 Now, your next task, and I want you to do this, so push pause if you have to. You have a neuron, you have the general parts of a neuron. You have dendrites, you have the cell body, you have the axon, and you have a synapse. I want you to challenge yourself and lay down some neurons to match this overview of the nervous system. How is a neuron going to be related to this little diagram that we just drew right here? While you are pushing pause, I'm going to keep going because I can't force you. Well, who knows what you're doing right now? Hopefully you've pushed pause and now you're back because you already pushed pause. Ready? This is how you're going to lay down neurons. First of all, you're going to have neurons out here that receive sensory information. You're going to have an axon in my cell body. Do you agree that we're going to probably have dendrites of some flavor that are receiving information from the environment? And then we're going to have an axon that carries that information in, through the axon, into the synaptic knobs and out across the synapse to someone else, another neuron. Look at the path. Look at this. We could have another neuron that actually receives the first information and then I have one neuron here. But if this is actually the brain and the spinal cord, dude, there's like, jillions of them in communicating with each other in a whole net and we'll try and comprehend this, but let's just keep it simple for now. There's some neuron that is processing the information in the central nervous system. And then delivering the message, right, to someone else, what? And then who's the message delivered to? Dog pounds to the effector, whoever that is. Could be a skeletal muscle, could be your heart, could be your eyeballs, your blinking eyeball, whatever. Somebody's going to do something and that's your effector and the information travels in that direction. Does the information ever, on this path that I just drew, does the information ever travel this direction, on this pathway? No, no, never, never. Information can only go one direction down the axon and that's a way from the cell body. So the information here is always, always, always traveling. Oh, you know I've got to find the perfect yellow, that direction, always. Now, we can throw in some crazy loop-de-loops and turnity turns and we can have multiple neurons in a pathway, like many, many neurons in a pathway, but the information is only going to go in one direction. I want you to think about this. We already labeled this little purple guy as the central nervous system. Central nervous system includes any neuron that stays in the brain and the spinal cord. So if a neuron is found in the brain or the spinal cord, then it is considered part of the central nervous system and there are some terms that we should probably be comfortable with that. But what's our alternative? If a cell is not part of the brain or the spinal cord, what is it in? That's the peripheral nervous system. So let's do peripheral nervous system. That's not brain or spinal cord. Now, here's the deal. These are my structural categories for how we can organize the nervous system anatomy. Look at my diagram here and do you agree that if the purple guy is going to stay fully in the central nervous system, it's going to stay in the spinal cord and the brain, then the peripheral nervous system neurons, my blue friend and my red friend, they have to actually have parts touching or entering the central nervous system and it's true. These neurons are still peripheral nervous system neurons even though they do have parts that come into the brain or the spinal cord. They're still considered peripheral unless they're entirely inside the brain or the spinal cord. So this little purple guy is considered a central nervous system neuron because it's completely inside. And again, it can travel up the spinal cord. It can go all over in the brain. There are multiple neurons in the pathway. Those are all in the central nervous system until the information comes out and is telling someone somewhere to do something. Then we're back into the peripheral nervous system again. So both of these areas are P and S. Don't say that too fast. We have some terms that hopefully will trigger some prior knowledge that you already have. First of all, a bundle of axons in the peripheral nervous system. So not just one axon. This is one neuron with one axon. If we take a whole bunch of neurons and bundle them together, a bundle of axons in the peripheral nervous system is a nerve. What? That's the definition of a nerve. It's a bundle of axons traveling through the peripheral nervous system. It's outside, it's a bundle of axons outside of the brain or the spinal cord. Guess what? I'm sorry about this. Sometimes anatomy is awesome. Sometimes not so much. The same thing in the central nervous system. A bundle of axons in the central nervous system is called a tract. It's exactly the same thing, but it's called a tract. We can have a bundle of axons traveling up the spinal cord, and that's a tract. We can have a bundle of axons traveling to different places in the brain, and that's considered a tract. The bundle of axons, I say it's traveling, it isn't traveling. It's holding still, but information travels through that bundle of axons. The bundle of axons can... Well, we'll go there in a second. If I had a bundle of axons out here, it's called... it's going to be a nerve. They're only tracts if the whole thing, the whole axon group is inside the central nervous system. Now, if we had a whole bunch of axons anywhere in the peripheral nervous system or the central nervous system, what are we also going to have a bundle of somewhere? You're also going to have a bundle of cell bodies. A bundle of cell bodies in the peripheral nervous system is called... I'm going to go up here so you can read it. A ganglion. Indeed, it is so. A bundle of cell bodies is a ganglion. A bundle of axons is a nerve. But of course, a bundle of cell bodies in the central nervous system is not a ganglion and this is going to hurt your brain even more so take a deep breath and clear everything out and feel happy and at peace with the world. Maybe pop some chocolate really fast. This is called... I'm really sorry about this. A bundle of cell bodies is called a nucleus. A nucleus in the central nervous system contains many cell bodies and each cell body has its own nucleus. So this is different from the organelle nucleus. This is actually a structure. It's exactly the same thing as a ganglion. It's just found in the central nervous system. All right. I think that that is good for you. I think we'll call it good. We'll talk about the afferent nervous system next.