Source: SOURCE: SODIUM-POTASSIUM PUMP; PUBLIC DOMAIN http://commons.wikimedia.org/wiki/File:Scheme_sodium-potassium_pump-en.svg
The nervous system, which is our brain and our neurons, and all the nerves that go throughout our bodies, acts as the body's communication system. It sends information to and from the brain, and allows us to control the rest of our body in different kinds of ways. So how do those individual neurons-- the cells that make up our nervous system-- how do they actually transmit that information, and create this kind of communication?
There are two different ways that a neuron communicates, and that's within the neuron itself-- within one individual neuron-- as well as between different neurons. Today, we're going to be taking a look at that message that's being sent inside of one neuron.
The process starts at the dendrites, which are the long, tree-like branches that extend from the cell body itself. What these dendrites do, is they collect different kinds of messages from other neurons surrounding it. But it doesn't necessarily do anything until it reaches a certain level of messages. So one message doesn't cause it to communicate its information. It has to be a combination of lots of them. And this is what we call the "Threshold of Excitation." Which is to say, the level, or the point at which the neuron is caused to fire, or a neural impulse is triggered.
Looking more closely at the axon, remember the axon is that tail like structure that extends off of the cell body, and acts as a sender or a transmitter of information. We see that the axon is actually covered in all these tiny little holes or tunnels, or what we call, "ion channels. " And what the ion channel does is, it allows the cell to control or pump the amount of ions, either inside of the cell, or outside of the cell.
An ion, remember, is a positively or negatively charged atom. So when the cell pumps a certain amount of sodium, let's say, which is Na, outside of the cell, and potassium inside of the cell, these electrically charged ions create a charge across the entire axon. And at a resting potential, which is when a cell is not firing-- in other words, when it's sitting and it's waiting to fire, or send a message-- the sodium is generally concentrated outside of the cell. And there's more potassium inside of the cell.
When the neuron receives a message from the dendrites-- enough to reach that threshold of excitation-- when it reaches that point at which it's going to fire, then the cell switches from a resting potential to an action potential. And this is when it sends that neural impulse. It fires. And what that means is that these ion channels all of a sudden throw themselves open, and the Na, the sodium, comes rushing inside of the cell, and the potassium comes outside of the cell afterwards. And what that does is, it creates more of a positive charge inside the cell, and then a negative charge outside of the cell. And this creates a sort of domino effect, where these channels open up and create that switch of electrical charges. And then the ones next to it also throw themselves open, and send sodium and potassium outside and inside of the cells. And then it continues to go all the way down the axon, until it reaches that axon terminal, that button at the very end of the cell.
When an axon fires, when a neuron fires, this is an all-or-nothing reaction. Either it fires completely and throws open all of these ion channels, or doesn't fire at all. So there's no partial. It's just like a light switch. On or off. And these neural impulses, these action potentials, can move very quickly. So their fast, but they're not necessarily instantaneous. Which is why it takes a split second for us to react to something in our environment.
After this occurs, after all of this exciting action potential happens, what the cell does is, it resets. Essentially the ion channels pump the potassium back inside the cell, and the sodium back outside of the cell. And it resets itself, and gets ready to fire again. So it goes back to a resting potential. And that is how the information is sent. That electrical charge is created that goes all the way across a neuron, and gets ready to send information to other areas and other neurons that are attached to.