 I'm super excited about this because we're going to talk, we're going to put some pieces together in an actual pathway to see an actual autonomic mediated response, reflex, pathway, the whole thing. Ready? Before we start, there's a couple of concepts or terms that you need to know. First of all, reflexes can be ipsilateral, ipsilateral or contralateral. And an ipsilateral reflex is one where the stimulus and response happen on the same side of the body. ipsilateral, the stimulus comes in on one side and the response happens on the same side. The contralateral reflex response is when the stimulus and the response are on different sides. And you might be like, dude, really? Never going to happen. But in fact, there's a lot of crisscrossing in the brain and very often information from one side of your brain controls things that happen on the other side of your body. So that would be an example of a contralateral pathway or a contralateral reflex. We're going to look at two different reflexes related to pupil constriction in your eyeball. And one of them is contralateral and one of them is ipsilateral. And they both involve pupil constriction. So there is the pupillary light reflex and that is an ipsilateral reflex. You shine a light in this eyeball and this pupil is going to constrict. And that makes perfect sense. That means that if you, like, stuck something here and you hid the light, I mean on this side where there's no light, if you hid the light from this side and you shine, you could actually watch this side and see, is there a contralateral response? I shine the light in this eyeball. Is there a contralateral response in this eyeball? Don't you kind of want to go check it out? There is. It's called the consensual pupillary response. Consensual pupillary reflex. And it is contralateral. Okay. We're going to look at the anatomy that enables this to be the case. It's so cool. Let me see if I have a blank canvas. Take a deep breath. These are your eyeballs. One eyeball, two eyeballs. Here are your pupils. Okay. If those are your pupils, I'm going to show you right now. This is my stimulus. Oh, light's coming this way. I've actually done like a transverse section right through the eyeballs in this person. In order to get this view, I'm looking down on this person. And that also means that this is the right side. Had to check. And this is the left side. So this is my right eye and this is my left eye. Everybody's cool with that. What is the sensory receptor that will respond to the stimulus of light that I have just delivered to these eyeballs? You know this. You're all over this action. The sensory receptor is, well, the photoreceptor cells inside the retina, the rods and the cones inside the retina. Everybody's cool with that? So light can come in from either side and it can stimulate the rods and cones in the retina. Now, we've stimulated. But what is the pathway? Obviously, we're going to have to go to the brain. And how are we going to get there? What is the afferent path that we're going to send an action potential down to deliver a message? What is that pathway? Okay, I can do this. It is going to be blue. Now, this is my optic nerve, you guys. And if you remember your anatomy, you will remember that the optic nerve actually crosses. And I'm making my optic nerves blue, but notice that I'm distinguishing between the right side and the left side. It's the optic chiasm where my optic nerves cross. But here's the thing that you did not know. You didn't know that some of the neurons that are picking up information and bringing it from the retina travel from the right eye to the left side of the brain. But some of them keep going on the right side. They like split. And same thing with my other, with my left side. They actually split. And all we're going to do is dump chemical, neurotransmitter onto some central nervous system nucleus, really. And so I'm going to draw that in two different shades of purple. Because I usually make my little, okay, this is kind of pinkish. Let's go with how about that color purple and that color purple. Can you tell the difference? It's supposed to be dark purple and light purple. But look, now I'm processing in the central nervous system. We could name this nucleus, no problem. What I want you to see is what happens to the axons from this guy. Watch, this is awesome. This central nervous system interneuron is going to go ahead and synapse on another interneuron. But it branches and it crosses and it's going to synapse on the same other neuron on the other side. What? Seriously. And my little dark colored one does exactly the same thing. Half of it comes and synapses on something on its own side and half of it crosses over and synapses on the other side. We have two opportunities right now for information to travel across the brain. This is definitely somebody who's going to do something. So watch. Let's keep track still. I think I'm going to go with pink. This is still in the world of, hmm, I love this. This is actually, okay, now I'm going to make it green. So never mind. We're going to make it light green and dark green. Okay, because this, you guys, is now the, okay, take a deep breath. It's now the pre-ganglionic neuron. What? In the central nervous system. I mean in the autonomic nervous system. And I'm trying to draw these two different colors so that you can see what's happening on each side. And now, not only that, but I'm going to go with, let's just do some, why not, why not like red and orange? Because now we've got our ganglionic neuron or post-ganglionic neuron and in the autonomic nervous system, what does it do? Dude, it dumps neurotransmitter onto the effector, which is what, oh my gosh, the muscles surrounding your pupil. You've got to be joking me. Okay, I got to make, I can't even remember what color I was doing. Should we do it this color? It's kind of a gnarly color, but sure. Oh my gosh, it's so cool. So look, if I were to draw a little line on here, I could go like this and I could say, hey, below this line is central nervous system. Above the line is peripheral nervous system. And here's my, okay, I'm going to start labeling in black. This guy is a pre-ganglionic neuron. These guys, that guy is an inner neuron. I just think this is so cool. Who is this guy? That's an afferent neuron. It's actually somatic sensory, right? You know that because it's light. It's visual conscious light coming in. Who else can we label here? Oh, totally. We've got to label our post-ganglionic neuron. We've got to label, oops, I'm still going with black. We've got to label our effector, which is the muscle surrounding the pupil and causes constriction. Now, here's the cool part. What if you just stimulated the right side? So pretend like nothing on the left side got stimulated. Look at the path I could follow. I could go like this. However, the light stimulus is going to split. And it's going to go to both sides of the brain. I know that yellow, you can't see that. It's going to go to both sides of the brain because it's split at the optic chiasm. And then it's going to travel down both sides of your cranial nerve number three, ocular motor nerve right here. And it's going to cause constriction of your pupils on both sides even though the light stimulus came in one side. Is that like mind-blowingly phenomenal? There's another autonomic reflex that you can mess with. And it's called the ciliospinal reflex. And that's actually an ipsilateral reflex that also affects the pupillary constriction. And I have no idea, like what's the pathway? Because if you tickle or pinch the back of your neck, you can get pupil constriction in, or no, it's dilation. You get pupil dilation in one eye. And I don't know what the pathway is. So if somebody wants to figure that out, that'd be really cool. How awesome is that? Do you think it's awesome or is it just me that thinks it's awesome? I really think it's awesome. And now that's it for the autonomic nervous system. When we come back, when you tune in for the next episode, we will do muscle, skeletal muscle contraction. And I'll try to take deep breaths because it's really cool too.