 Let's talk about the parasympathetic branch of the autonomic nervous system. Remember, the parasympathetic branch is visceral motor. Just like all motor pathways, the information ultimately has to come from the integrator. It either comes from the brain or the spinal cord. If you remember from anatomy, the location of the cell bodies that exit the central nervous system, we know that. We actually know that if we're talking about parasympathetic neurons, they're either going to come out of the sacral region, can't help it, got to write it down, or the cranial region, which means this is an anatomical characteristic of the parasympathetic nervous system. The neurons come out of cranial nerves, 6, 7, 9, and 10, or out of spinal nerves in the sacral region. The sympathetic nervous system is different than that. Okay, so that's anatomy. We also know that physiologically, the parasympathetic branch of the autonomic nervous system causes a lowering, a calming, a decrease of stress. Calm down, eat some food, make some babies, relax, it's all good. That's the message from the parasympathetic nervous system. Now, this stuff is all anatomical and structural. The physiological part happens when we know that there are two neurons in the pathway. So I'm just going to do it down here for ease. We have a cell body embedded in the central nervous system. We're in one of our specific locations. And this is called the pre-ganglionic neuron. The pre-ganglionic neuron has a really cool characteristic. Are you ready for this? It dumps a specific neurotransmitter. Guess what it is? Acetylcholine. It dumps acetylcholine on the... Depending on how you want to say it, your textbook calls this the postganglionic neuron. The anatomy textbook calls it the ganglionic neuron. I prefer post, even though this cell body is what makes up the ganglion. Because remember, a ganglion is nothing more than a group of cell bodies that are found in the peripheral nervous system. A group of cell bodies of neural cell bodies found in the central nervous system are called a nucleus. So if we're dumping acetylcholine onto our postganglionic neuron, then we have to have acetylcholine receptors, yes? And what do you remember about acetylcholine receptors? We have two flavors. Guess what? The postganglionic neuron contains nicotinic, acetylcholine receptors. That's the specific flavor of receptor that we have there. And it's, I say always, I'm sure there are some exceptions, but if we're talking parasympathetic, then our first synapse is going to be flooded with acetylcholine and met with nicotinic receptors. And I want to remind you also of how nicotinic receptors work. They're basically little closed sodium channels that have a binding site for acetylcholine. So in comes an acetylcholine molecule. If it binds to the nicotinic receptor, it opens up a sodium channel, which is the receptor. The receptor is the sodium channel. Sodium rushes in and creates an action potential in the postganglionic neuron. That passes the message along. Now here's the other cool thing. Guess what happens next? That is dumped on the effector. I gave you a huge hint, a colorful hint. Acetylcholine, I just drew the exact same thing. However, check out the receptor. Is that the same receptor? The receptors on the effectors are muscarinic. Muscarinic acetylcholine receptors. And hopefully you remember that muscarinic acetylcholine receptors stimulate that whole G protein second message cascade madness of madness. All those crazy details that we actually aren't going to be spending time looking at every single one of them. But dude, look, somebody did. This is awesome. So this is my receptor binding to acetylcholine and stimulating a response accordingly. Those are two important characteristics of the parasympathetic branch of the autonomic nervous system. Shall we look at the sympathetic branch? Yes, I think we shall.