Hi Ninja Nerds, in this video today we are going to talk about the stretch reflex. Let's go and get started. Hi Ninja Nerds, so when we talk about the stretch reflex, obviously another way that we can kind of define the stretch reflex is kind of like the muscle spindle reflex. Okay, so we're going to talk a little bit about muscle spindles, but in order for us to talk about muscle spindles, we got to kind of just quickly go over a little bit of anatomy of skeletal muscle.
So, when you take skeletal muscle, there's two types of fibers that we have to differentiate between. One here is in this red color, right, nice and striated all the way about. This guy here is called your extrafusal muscle fibers.
Okay, so these are called your extrafusal muscle fibers. Now the big thing to remember about the extrafusal muscle fibers is that there's two big things. One is the extrafusal muscle fibers actually attach to tendons. And those tendons obviously connect the muscle to the bone.
Okay, so because of that, whenever the extrafusal muscle fibers contract, they can pull on the tendon, which can pull on the bone. So that's going to generate movement. So the big thing I want you to remember is that extrafusal muscle fibers connect to tendons and they generate movement.
Now, let's say that we kind of take in here and we kind of take like a little like thing and spread open the extrafusal fibers. Inside there's this little connective tissue capsule. You see all this green tissue here?
Inside there's this little connective tissue capsule. So we'll put CT, capsule. The connective tissue capsule. kind of basically kind of encases these really important fibers inside and these are called your intrafusal muscle fibers. So again these ones all here are called your intrafusal muscle fibers.
The big thing I want you to know about the intrafusal muscle fibers is that these don't really connect to tendons. These are what are called proprioceptors. What does that mean? Proprio receptors is they basically tell you the position of your muscles, your joints, the ligaments, the tendons, all of those things in a three-dimensional space. So it allows for me basically to close my eyes and be able to know where my hand is, to know if it's extended, to know if it's flexed.
That's the job of these beautiful muscle spindles. Okay, now that's why I actually have to talk about. Intrafusal fibers, right?
Are these, there's two types. We'll talk about them in a second. Nuclear bag and nuclear chain.
But if you take a bunch of intrafusal muscle fibers together, a bunch of intrafusal muscle fibers. So we'll say here, we'll put kind of put down here a bunch of, I'm going to put IF, intrafusal fibers is a muscle spindle. Okay. That's important.
So I want you to remember that term. So whenever I say, Intrafusal fibers and muscle spindles, they're somewhat interchangeable, but just remember that muscle spindles are a bunch of intrafusal fibers. Alright, sweet. So we know extrafusal fibers connect to tendons, generate movement.
In between the extrafusal fibers, if you kind of separate this, you have a connective tissue capsule. Inside of that is intrafusal fibers, a bunch of intrafusal fibers, or a muscle spindle, and these are proprioceptors. Alright, so let's go ahead and talk about a couple basic functions of these. Alright, so now what I want to write down here is extrafusal fibers, right?
So we know basically if we take a structure of a skeletal muscle and you were to have to point to each one, we know which point each is. But now we have to talk about what is the function of these extrafusal fibers. We already talked a little bit about them, right?
But we know, what is it? They connect with tendons. And not only do they connect with the tendons, but whenever these extrafusal fibers contract and shorten, they can generate movement. So these are the ones that are going to generate movement.
That's very, very, very important. Okay. All right.
So we know the function of our extra fusel fibers. The other ones that I needed to talk about is the intra fusel fibers, but a bunch of intra fusel fibers are what they make up a muscle spindle. All right. What do we say? The intra fusel fibers are, they are.
proprioceptors, right? So let's write that down. They are proprioceptors. So in other words, they tell us the position of our muscles, our tendons, our joints, our ligaments, all of that in a three-dimensional space. But if we were to really be specific with the intrafusal fibers, The types of sensations that they pick up is actually going to be picking up length.
So they pick up the degree of stretch of the muscle. And they also pick up the velocity. Okay? So the degree, the speed at which the muscle is being stretched. They pick up the degree of stretch and the speed at which it's being stretched.
There's two types of... kind of intrafusal fibers and we're going to talk about those in more detail next those are called your nuclear bag fibers okay these are the bigger ones and there's actually technically two types we're not going to get into a lot of detail but just to know there is what's called dynamic and static nuclear bag fibers just no nuclear bag fibers though and the other one is called your nuclear chain fibers okay so now we know the basis of kind of being able to pick out the anatomical position or what they look like and kind of a diagram about these different types of fibers and we know their basic function. Let's dig into them a little bit more now really focusing on the intrafusal fibers. All right so now what we got to talk about is these different types of intrafusal fibers because really this stretch reflex is really dependent upon these structures. So I already kind of mentioned one of them you see this fat cell right here this little chunky guy this is called our nuclear bag fibers.
Okay, so these are called your nuclear bag fibers. Now, the nuclear bag fibers are larger than the nuclear chain fibers. And the reason they're called nuclear bag fibers is that the central point of this actual fiber is where you find a lot of the nuclei.
So the nuclei are kind of centrally located in this fiber here. That's one of the reasons that they call it nuclear bag fibers. So it's larger than the nuclear chain and all of its nuclei are more centrally located in this bulbous portion. Okay?
Second thing that I need you to know, the nuclear bag fibers actually pick up two types of sensations. We already said that they generally, they pick up length, so the degree of stretch, and the velocity. The nuclear bag fibers, these are the ones that are more sensitive than the nuclear chain fibers to the length, so the degree of stretch, and the velocity.
Whereas the nuclear chain fibers, they are more particularly sensitive to only length, the degree of stretch. Okay, now the next one here, what do we have for this guy? This is our nuclear chain fibers. Okay, so these are our nuclear chain fibers and why are they called nuclear chain fibers? They're called that because their nuclei are arranged in a linear or chain like fashion in the central portion.
Okay, so nuclear chain fibers. They are pretty much only sensitive to length, the degree of stretch. So when I ask Which of the fibers is more sensitive to both length and velocity?
You say nuclear bag. Which one's more specifically just length? You say nuclear chain.
Next thing we need to know, whenever we use these fibers, okay, it's particularly in what's evolved, it's called a stretch reflex. So let's pretend, for example, that I'm going to take both of these fibers, and I'm going to stretch them. Okay, so usually the way that you do that clinically is you take your reflex hammer, and you tap on a tendon. What that tendon does is it stretches the muscle.
And guess which muscles are actually going to be particularly stretched that we care about here? These intrafusal fibers, the nuclear bag and nuclear chain. When you stretch them, so let's pretend here we stretch these bad boys.
When you stretch them, what happens is it actually causes these little fibers here, these little nerve fibers that are wrapping around this intrafusal fiber, to become really taut and stretched. activate particular little sodium channels in the actual sensory nerve index. And we'll show that in an example in a second.
But what are these fibers that pick up the degree of stretch? These sensory fibers that pick up the degree of stretch based upon whenever there's lengthening or stretching of the fiber, these here, these green fibers, these are called your type 1A fibers. Okay, type 1A fibers.
You know what else they love to call these? They love to call these sometimes. anulo spiral endings. Alright beautiful. Okay now you're going to see the same thing here on the nuclear chain fibers.
Whenever you stretch the nuclear chain fiber It's going to tighten up these type 1a fibers and activate them. Okay, so whenever you stretch, you activate the fibers. Which ones? In this case, your type 1a fibers.
And these are heavily myelinated. They're going to travel pretty fast. Now, you got these pink ones that you see here on the nuclear chain fibers, but you don't see it on the nuclear bag fibers.
These are very, very important to remember. You only find these on the nuclear chain. If you really want to be specific, You can also find these fibers on what's called the static nuclear bag fibers.
I don't really care about that. I just want you to know for the most part, these fibers here, these pink fibers that also determine the degree of stretch that are more located on the ends of the fibers, particularly the nuclear chain fibers. These are called your type two fibers. Another name for the type two fibers are called flower spray. endings.
Okay, so flower spray endings. So again, type 1a found on both nuclear bag, nuclear chain. Type 2 primarily, I only want you to know, found on nuclear chain. All right, good.
What do these fibers do? They pick up sensory information. So what I want you to remember is that both of these fibers are sensory fibers. What are they?
They're sensory fibers. They pick up sensory information. So again, what are the type 1a fibers here?
These are sensory fibers. They pick up the degree of stretch or the speed at which these fibers are being stretched. Beautiful.
The next thing that we have to also understand, whenever you stretch these fibers, right, it can activate these type 1a and type 2 fibers. But you know what else is important? In these nuclear bag and nuclear chain fibers, on their ends, on their polar regions here, you see how this is your polar region here? They have these little myofilaments, actin and myosin. What does the actin and myosin allow for?
Contraction. But whenever these muscles are contracted, guess what they do? They stretch the muscle spindle. That's why they're located at the ends. So whenever these little myofilaments are stimulated, whenever they contract, they actually pull the central region, right?
So whenever you have these ends contracting, it's going to tighten or pull on the ends stretching out the fiber. What actual nerve stimulates the contraction at the ends here of these interfusal fibers? These maroon colored neurons here are called your gamma motor neurons and they release acetylcholine at that synapse there and cause the actual actinomycin to lead to that cross bridge contract and whenever both these ends contract It stretches, it lengthens the fiber, not shortens it.
That's what's really important. So again, what are located on the ends here? These are your gamma motor neurons.
What are these? Motor supply to the intrafusal muscle fiber. So again, what are these here? I'm just going to put gamma motor neuron, gamma motor neuron. And again, these are your motor supply to the intrafusal fiber.
They cause the fiber to contract. Let's dig in a little bit more into the detail on that and actually look at that more on the cellular level now. Alright, so now we know about the extrafusal fibers.
We know a little bit more about the detail about the sensory and motor innervation and just the basic anatomy of the intrafusal fiber. Now let's put into play kind of a little bit more in detail by getting into like the cellular level at these actual intrafusal fibers and how it can, the sensory fibers are activated and how the motor neurons actually cause contraction at the polar ends, right? So, here we're just picking one of the fibers.
We could have picked the nuclear bag or nuclear chain. I just picked the nuclear bag here. So, here we have a nuclear bag fiber, okay? Now, what do we say is really the trigger for these sensory neurons that are wrapped around the central portion? Remember, type 1 are surrounding the central portion of the fibers.
Type 2 are a little bit more towards the ends, right? And only found on the nuclear chain. Well, first stimulus here is stretch. So you need some degree of stretch. So whenever they're stretching, it actually causes these sensory fibers, the type 1A fibers.
The same thing would be for the type 2 fibers. I'm just only showing it for the 1A right now. But whenever you stretch this fiber, it actually stretches these sensory fibers wrapped around them.
Now you know what's really cool? If you actually zoom in. So if we were to zoom in on one of these sensory fibers at the nerve terminal point. What happens is, is that as you stretch, so there's some degree of stretch, right? That stretch, you know what it does?
It stimulates these little purple channels on the sensory neuron. You know what these channels are? These are called mechanically... Gated ion channels. Oh, that's cool.
You want to know why? Guess what is a mechanical stimulation? Stretch.
So as you stretch these actual interfusal fibers, you stretch these actual sensory neurons, and it actually opens up these mechanically gated ion channels. Guess who starts flowing in? Sodium is going to be like, uh-uh, baby, I'm coming in. And sodium starts flushing in.
from the extracellular space into the intracellular space. Now we know that as sodium starts entering into the cell it makes the cell more positive. Usually the cell has a something called resting membrane potential.
But as you bring in lots of these sodium ions due to that stretch stimulus guess what it does? It takes the resting membrane potential to what's called threshold potential. And once we reach a particular voltage or threshold voltage guess what that does?
It activates these little orange channels that are present on the axon. You know what these are called? Voltage-gated ion channels. And they only open whenever you hit a specific voltage or threshold potential. We hit that threshold, guess what happens?
These voltage-gated ion channels open, and guess who starts flowing in? Sodium, but at a much faster rate. And as sodium starts flooding into the cell, it makes the inside of the cell super positive. And that generates action potentials. And those action potentials are going to travel down what type of neuron?
They're going to travel down the type 1a or the type 2 sensory fibers. That's so cool. So now we know that if there's a degree of stretch, that's what leads to this whole chemical process here.
And the same end, remember, the stretch can be initiated by what kind of things? One we said is the patellar, any kind of tendon reflex, deep tendon reflex. The other thing that we said could initiate stretch is whenever those gamma motor neurons innervate the ends, right? So we had those gamma motor neurons.
They release acetylcholine and stimulate the polar ends to contract, which stretch out that fiber. We're not going to go into crazy detail. You want to know why? Because we've already talked about this in our whole neuromuscular junction video.
But just to give you guys the whole point, here's our gamma motor neuron. Once there's action potentials being sent down this neuron, it leads to calcium entering into this whole bulb here and activating the little vesicles to fuse. And once the vesicles fuse, we know that it's going to release all the acetylcholine.
The whole point here is that whenever acetylcholine binds onto this end here, we're kind of zooming in on the end, it binds on here and causes sodium ions to start rushing into the cell. As the sodium ions rush in, it leads to an action potential, travels down the Sarcolemma down the t-tubules activates that sarcoplasmic reticulum, pushes out the calcium, and then the calcium binds on to the whole cross-bridge structure there and leads to the contraction. And whenever it contracts, it contracts. Remember, whenever it contracts at the ends, it stretches the fiber.
When you stretch the fiber, what do you do? Activate the sensory fibers. So it's this constant vicious cycle.
So now that we understand that, let's actually look at the reflex and how this all kind of comes together. Alright, so now let's take everything that we know about kind of the anatomy of the overall skeletal muscle and those muscle spindles and put into action the actual reflex itself. So, the reason why this is such an important thing, and I'm kind of really harping on it here, is because this is a clinically relevant reflex, okay? Now, it's kind of a misnomer sometimes when people say that, you know, your deep tendon reflexes, they think, oh, the Golgi tendon organ is involved. It's actually not.
deep tendon reflexes involve your muscle spindles. That's actually what's really being involved here. So now let's put into practice all of the stuff that we learned. So let's say that you guys are coming in and you're going to be doing a patellar reflex on somebody, right? So what are you going to do?
You're going to tap on the patellar tendon. When you tap on the patellar tendon, what it does is it actually pulls, it kind of like stretches out the muscle. Okay, now what did I tell you whenever you stretch the muscle you're stretching two types of fibers the extra fusel fibers and the Intrafusal fibers, which is the one that's really the sensory component the extra fuse or the intrafusal the intrafusal fibers Whenever you stretch them you guys remember what does it do it activates the type 1a and type 2 fibers by opening up their Mechanically gated ion channels and generates the whole action potential So what are the two fibers here that are going to be coming? So whenever you hit the patellar tendon, what muscles are you actually stretching?
You're actually stretching the quadriceps muscles, okay? So whenever you stretch those quadriceps muscles, the sensory fibers that are going to be involved with them, okay? So again, your type 1A and what else? Type 2 fibers. 1A is annular spiral, type 2 is your flower sprandings.
Those are going to pick up the sensations of stretch. They're going to travel down the sensory neuron to your dorsal root ganglia. As they come into the dorsal root of the spinal cord, so here we have a cross-section of the spinal cord. Here's our dorsal, our posterior gray horn, anterior gray horn, right?
As they come into that posterior gray horn, what can happen is, is they can synapse on two points here, right? So one is the sensory neuron can either synapse directly onto a motor neuron or it can synapse onto a little neuron between the sensory neuron and the motor neuron. So here, we're going to draw the motor neuron here, for the sake of not getting the colors all kind of mixed up here. We're going to draw this here in a blue color. Now, if the sensory neuron directly synapses onto the motor neuron, that motor neuron is then going to do what?
It's going to go out to the muscle and cause the muscle to contract right if you have one sensory neuron leading to one motor neuron what is that called that's called a monosynaptic reflex so this is actually called a monosynaptic reflex and you know what's interesting it's all occurring on the same side so it's actually called a ipsilateral monosynaptic reflex right now Why do we have these motor neurons activated? Well whenever you tap the patellar tendon, you stretch the muscle. What is the job of these muscle spindles? The big job is to prevent overstretching of the muscle.
How do you prevent overstretching or lengthening of the muscle? You shorten it. How do you shorten a muscle?
In this case you're going to have to contract the muscle. So in this case I want my quadriceps muscles to contract and shorten. So that is why that occurs.
Now let's come to the other aspect here. The other thing that can happen is that this sensory neuron can also act on a little neuron between here. So here we're going to have another neuron here and it's going to be between the sensory neuron here and the motor neuron.
What is this little pink thing called? This is called our interneuron. This is called the Interneuron. Now in this case usually let's just say for this case the interneuron is going to be inhibitory.
So what it's going to do is it's going to inhibit this motor neuron. And now this motor neuron is not going to be sending potential stimuli to the skeletal muscle, right? So if this is not sending any signals to the skeletal muscle, in this case what muscles are on the opposite side of your quadriceps?
Your hamstring muscles. In order for your quadriceps muscles to shorten and contract you need the antagonistic muscles, the hamstring muscles, to lengthen and relax. So that means that these motor neurons are going to go to this muscle here and inhibit the muscle contraction. Whereas the motor neurons going to the quadriceps muscles are going to stimulate muscle contraction.
Okay? There's a really important term for this whenever you're contracting the muscles and shortening the muscles that are basically stimulated from the stretch reflex and inhibiting or relaxing the muscles the antagonistic muscles this is a very important term i want you guys to make sure you write down son of a gun this is called reciprocal inhibition it's called reciprocal inhibition now here's the thing we only really talked about one motor neuron at this point This motor neuron that innervates the extra fusel, the red fibers, what are those called? These are called your alpha motor neurons.
So we only really talked about the alpha motor neurons in this case. Guess what? The same thing happens here with the gamma motor neurons.
So we're going to kind of basically introduce that whole concept, but the same thing applies. You have the patellar tendon, you tap it, right? When you tap the patellar tendon, you stretch these quadriceps muscles. When you stretch the quadriceps muscles, you activate not only those extra fusils to stretch, but the intrafusils to stretch. That activates the fibers, right?
Those sensory fibers, which are basically picking up those degree of stretch and speed. And that's called your type 1A and 2 fibers. They're sending their sensory impulses into the spinal cord. Now, when they send their sensory impulses into the spinal cord, they can synapse again onto either the motor neuron directly.
And in this case, let's make it... the color that we made it before which was this maroon color here's our gamma motor neuron or we can have it synapse on a interneuron which we did in pink here and this interneuron is going to be inhibitory so it's going to inhibit these gamma motor neurons and it's also going to in this case stimulate these gamma motor neurons now if it stimulates the gamma motor neurons in this case we want to stimulate the gamma motor neurons that are going to the quadriceps muscles Same thing with the alpha motor neurons that went to the quadriceps muscles. We're going to want these to be stimulatory. But they're not going to stimulate the extra fusel fibers to contract and generate movement.
They're going to stimulate the muscle spindles to contract. So that we maintain a degree of sensitivity and sensory input via the sensory fibers. And I'll explain what I mean. It might seem a little confusing. But again, the gamma motor neurons.
are going to stimulate the muscle spindles just as the alpha motor neurons will stimulate the extra fusel fibers of the quads. In the same way, we're going to inhibit these gamma motor neurons that are going to the muscle spindles of the hamstring muscles because we don't want those to be contracting and increasing the sensory inflow to the spinal cord, okay? There's a very particular name of why we have alpha and gamma motor neurons.
contracting called alpha gamma co-activation. Another thing that I have to add in here because it's clinically relevant is that you have these little tracks here located in the lateral white column and these are called your corticospinal tracks. What is this called? Your corticospinal tracks.
These contain your upper motor neurons right and these upper motor neurons can synapse and modulate the activity of these gamma motor neurons. Why is that important? Let me just quickly explain because it's going to become clinically relevant.
If you have a lesion that damages these corticospinal tract neurons, what happens is these corticospinal tracts generally have an inhibitory signal to these gamma motor neurons. So if you damage the corticospinal tracts, which is that's considered an upper motor neuron lesion, What that leads to is it leads to what's called gamma motor neuron activation. And this is going to lead to, if we have gamma motor activation, it's actually going to be more activated than usual because the upper motor neurons are designed to inhibit these gamma motor neurons. So if you have a lesion, you're taking away the inhibitory output.
The gamma neurons are going to hyperfire. And if they hyperfire, what that does is that causes the muscles to have a lot of tone. And it can lead to what's called hypertonia. And it can also lead to something called spasticity, which we'll talk about in our lesion videos. Okay?
So it's very important to understand that. All right. So the reason why I kind of want to take some time to explain this alpha, gamma, motor neuron co-activation process is because it's common questions that come up on exams. So let's say pretend here we have we're focusing on one type of structure here. So these kind of red structures here, this is your extra fusel fibers.
I'm just going to put EFF and then here is going to be this blue one, your intrafusal fibers, and then these are going to be your sensory neurons, right? So we'll put the 1A fibers here since it's around the central portion of the nuclear bag. Let's say that this is normal. Okay, so this is a normal muscle.
All right, so no contraction whatsoever at this point. So now what we're going to do is we're going to apply some type of stimulus. Okay.
And let's say that over here on this side, what we're going to do to this normal muscle is we're going to stretch it. Okay, so what I'm going to do is I'm going to stretch the muscle. And then over here, I'm going to contract the muscle. Okay.
So obviously when we stretch the muscle, we're going to lengthen it. When we contract it, we're going to shorten it and squish it, right? What I want us to only focus on here is the alpha motor neurons causing the contraction.
And we'll talk about that in just a second. And here's what I want you to remember. When you stretch the muscle, you stretch the extrafusal fibers, right?
And you stretch the intrafusal fibers. Remember, when you stretch the intrafusal fibers, what does that do to the sensory fibers? Does it stimulate them?
Or does it actually inhibit them? It stimulates them. So these guys are going to fire.
So again, I want you to remember, remember you stretch the intrafusal fibers, it activates the type 1 and type 2 fibers, and they stimulate and they send action potentials very quickly. Now, if you contract, if you only contract the extrafusal fibers and shorten the muscle, I want you to imagine the intrafusal fibers, they are not contracting. Now, if that happens where the extrafusal fibers contract, shorten the muscle, but the intrafusal fibers don't contract and stretch the muscle, guess what happens to the type 1a fibers? They are not going to be stimulated.
So here's your type 1a fibers. If they are, the intrafusal fibers are not contracted, okay, and they're not stretched. The type 1a fibers are going to have very little. action potentials generated around them.
You want to know why? Whenever the extrafusal fibers contract and the intrafusal fibers don't contract and stretch, these little sensory neurons, the type 1a fibers, become very slack. That's kind of the term that you're going to hear a lot. They become really slack, they become loose. And whenever they're not tight or taut, they're not stretching and opening up those mechanically gated ion channels.
So if they're really slack, they're not going to open up those mechanically gated ion channels, and they're not going to lead to lots of action potentials down the type 1a fibers. How do we combat that? Well, here's what we do. We come down here, we have our, again, our contracted muscle, right? So again, I want you to kind of remember here, here is the extrafusal fibers contracting, okay?
And the intrafusal fibers are not contracting. The muscle shortens. What happens to the intrafusal fibers? They don't contract, they get all kind of like fat here and bulbous.
What happens to these little sensory fibers? Do they become activated? No.
So the type 1a fibers, what's going to happen to them? There's going to be decreased action potentials, okay? Because they're not being stretched.
How do we combat that? Well guess what we're going to do? We're going to have the extra fusel fibers and the intrafusal fibers contract at the same time. Who causes the extra fusel fibers to contract?
The alpha motor neurons. Who causes the extra fusel fibers to contract? Alpha motor neurons.
who causes the intrafusal fibers to contract. The gamma motor neurons. Now watch what happens when both of these neurons are firing. Extrafusal fibers contract, they shorten the muscle.
Gamma motor neurons contract the intrafusal fibers, they stretch and lengthen. What happens to these little sensory neurons? Are they slack anymore? No, they tight, baby.
And if they're tight, you're gonna activate those mechanically gated ion channels around them. If you activate those mechanically gated ion channels, What's that going to do to the type 1a fibers? It's going to cause them to be generating lots of action potentials. So there's going to be increased action potentials. Okay, this is called alpha gamma co-activation.
And it is very, very important because if your alpha molyneurons are causing your extra fusel fibers to contract and shorten, but they're not you're not having these sensory fibers send signals it's going to affect the entire proprioceptive process so you need both the alpha motor neurons to contract to shorten the muscle and the gamma neurons to stimulate the intrafusal fibers to contract so that we're sending proper sensory signals to the spinal cord. Alright Ninja Nerds so in this video we talk about the muscle spindle or the stretch reflex pathway i know we talked a lot about a lot of stuff in this video and i hope it helped i really do I hope it made sense. I hope that you guys really did enjoy it and you learned a lot from it.
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