Sunday, February 12, 2012

Action Potentials - What Make Your Brain Work

Since I'm in Human Physiology I thought I would post some stuff about what I'm learning - mostly so I can remember it.  Please comment at the bottom, it's great to know if this is helpful to people out there!

The way that neurons send electrical impulses is incredible.  It's really a bunch of ions diffusing in tiny fractions of a second and this is responsible for all activity in the brain (and the heartbeat as well - the heart is technically muscle tissue but it behaves an awful lot like nervous tissue).

So there are a bunch of sodium ions (Na+) and potassium ions (K+) in the intracellular and extracellular fluid of neurons.  At rest, the cell is busy creating a concentration gradient by keeping the Sodium Potassium pump running strong.  It pumps 3 Na+ ions out and brings 2 K+ ions in.  Since more of the + charged ions are leaving than entering, the inside of the cell has a negative charge.

So, as I mentioned this is making a concentration gradient.  You know how when you stir salt or sugar into water, it disperses evenly in the water (aka dissolves)?  So solutes naturally try to spread themselves out evenly.

If there are a whole bunch of them on one side of a membrane and not many on the other, they want to cross the membrane to even things out.  (Eeesh, I'm anthropomorphizing ions for heaven's sake!)

To visualize this, imagine being stuck in a small classroom with 60 students.  The second the bell rings, everyone wants to rush out of the room into the less crowded hallway.  That would be called "diffusion" or "moving down the concentration gradient."

So the Sodium Potassium pump is like purposely stuffing as many students into the small classroom as possible, so that when they do open the door at the right moment, students will come rushing through the door which creates energy!  This is the same thing that the electron transport chain in Mitochondria does which creates the majority of your energy for living!  Pretty amazing.  (Click to watch a super cool video of the Mitochondria doing it's thing - by BioVisions.)

It's also the same concept behind hydro-power.  People build a dam to hold back a bunch of water, then they create a small opening and let some of the water rush out which turns a turbine and generates electricity.
So, an action potential is graphed like this:

This is what is going on with a nervous impulse.  Basically, the cell is resting at a negative voltage (-70 mV) because of all those sodium ions being pumped out, plus most of the stuff in a cell is negatively charged anyway.  If it gets up to "threshold" voltage, then the voltage gated sodium channels open and positively charged sodium rushes into the cell, causing the voltage to go from -70 to +30.  That's called depolarization.

At the top of that action potential, the voltage is at a point where no more sodium wants to come in, that channel closes and the potassium channel opens allowing K+ to rush out of the cell, which then brings the cell back to the negative voltage.  This is called repolarization.

The K+ channels close kind of slowly, so that's why there's the little dip at the bottom, because the voltage drops below the resting point since it takes a little longer for the K+ channels to close, and then it goes back up to resting at -70 mV.  When this electrical impulse reaches the end of the neuron's axon, it releases chemicals called neurotransmitters which a neighboring neuron can then pick up and start sending another electrical signal through that neuron, and on and on.

And THAT is how all your thinking and everything takes place.  Amazing, huh?!!?

To learn more about neurons, go to this post:
Click to learn more about neurons

And for a really great explanation of how the message goes from neuron to neuron see this post:

Neuron Communication - click to learn more

If you found this interesting or useful, or if you have any suggestions or comments, please leave a comment below!  Thanks!


  1. Was this helpful? Have a suggestion? Please comment! Hundreds of people visit this post per day, it would be great to have feedback on its quality - thanks!

  2. That was fantastic! So much better than on science TV shows on The Brain. They don't seem to want the biology. Your description is so nice and clear and once we understand that electricity stuff then we can forget about it. Because it doesn't actually explain anything. It's just how the cell send out that a chemical. So that was great. But sorry to say, you're a neuron picture with it is awful. Trippy sci-fi stuff. Check out this on Wikipedia the third picture down, it's totally killer.

    1. Hey so I actually found this other post of mine to be a better place to add that graphic. You should look at this post too, it has more information and a couple of really great videos as well. This Action Potential post is more about the micro scale what is happening along the membrane, and the other post takes a "bigger picture" of the entire neuron.

  3. Thank you connsuits, I think I will add that neuron picture, you're right it's a good one. I have several posts about neurons and action potentials so some of the other posts may have better stuff too, you can check those out. I'm glad you found the post helpful, it was an "aha" moment for me, so I had to share with the masses. :)