelmed by all the K+ flowing around it. But if the Na+ can get the membrane potential to around -40 (the membrane threshold), the cool stuff starts to happen.
Na+ flowing into the cell starting having positive feedback and opening more and more Na+ channels, and Na+ comes rushing into the cell. This is known as "depolarization". And the membrane potential rises and rises, shooting up toward the favorite potential of Na+, around +55 mV.
Na+ is flowing in to the cell like crazy, and all the Na+ channels availible are open. But the channels can only remain open for so long. After a bit they go into their inactive phase, where no Na+ can get in. The membrane potential never reaches +55, it usually tops out just below +40 mV.
The Falling Phase
Around this time, K+ recruits more channels and THEY start opening, K+ goes rushing in, and the membrane potential drops like a stone. During this time, most of the Na+ channels are inactive, and there is nothing to stop the K+ from doing as it likes.
Because all the Na+ channels are inactive and there are extra K+ channels open, the membrane potential will overshoot a bit, going down to around -80 mV before the extra K+ diffuses away from the area. Then the membrane potential rises back to around -70 mV, ready for the next action potential.
The whole result will end up looking like this:
The refractory period
This is the period that overlaps two others, part of the falling phase, and part of the undershoot. Basically, during this period, Na+ channels are tired, they've had enough, and they aren't opening for anybody. An Na+ channel actually have three positions: open, closed, and inactive. When the channel is inactive, there's actually a little inactivating particle that blocks the channel so nothing's going through it.
(Courtesy of the Dept of Biology at the University of Newfoundland)
So during the refractory period when many of the Na+ channels are inactive, it is impossible to get enough Na+ channels open to start another action potential. No power on earth is gonna make that area of the membrane fire at that moment. This is where my textbook brought in the analogy of the toilet. When you flush a toilet, there's a period of time when the water is going down the drain and just after, when no matter how you pull on the lever, the toilet won't flush again. That is the toilet's refractory period. And then, when you CAN flush, the resulting flush will be a lot weaker than if you'd waited til the bowl was full. The same thing happens with neurons. When enough Na+ channels are ready to fire, but most still aren't, any resulting action potential is going to take a lot more effort and be a lot smaller than the preceeding potential. This is because you're trying to get the membrane to depolarize during the refractory period.
Isn't an action potential cool?! It's like a big wave, the Na+ starts to come in a builds up, and WHOOSH, all the Na+ comes and the potential shoots up! Then that energy is expended and the K+ takes over, and "SLOSH" the rest of the potential slides back into the ocean, dips under for a bit, and is ready for another go. Man, that's cool! I'm still geekin' out about it!
So you're probably looking at this potential and saying, great, but that's a spike. How does it GO anywhere, how does it travel down neurons? Action potentials themselves don't travel, they propagate. It's really simple when you think about it. When all the Na+ channels open and Na+ flows in, it doesn't just stay localized, it diffuses a little. And that diffusion is enough to get another action potential started further down the line (it won't go backwards because it would run into the refractory period of the previous action potential).
(image via Beta Rhythm)
So there you go, the very, very basics of an action potential. I'll be referring to this post whenever I talk about them, so you can come to it and get an idea of what's going on. Not all action potentials are the same, though, and when you get to the end of a cell, you start chemical signalling, which is signalled by calcium instead of Na+ or K+. And chemical signalling (neurotransmission) is a whole different ball game. Maybe next time.