 Hello friends, today we are going to discuss a very important topic, comes in first year MBBS exams also, a very favorite topic for MCQs as well. But it is a bit difficult one, that is stretch reflex. So let us see the fundamentals of stretch reflex and its applications as well. See, first as the name suggests, it is a reflex and like all the reflexes, it will have a stimulus, there will be receptor, afferent, center, efferent and there will be a response. So what are the components of this stretch reflex? Stretch reflex operates in a muscle, skeletal muscle. So suppose this is a skeletal muscle, that is the contractile component of the skeletal muscle, where there will be lot of muscle fibers. So these are the contractile muscle fibers which will extend from end to end of a muscle and the ends of these muscle fibers will continue as a tendon. Now all skeletal muscles have a sensory component as well, that is the muscle spindle. So in the center of the skeletal muscle, there is a sensory component which is the muscle spindle and it has two types of fibers, that is there is a nuclear bag fibers. So this first one which I do, it is like a bag, you see central part it is like a bag. So this is a nuclear bag fiber and then there is nuclear chain fibers. Okay, nuclear bag fibers and nuclear chain fibers. So these are the receptors for the stretch reflex. Now remember there are two components of these nuclear bag fibers, that is static nuclear bag fibers and dynamic nuclear bag fibers. So this is the receptor for the stretch reflex. So that means this receptor, static receptor is important for detecting a static stimulus and dynamic receptor is important for detecting a dynamic stimulus. And what is the stimulus? Stretch is the stimulus. So we will see what is the static and dynamic. On the other hand, nuclear chain is only detecting static stimulus. Okay. So what I do is a nuclear chain, there is C. So C I make it as S. So it becomes a static nuclear chain is only static nuclear bag both static and dynamic. Another way to remember this B becomes D right. So nuclear bag is static as well as dynamic. Now these nuclear bag and nuclear static fibers are supplied by efferent nerve fibers and what are these efferent nerve fibers? There are one A fibers and there are two fibers. Now these names come from Liott's classification of sensory nerve fibers. So one A fibers and two fibers supply these nuclear bag and nuclear chain and which one these fibers supply? One A supplies all the fibers that is nuclear bag static dynamic and nuclear chain static. One A is for all right. So they supply the central part of these muscle spindle both types of muscle spindle and both static as well as dynamic component. On the other hand, type two fibers supplies only the static component that is static component of both nuclear bag as well as nuclear chain fibers. So that I'm not going to draw but remember here that these fibers are actually one A and two fibers. One A fibers are supplying all the types of fibers static dynamic in nuclear bag as well as nuclear chain and two fibers it is supplying only static component of nuclear bag and nuclear chain fibers. So these are the efferents of the stretch reflex. Then these efferents actually make contact in the spinal cord. So let me just draw a spinal cord here. We'll get another neater diagram later. So they just go into the spinal cord and from here they actually climb. These form the dorsal column of the ascending tract. But one branch of these fibers goes and makes contact with the alpha motor neurons right. So motor neuron that is the Bell-Megendi law motor neuron arises from the ventral root of the spinal cord. So that is why we are bringing it to the ventral side and this alpha motor neuron supplies the external fibers which are contractile right. So remember I told you before that there are contractile muscle fibers which run from end to end. These contractile muscle fibers are known as extrafusal muscle fibers okay and these sensory component that is the spindle shaped sensory component that is known as intrafusal muscle fibers. So you see this alpha motor neuron is supplying the contractile fibers. It is not supplying the sensory fibers. So sensory component is within and the motor component the efferent is outside to the extrafusal fibers. However there is another motor component that is the gamma motor neuron. So here only there is roots for the gamma motor neuron which supply the ends of these intrafusal fibers. Understanding ends of the intrafusal fibers are supplied by the gamma motor neuron. So this intrafusal fiber it is sensory in the central part but there is motor component it has contractile ends which are supplied by the gamma motor neuron. So this is the basic structure of stretch reflex. Let's see a neater diagram now. So here we are showing the muscle spindle containing nuclear back and nuclear chain fibers and there are one a fibers which are supplying both right and two fibers which are supplying only the static component and these are going to the spinal cord. Then there is alpha motor neuron which is supplying to extrafusal fibers which are basically contractile and there is gamma motor neuron which is supplying the ends of these muscle spindle. So these muscle spindle ends are actually contractile. So these two are the efferents. So with this basic structure now let us move on to the actual stretch reflex. What is stretch reflex? As the name suggests stretch reflex basically stretch is the stimulus right then there are efferents. What is the center? It is the spinal cord which go to the spinal cord and there is activation of the efferents that is the alpha motor neuron only. We are talking about simple stretch reflex. There is activation of alpha motor neuron because as I told you afferents are making direct contact with the alpha motor neuron and this stretch causes the activation of the alpha motor neurons and there is contraction. So whenever the muscle is stretched it is leading to contraction of the muscle. So this is the simple diagram for a stretch reflex. Suppose the muscle is stretched then when the muscle is stretched you see what will happen these intrafusal fibers will also be stretched along with the muscle because they they are passive okay. So with the stretch of the muscle spindle there will be increase in efferent activity in these 1A and 2 fibers and they are going to make contact with the alpha motor neuron. So here you see it is a single synapse that is why stretch reflex is monosynaptic and it is the only example of monosynaptic reflex in body. So it directly makes contact with the alpha motor neuron which supplies the extrafusal fibers and there is contraction. So this is the simple stretch reflex but remember we talked about the dynamic and static components. So what is that actually? What are these 1A and 2 fibers carrying? See what is a static component is suppose this is your arm okay somebody's arm very line diagram we will draw and it is at 90 degrees. So there is a particular length of the muscle right suppose this arm is stretched to this position then the length of the muscle is increased suppose the arm is flexed in that case suppose it is in this position then length of the arm is decreased okay. Now when the muscle is stretched right suppose this position this muscle spindle will stretch and it will carry the information to the alpha motor neuron and the muscle will contract back again to its original length. Similarly when the muscle is flexed okay that means for flexing there will be shortening of the muscle then this afferent activity is going to decrease why it is going to decrease because when the muscle is shortening along with that this intrafusal fiber length is also going to shorten here we are talking about whenever the muscle length is decreasing the intrafusal fiber is shortening we will talk about voluntary contraction little later that why we are able to voluntary flex the muscle why it doesn't happen okay. Anyways so when there is decrease in the length of the muscle this will shorten the muscle spindle and this afferent activity in these fibers is going to decrease okay and when this decreases what will happen here the activation of the alpha motor neuron is also going to decrease thus causing decreased activation of these extrafusal muscle fibers so there will be relaxation and hence the muscle will come back to its original position so stretch reflexes working both ways increase in the stretch is causing contraction decrease in the stretch is causing relaxation okay so this is very simple you should remember so basically this stretch reflex is acting as a negative feedback loop where it is maintaining the length of the muscle okay negative feedback loop but till now we are talking about static stretch only see I told you that suppose this is the muscle position but I never talked about the dynamic component what is dynamic component dynamic component is rate of the movement how fast the muscle length is being changed from this particular length to this particular length so how fast the muscle is being moved similarly here how fast the muscle is being moved that is the dynamic component and that information is being carried by our 1a fibers so basically two fibers are mainly for the static length that currently what is the length of the muscle and 1a fibers is talking about the velocity of change in the length of the muscle from one position to other when it is going how fast it is going so that information is also very important because it kind of tells the brain and anticipatory information that how fast the correction should be done also so that is the meaning of static and dynamic component now we have talked about the static and dynamic component of further afference but one thing more you remember here that even the gamma motor neuron which are supplying the contractile ends of these intrafusal fibers they are also of two types that is the static and dynamic okay so they are also because this information is going to the alpha motor neuron also to the top to the brain also it is going so to make corrections later we will see that how voluntary contraction this gamma motor neuron is also activated so for that also we need a static and dynamic component so with this let us see certain facts about the stretch reflex one that this stretch of the muscle when it causes a stretch of the muscle spindle that is known as loading of the spindle so sometimes you should remember here loading of the spindle stretch of the spindle is known as loading of the spindle because it is increasing the afferent activity on the other hand the decrease in length of the muscle unloads the spindle so that is known as unloading of spindle so that is one second the reaction time for this stretch reflex is 19 to 24 milliseconds what is the reaction time meaning of reaction time reaction time is basically time gap between the stimulus and the response right so from the time the muscle is stretched till the time the muscle is contracted by the alpha motor neuron that is the total time taken for the reflex so that is known as reaction time and it is 19 to 24 milliseconds now what is the importance of this 19 to 24 milliseconds when you see for any negative feedback loop there is some time involved why because there will be some time in the activation of the afferents then carrying of this information to the spinal cord some conduction velocity this fiber will be having so some time it will take to travel from one portion to the spinal cord then there will be some component involved in this release of the neurotransmitters activation of the alpha motor neuron and then there will be again velocity of this alpha motor neuron for getting the information reaching the information to the extra use extra fusel muscle fibers right so this is the entire time required for the reflex but if we subtract the time taken for traveling of the impulse then we can get to know that how much time this particular reflex is taking at the level of the spinal cord and that is known as central delay and this central delay is found to be 0.6 to 0.9 milliseconds that means out of this 19 to 24 milliseconds only 0.6 to 0.9 millisecond time is within the spinal cord rest all the time is taken for conduction of the impulse from the muscle spindle to the spinal cord and again bringing the impulse from the alpha motor neuron to the muscle so this is the central delay again what is the importance of this see minimum time for synaptic delay what is synaptic delay synaptic delay is basically the time taken for the neurotransmission in one synapse the vesicles have to be released the neurotransmitter has to act on the nerve terminal so it is a chemical response and there because of this there is some synaptic delay and that minimum delay is 0.5 milliseconds so if we know that this is the minimum delay that means in this stretch reflex there can be only one synapse if there are two synapse how much will be the delay it will be greater than at least one millisecond isn't it here we are talking that the central delay is only 0.6 to 0.9 millisecond which is less than one millisecond so the number of synapses involved in a stretch reflex is single only so that is why it is a monosynaptic reflex I mean that's how we found out that a stretch reflex is a monosynaptic reflex by determining the total time required for a stretch reflex and the central delay okay now let's move on to the next component of stretch reflex that is alpha gamma co-activation now you see I gave you that example that suppose this is the muscle length and if it is changed to a different length or different position see when the muscle position is changed when the arm is moved what will happen the muscle length will also change so basically length and the position are kind of synonymous that is why these one and two fibers are important for proprioception they give the information about the position of the arm anyways so I told you that as the muscle length will change this stretch reflex will tend to bring the muscle back to its original length right so if this happens every time we move our arm we will never be able to move because there is a reflex operating it will always bring the arm back to its original position so how to move voluntarily well this is because of alpha gamma co-activation every time we wish to contract the muscle there is alpha gamma co-activation and every time we have to relax the muscle there is alpha gamma co-inhibition so we use the word co-activation for the contraction which is happening so what is that I will just to redraw that diagram quickly so this is suppose extrafusal muscle fiber and this is the intrafusal muscle fiber and it is having the contractile ends right so when we want to contract the muscle what we do from the spinal cord there is basically information to both alpha motor neuron and also to gamma motor neuron so both get excited together that is the term alpha gamma co-activation so what happens because of this see when the alpha motor neuron will be activated there will be shortening of the muscle right now as I told you before if the muscle shortens there will be passive shortening of the intrafusal muscle fiber also stretch reflex will be activated no with alpha gamma co-activation it doesn't happen like that because when gamma motor neuron gets excited and it contracts the ends of these intrafusal fibers what happens if the contractile ends contract right then there will be stretch of this intrafusal muscle fiber so it will become something like this so instead of contracting because of the ends contracting there will be stretch of the intrafusal muscle fiber so it is like when the extrafusal muscle fibers shorten or contract the intrafusal muscle fibers are kept stretched they are not allowed to shorten by activation of the gamma motor neuron so that is why for voluntary contraction there is alpha gamma co-activation so that is one application of alpha gamma co-activation in body there is another one also see there are many top fibers which are affecting the activity of gamma motor neuron like there is a pontine reticulo spinal tract which is stimulating this gamma motor neuron and there is medullary reticulo spinal tract which is inhibiting this gamma motor neuron now suppose if the muscle is stretched and also there is I mean suppose if the muscle is stretched there is a particular position in which it is maintained suppose somebody is standing the length of the leg muscles will be some value right so length of the muscle or the position of the muscle is some value now if during that time this pontine reticulo spinal tract stimulates this gamma motor neuron what will happen in that particular length only there will be increased stretch of this intrafusal muscle fiber and with that stretch what will happen that there will be increased activation of the alpha motor neuron and there will be increased contraction of the extrafusal muscle fibers understanding so basically it is increasing the tone of the muscle in a particular length only so this gamma motor neuron activation what we say is increasing the sensitivity of stretch reflex increasing the sensitivity of stretch reflex for a particular position also so that is how the various descending tracts actually influence the activity of gamma motor neuron to maintain muscle tone and posture so that is the second application of this gamma motor neuron third see this stretch reflex basically is used clinically in assessing the motor system so suppose in a patient of paralysis we need to assess the integrity of this reflex arc and that is done by assessing various jerks in body jerks means these stretch reflex there is the knee jerk biceps jerk triceps jerk so all these reflexes we assess at the level of the muscle and what we say in that it has to be supported the muscle has to be supported why we say because we don't want the voluntary contraction there we don't want that alpha gamma co-activation to be together there right because we are assessing only the stretch reflex so that that time we hit actually the muscle on the tendon and by hitting the muscle on the tendon we are actually stretching this intrafusal muscle fiber now sometimes it becomes difficult to elicit this stretch reflex in that case we ask the subject to do gender six maneuver so gender six maneuver what happens that there is increased activation of this gamma motor neuron thus increasing the sensitivity of the stretch reflex and then only we are able to elicit this stretch reflex so that is another importance of only gamma motor neuron alpha gamma co-activation for voluntary contraction of the muscle then gamma motor neuron activation increases sensitivity of a stretch reflex gender six maneuver again increases gamma motor neuron activity so that increased sensitivity of the stretch reflex occurs then there is another effect on this gamma motor neuron suppose in case of upper motor neuron lesions in upper motor neuron lesions what we get we get hyperactive stretch reflex why is it happening because the negative influences on the gamma motor neuron get disrupted so this gamma motor neuron which is generally under more inhibitory control from the upper motor neuron that gets interrupted and that releases the gamma motor neuron activity and it becomes more active causing increased sensitivity of the stretch reflex that is why we get hyperactive stretch reflexes in case of upper motor neuron lesion then there are some other factors which also increases gamma motor neuron activity and these are like anxiety unexpected movements all these also increase gamma motor neuron activity that is why in anxiety also we get hyperactive tendon reflexes again another term for stretch reflex jerks okay so that was all about the stretch reflex hope you have understood the concept thoroughly thanks for watching the video if you liked it do press the like button share the video with others and don't forget to subscribe to the channel physiology open thank you