 Okay, we're going to map a whole pathway, a whole reflex onto the anatomy that we already know. You hopefully see some central nervous system here. We've got some brain. We've got some spinal cord. Does this look totally familiar? You're all over this action. We even have some more anatomy that you should be totally comfortable with. In fact, what does this look like? Of course, it looks like a leg dog, and this is your knee. Of course, it is. So tell me the name of this bone right here. That's your tibia. Here's your foot. Look, what do you think that attachment is? I mean, that bump on that tibia. Tibial tuberosity, right? And the tibial tuberosity has the quadriceps or the patellar ligament attached to it, which has all the quadriceps muscles attached to it with the patella embedded inside, right? You already know all this anatomy. So what do you think this little skeletal muscle represents? This is my quadriceps group. Now, you know that the patellar reflex is that reflex that happens when you start out with a whack. A whack is your stimulus. A whack of from what? A whack from a reflex hammer, of course. That's an upside-down reflex hammer? Sure. And, dude, we're just going to undo that one because we're going to throw our reflex hammer here, and it looks more like this, doesn't it? Doesn't that look totally like a reflex hammer? And we're going to whack right about her. And that, okay, are you ready? That's my stimulus. You're totally cool with that. I mean, if you go whacked upside the head with anything, be it reflex hammer or a lecheron, it's a stimulus. There's a message that is going to be sent to your central nervous system. It's relevant to be aware that there are sensory receptors. Now, take a deep breath and imagine this. What if I truly did whack the patellar ligament? Can you agree that that's going to push in just slightly? And that's actually going to stretch the quadriceps muscle group just slightly? I mean, it's so tiny that we would think, dude, really that stretched something, but can you agree that if I push this in here, that, yeah, it's all connected to the quadriceps group. The force applied at the patellar ligament is going to stretch the quadriceps tendon just imperceptibly. There are little stretch receptors. Sensory receptors specifically, their function is specifically what I want to say designed to pick up stretch. So they look like little tortly witzes, which is a very scientific term. And they're connected, they look like this, and they're connected to dendrites of who? A sensory neuron. What kind of sensory neuron do you think those guys are? They're bringing sensory information from your skeletal muscles. I would imagine that those are somatic sensory fibers. What direction is information traveling through this single pathway? It's traveling toward the central nervous system. It's an afferent stimulus. It's an afferent message. Where is it going to go? Do you agree? It's going to, eventually, it's going to pass through an anterior ramus somewhere to get to the spinal cord. And in fact, we could map this little pathway and we know that this anterior ramus, where are we right now? Dogs of a feather, what is this right here? That's my spinal nerve, right? We know that the sensory information from the quadriceps group travels through the spinal nerve associated with, let's just say L3. Lumbar vertebra 3 is the level that this information is going to travel through the spinal nerve. So it went through the anterior ramus, through the spinal nerve. This is where the little pathway is traveling. Then, where does it go? Of course, it goes into the dorsal root ganglion. L3, and where does the little fiber continue? Where does it end up? It ends up, oh, I think that I want to make that a little bit more clear. It's going to end up in the anterior gray horn. Why? How do you know that? How do I know that? I know that because this is a very simple reflex. In most reflexes, there isn't processing. There isn't, there is integration. There is, we are going to the spinal cord, but you would agree that this is peripheral nervous system, and guess who responds? It's not even, like we're not even messing around. We immediately synapse with a somatic motor neuron, and the information travels out. Now look at this. Where is the somatic motor neuron traveling through? Where is its axon traveling through? Where is its axon located? In the spinal nerve. The same spinal nerve. Is that always the case? Well, if you think about it, I mean, there are ways that this stimulus, the whack on the patellar tendon, like if you didn't know that your doctor was checking your reflexes, and she just came in and whacked you on your patellar ligament, and you were like, that information went to your brain, and your brain processed it and thought, what the hell are you doing, lady? Now I'm going to have to whack you back with my electronon. Then, you know, that that's going to be a whole another pathway, which is possible. Probably wouldn't prevent your patellar reflex from happening anyway. Here comes the motor information, and guess what the motor information is going to say. Head, dog, pounds, contract. And in fact, the quadriceps group contracts. Just the here came the message, and it does it. You don't really think about it much, but we get a contraction and your leg kicks out. An effect happens. Are all actions like this? No way. Most of them aren't like this, because what happens in here? What happens in this zone? Oh, holy, let's pass some information up and down. And while we're at it, dude, let's just go to the brain. And let's do stuff around in the brain, and then we'll decide what to do with that and go to all different places. Today, today we're looking at these pathways. We're looking at all our spinal nerves. We have a set of cranial nerves. We have spinal nerves that combine and braid each other and become named nerves like the sciatic nerve or the axillary nerve or the femoral nerve. And we've actually seen a bunch of those guys already. That's our goal. Our first thing that we're going to look at, let's look at all of our spinal nerves. I would argue that these guys are probably the most straightforward. Here's an example of one. This would be spinal nerve L3. Let's go see how all of our spinal nerves are organized.