Neurology | Spinal Cord: Stretch Reflex | Muscle Spindle

hi ninja nerds in this video today we are going to talk about the stretch reflex let's go and get started all right ninja so when we talk about the stretch reflex obviously another way that we can kind of just 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 extra fusel 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 intra fusal 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 proprioceptors 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 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 all right sweet so we know extra fusal fibers connected tendons generate movement in between the extrafusal fibers if you kind of separate this you have a connected tissue capsule inside of that is intrafusal fibers a bunch of intrafusal fibers are a muscle spindle and these are proprioceptors all right so let's go ahead and talk about a couple basic functions of these all right 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 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 extrafusal fibers the other ones that i needed to talk about is the intrafusal fibers but a bunch of intrafusal fibers are what they make up a muscle spindle all right what do we say the intrafusal 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 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 as if 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 in 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 what now 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 fatso 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 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 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 of 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 involved 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 is it actually causes these little spin these little fibers here these little nerve fibers that are wrapping around this intrafusal fiber to become really taut and stretched and activate particular little sodium channels in the actual sensory nerve index and we'll show that next 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 1 a fibers okay type 1a fibers you know what else they love to call these they love to call these sometimes annulo spiral endings all right 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 fiber 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 2 fibers another name for the type 2 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 actual 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 intrafusal 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 crossbridge contract and whenever both these ends contract it stretches it lengthens the fiber that 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 all right so now we know about the extra fusible fibers we know a little bit more about the detail about the sensory and motor innervation and just the basic anatomy of the intravasal 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 back 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 one error surrounding the central portion of the fibers type two 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 intrafusal fibers you stretch these actual sensory neurons and it actually opens up these mechanically gated ion channels guess who starts flowing in sodium gonna be like uh um 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 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 oh 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 like 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 instant contract which stretch out that fiber we're not going to go into crazy detail we 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 you know 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 crossbridge 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 all right 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 the you know your deep tendon reflexes they think oh the golgi tendon organ is involved it's actually not the 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 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 extrafusal fibers and the intrafusal fibers which is the one that's really the sensory component the extra fees are 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's annular spiral type 2 is your flower spray endings 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-sectional 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 maxed mixed up here we're going to draw this here in a blue color now if the sensory neuron directly in synapses on to 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 mono synaptic reflex and you know what's interesting it's all occurring on the same side so it's actually called a ipsy lateral 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 over stretching of the muscle how do you prevent over stretching 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 motorons 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 extrafusal 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

fusibles to stretch but the intrafusals to stretch that activates the fibers right those sensory fibers which are basically picking up those uh 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 uh the color that we made it before which was this maroon color here's our gamma motor neuron or we can have its synapse on a

interneuron which we did in pink here and this inter neuron 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 mode neurons in this case we want to stimulate the gamma motor neurons that are going to the quadriceps muscles right 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 extrafusal 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 would stimulate the extrafusal 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 it's called alpha gamma coactivation 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 tracts what is this called your cortico spinal tracts these are 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 interactivation it's actually going to be more activated than usual because the upper motor nodes are designed to inhibit these gamma motor neurons so if

you have a lesion you're taking away the inhibitory output the gamma motorons are going to hyperfire and if they hyper fire 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 extrafusal 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 now 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 without wherever you stretch the intrafusal fibers it activates the type 1 and type 2 fibers and they

stimulate and they extend 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 mechanical irrigated 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 extrafusal fibers and the intrafusal fibers contract at the same time who causes the extrafusal fibers to contract the alpha motor neurons who causes the extrafusal 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 extra fusible fibers contract they shorten the muscle gamma motorions 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 going to 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 motors are causing your extra fusible fibers to contract and shorten but they're not you're not having

these uh sensory fibers send signals it's going to affect the entire proprioceptive process so you need both the alpha motors to contract to shorten the muscle and the gamma motor around us to stimulate the intrafusal fibers to contract so that we're sending proper sensory signals to the spinal cord all right ninja 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 if you guys did please hit that like button comment down in the comment section and please subscribe also down in the description box we have links to our facebook instagram patreon all that stuff if you guys also would be willing to donate we truly appreciate it helps us to continue to keep making videos for you guys enjoying them all right nigerians as always until next time [Music] you