 I'm going to draw you a chemical synapse. So let's look, let's imagine that we have our neuron and we have our axon terminal and everybody's cool with this. This is all, this makes perfect sense and man, that is an attractive axon terminal. The axon terminal is not touching the next person in line. The next person in line could be another neuron or it could be an effector. I'm just going to make a rather amorphous, like who knows what this thing is. It's either another neuron or an effector. What do we know is traveling down this neuron because life is so exciting? Let's call it, dude, an action potential is coming and remember an action potential is nothing more than this weird wave-like switching of the membrane potential from negative to positive and back again. And remember that as the action potential traveled down the axon, as the membrane potential changes, voltage-created sodium channels and potassium channels are stimulated to open because of the change in membrane potential, they're voltage-gated channels. That's important because we've got a new voltage-gated channel that's coming into play here. We haven't seen this guy yet, but it's a gated channel, it's got a lid on it and I'm going to give you a little hint here. Oh, that's a lightning bolt. And the lightning bolt indicates that this is a voltage-gated, anybody want to take a wild guess, calcium channel. Interesting. And I'll tell you right now that there is a high concentration of calcium outside the cell in the extracellular fluid and that tells you that there's probably, well it doesn't tell you, we can probably guess that if we've got calcium channels and a high concentration outside, that we have a low concentration of calcium inside and that's true. So here comes the action potential. You know where this is going. The first thing that happens is that voltage, that's voltage-gated calcium channels open. Why? Dogs, the membrane potential just changed, the action potential arrived and caused the opening of these calcium channels. What's calcium going to do? Calcium rushes in. Okay, who cares? I'm telling you right now, nobody cares. So what? So calcium goes in. I don't like, make me go, oh, of course, except, this is so cool. Inside the axon terminal, you also have these little bubbles of juice. And I'm going to draw the juice like a, like a this. Guess what the little green things are. That's neurotransmitter dogs. That's a chemical. And they're, the chemicals inside this little bubble in the axon terminal and in comes calcium because the action potential arrived. Now, through the magic of chemistry, the calcium, I don't know the mechanism of this, the calcium comes in and somehow stimulates. There's some kind of chemical reaction that takes place, some kind of shape change that happens and guarantees someone somewhere knows the exact mechanism of this and calcium is going to bind. Calcium, I'm going to say it binds to neurotransmitter bubble. Vesicle is probably a more scientifically lovely word. And guess what happens when calcium binds? Exocytosis of neurotransmitter, where? Dude, all we're going to do is dump neurotransmitter through the process of exocytosis into the synapse. Remember, the synapse is this space. This right here is my synapse. Now, my electricity can't cross the synapse. My kids have this awesome electrical set that they can build all these crazy circuits and they totally make these switches and they push this button and that light goes on but if you push this button then that light goes off and this one blinks and the green light and the red, whatever. They make all these really complicated circuits and what they, without a doubt, know is that you can't pass electrical information across the space, you have to have a connection between two things if you want to pass electrical information, which is why we have to go to a chemical message. So exocytosis of the neurotransmitter happens into the synapse. Now what? Well, my friends, all sorts of things can happen. I mean truly, depending on the neurotransmitter and depending on the effector, we have a myriad of different options. First of all, in most cases, we will have a receptor. I say most cases, it would be really pointless to dump neurotransmitter into a synapse if you didn't have some kind of receptor on whatever is next that can connect with the neurotransmitter. Once we connect, oh my gosh, so you get neurotransmitter plus receptor. And then it's like, choose your own adventure, man. What do you want to have happen? Depends on your receptor, depends on the neurotransmitter, but you can actually stimulate the opening of sodium channels. What's that going to do? That's actually going to stimulate an action potential in the next cell. So you could stimulate the opening of potassium channels. What's that going to do? Oh my gosh, what a cool application question that would be. If you open potassium channels, that's going to prevent an action potential. That's going to make the membrane potential more negative because potassium is going to rush in, out. Potassium is going to go where it's not supposed to go. It's going to go where it's not supposed to go. Sodium comes in, potassium is going to rush out. So potassium rushes out and prevents an action potential in the effector. You also can have the binding of neurotransmitter to a receptor and then you have some like direct action of the effector such as the contraction of a skeletal muscle and that's coming too. We're going to learn the mechanism of that whole process as well. All right, let's take a little bit of a closer look at the neurotransmitter. Once the neurotransmitter is in the synapse and is having an effect, there are some things that we need to think about.