Improving science literacy – part 4

Action potentials (aka nerve impulses)

Many times people don’t understand scientific concepts because they seem very difficult. Take nerve conduction for example. You have a bunch of neuroscientists throwing around jargon like chemical synapse, neurotransmitters, voltage and ligand-gated ion channels, Nerst equation, blah blah blah. Yes, it’s true, that as a practicing neuroscientist you need to know what those things are, how they function, and why they are important, but that doesn’t mean laypeople have to understand all that junk to have a general idea of what’s going on. That is why I’m here.

As always, this is very general and stylized. Essentially, I’m skipping over nuances, so this explanation is far from rigorous but good enough for any non-biologist out there.

A few basics first.
1. Na+ and K+ are used to represent sodium and potassium ions. Essentially, they are single sodium and potassium atoms that lost a single electron and therefore have a single positive charge (an electron = a single negative charge, so when you lose one it makes a net positive charge on the atom).
2. This takes place on the neuon’s axon – which is a long tube of cell membrane.
3. Embedded in that cell membrane are channels that let only very specific ions through (sodium and potassium ion channels).

Cool? Any questions? Good, let’s begin.

action_potential_propagation

The first thing you should notice when looking at the picture is that the axon has little positive and negative symbols on the inside and the outside of the cell membrane (the red colored portion of the axon). This is a condition created by your body to make sure neurons work correctly. They force an abnormally high concentration of Na+ outside of the cell (extracellular) and an abnormally high concentration of K+ inside the cell (intracellular). However, there is still relatively more Na+ outside of the cell which leads to it’s overall positive charge.

Now, I’ll skip over the part of how an action potential is created because, for the most part, it’s so similar to what follows, so let’s just skip that for right now. So, the first thing you have to understand is that ion channels are not always open and that different ones open and close at different rates. Na+ channels open and close quickly while the K+ channels open and close at a slower rate. Both of these types of channels are “voltage-gated” which is simply code for “they open when the cell membrane changes voltages.” This can happen naturally or artificially and is what I was referencing in the opening sentence of this paragraph (for this example just assume it happens).

This is where it gets a little complicated and it’s good to have a picture on hand (the top one is easier). First, the intracellular voltage (the only one we’re concerned with – but still the other side of the same coin) increases slightly from highly negative to less negative. This triggers only the Na+ channels to open and Na+ to flood into the cell causing its voltage to jump high into the positive zone, but many of those channels are closing and halting the influx of Na+. While this is going on K+ channels are beginning to open and let out a large amount of K+ ions which then decreases the intracellular voltage and eventually returns it back to normal.

A shorter version: Na+ comes into the cell making it more +. After that, K+ leaves the cell making it more – (because removing a + is essentially the same as adding – charges).

And that, in a very small nutshell is how nerves propagate signals throughout your entire body. Amazing stuff, ain’t it?

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