hello and welcome to the review of chapter 55 of guyton and hall's medical physiology textbook in this chapter we go over how the spinal cord controls our motor functions if you enjoy the video please don't forget to give it a like and subscribe to the channel otherwise we'll push forward with the rest of the chapter so to begin with since it's the start of the unit on motor function it talks about how the motor responses begin in the spinal cord for very simple muscle reflexes but they will also extend out into the brain stem for more complicated responses and then finally the most complicated muscle skills are controlled in the cerebrum so in terms of the organization of the spinal cord which is the focus of this chapter and its role in motor function we really start at this figure here figure 55 1. and as you can see the sensory root is always on top of the spinal cord and this is called the dorsal root where the sensory nerves come in and innervate into the gray matter now there are some interneurons and then for the local reflexes these will just connect straight to the anterior motor neurons which will then go out to control the muscles so there will be a sensory input that will instantly go out into your motor neuron to then innovate your muscle there will be some connections to then go up to the higher centers of the brain for more complex movements but we will mostly be worried about this reflex of our here in this chapter so in terms of the motor neurons we do have two types of motor neurons we have the alpha motor neurons and the gamma motor neurons now alpha motor neurons they are the ones that control the motor units so the larger muscle groups out to your skeletal muscle whereas your gamma motor neurons they innervate these special type of muscle fibers called intrafusal fibers and intrafusal fibers are these tiny little muscle fibers that are within the actual muscle body itself and they are involved with controlling muscle tone so they're just these specialized units as shown here within the actual skeletal muscle itself and if we break it down this is the intrafusal muscle fiber here with the extrafusal fibers representing the actual skeletal muscle now this little box here just talks about renshaw cells which transmit inhibitory signals which will act like the lateral inhibition that we always talk about throughout this neurological unit where it will inhibit that signal from spreading widely and make sure it's fine-tuned so it goes to where we need it to go now we've talked about the motor neurons so the alpha and the gamma but now we are going to focus purely on the control of motor function through these two types of special sensory receptors talking about the muscle spindles and the golgi tendon organs now the muscle spindles they are the ones we talked about previously which sit in the middle of the muscle fiber and they sense the change in the length of the muscle unit and the rate of the lengthening whereas we also have golgi tendon organs which actually sit more in your tendons and they sense the actual tension in the tendon and the rate of change in tension and all of these control mechanisms are on subconscious level control these reflexes happen automatically so they were able to move and function in a smooth manner so we're going to start with the muscle spindle and the muscle spindle is this intrafusal muscle fiber which are innervated by the gamma motor nerves now as we mentioned they have two mechanisms of actually being excited that's if the whole muscle actually stretches and lengthens or if the actual intrafusal fiber only stretches so only this portion lengthens rather than the entire muscle unit lengthening and that is sensed through two main efferent neurons called primary efference and secondary afferents now before talking about the two types of these sensory neurons we should start by actually showing the two types of intrafusal muscle fibers within the muscle spindle so we have these nuclear bag fibers which are more of a collection of muscles fibers in one region and then we have the nuclear chain fiber so as you can see it's more spread out now the primary fiber or the primary efferent innervates both of these guys and the secondary afferent only innervates the nuclear chain fiber so that's the main difference there the other main difference is although both of these afferents sense the lengthening of the muscle spindle only the primary afferent actually also senses the rate of lengthening so if it's going to lengthen very fast versus very slow only the primary afferent is able to actually identify that whereas a secondary efferent is only involved with knowing whether or not the muscle spindle is lengthened or not which the primary afferent can also do and being able to identify the rate of lengthening by the primary reference that is called the dynamic response and then just identifying when the muscle spindle is lengthened or not is called the static response remember that's what both can do both the primary and secondary afferents can identify that now once it is stretched then the sensory neurons are going to be excited and send that signal up into the spinal cord which will then come back through the gamma fibers now the gamma neurons or the gamma motor neurons we have two types we have the gamma d and the gamma s now the gamma d fibers are mainly for the dynamic response that is sensed by our primary reference remember so the rate of change that there's a fast change in the lengthening in our muscle fibers there will be a lot of signals going through those primary afferents coming back through the gamma d motor fibers they will then innervate and tell the muscle spindle to contract whereas the gamma s motor neurons are more involved with the static response so if there is a lengthening to that muscle then that will be sent up into the spinal cord come back out through that gamma s which will then tell the muscle spindle to contract at a sustained point to keep it controlled so the key there is that you get positive signals when you get in lengthening and then you get negative signals when there is shortening so there is actually a feedback loop in your muscles themselves telling itself when it is lengthening and when it is actually shortening so then it is able to stabilize itself and this entire pathway is shown in the stretch reflex which is a monosynaptic pathway so as you can see we have our muscle spindle that is innervated by our sensory neuron and if this gets stretched that will be sensed and excite that neuron which will then synapse to the afferent motor nerve which will be a gamma motor nerve which will then innervate and cause contraction of the muscle itself so then if there is a sudden lengthening that then gets contracted to stabilize the muscle and as it talks about a little bit later on that's a very good example of the patellar reflex where if you hit the hammer on your patella you cause a lengthening of these receptors which then gets sensed and then tells the motor nerve or the quadriceps to actually contract so then you end up kicking out your leg and the real purpose of the muscle spindle and also the golgi tendons is to actually smooth out our contractions and that's shown in this diagram up the top here where a is a person with normal sensory receptors and b is when that has been denovated so as you can see the force of contraction is smoothed whilst without these sensory receptors then there is a very jagged contraction of your muscle because you're unable to really know what's happening within your muscles as they're contracting or as they're relaxing so since you don't know the rate of change in your muscle length then you're unable to correct for it and smooth out that contraction and that's called signal averaging and this is so important that every single time the brain itself tells a certain muscle to contract it also sends a signal to the gamma motor neurons which are innervating the muscle spindles in addition to those alpha motor neurons which are innervating your large muscle groups so you actually have co-activation of both your extrafusal and intrafusal muscle fibers and that allows us to have that smooth contraction and control with help from these spinal cord reflexes and those signals come from what's called the bulbo reticular facilitary region of the brain stem and they can also be transmitted from the cerebellum basal ganglia and cerebral cortex so that really helps with these antigravity contractions so then we're able to stand upright without having these jerky movements and our muscles to just keep our body stabilized in a certain position this also helps to stabilize during an intense action so for instance if we want to hold our knee joint in a particular angle then you will actually send down the signal to both of your gamma neurons on both sides of the joint so then there's a reflex excitation of both skeletal muscles so your muscles tense up and hold that joint in a particular position so that is our muscle spindles now our golgi tendon reflex is very similar but the main difference is that instead of sensing the length of the muscle we are now sensing the tension or the rate of change in tension of the actual tendon remember the tendon is where the muscle connects to bone and then the muscle spindle is located actually within the actual muscle fiber itself so golgi tendons are more up in this region in the muscle tendon and they do a very similar mechanism to our muscle spindles where they have a sensory nerve that comes out and then there's a reflex arc back to our muscles but the big function of our golgi tendons is to actually inhibit the contraction of our muscle because we're trying to prevent excessive tension on the tendon or if we have too much tension then we may damage the muscle we may damage our tendon so if there's excessive force through this golgi tendon organ that sends an inhibitory signal to our muscle fibers to cause relaxation that actually helps to spread the load out to all different muscles which all you know each joint has multiple muscles innervating it so each golgi tendon organ is able to relax the ones which are tensing too much so then the other ones are able to tense a little bit harder and that really prevents the damaging of our muscle fibers in both of these receptors the golgi tendon organs and the muscle spindles they actually send those signals up to the higher centers of the brain via the spinal cerebellar tract so then we are able to know exactly where our body is positioned in space and time so that all leads us to these flexor reflexes and withdrawal reflexes and the point behind these is that if there is a painful stimulus in one of our limbs that is sensed and then there is an instant reflex arc with some interneuron complexity within the gray matter to then send out an excited signal to our flexible muscles to pull away our hands and the inhibitory signal to our antagonistic muscles to prevent us from keeping our hand there which is called reciprocal inhibition and then also something called after discharge meaning that although we're able to quickly pull our hand away there is a little bit of reverberation within our into neurons which keeps that signal going so we're able to make sure we keep our hand away from the painful stimulus for a little bit longer than what is maybe actually needed which is an extra safe mechanism now that actually also gets spread over to the contralateral side which is called the crossed extensor reflex to then push away and extend so that we're able to push away from that painful stimulus so this reflex arc is simple and complex at the same time where we just have the sensation that instantly goes in reflexes back through the spinal cord without having to involve your higher centers to then pull away from a painful stimulus but it's complex because there's a bit of reverberation within these interneurons and there's also a cross over to the other side for our crossed extensor reflex and so then we get both after discharge and the pushing away or extension away from the painful stimulus and it's not as simple as just one muscle being flexed and one being inhibited because if there is a painful stimulus on the inside of your arm not only will you flex but you also add duct as well so you pull your hand completely away so since there's so many muscles within each of our limbs it's a little bit complex within this gray matter with the interneurons to make sure we pull away in the appropriate direction so then we actually get to our kind of postural and locomotive reflexes meaning that when we actually put pressure on our foot or they give the example of animals throughout the rest of this chapter so pressure on the footpad results in a reflexive extension of that limb to make sure we don't fall over and that's same if the pressure is on one particular side then we will actually increase the extension on that side which is called a magnet reaction even if the spinal cord is severed animals still have that reflex to try and get up and stand and hold posture purely because we have these reflex arcs within the spinal cord and we don't need that signal going up to the brain in order for the basic neurological phenomena to occur and that's the same with stepping and walking so even if you separate the spinal cord and even transect through the middle then as long as this pathway is still present in this one limb the sensation and then the motor then you're able to actually create still a walking motion in that leg so it's quite a primitive or simple method for walking where you're able to actually still move the limb and still appropriately adjust where you need to extend your limb based on the sensation that you're feeling on your foot or on the foot pad and that's the same if something touches the top of a foot as you're walking instead of just pushing forward you'll actually pull your leg up to actually put your foot on top of a platform and that's called the stumble reflex obviously if you don't have that severing through the middle of your spinal cord then there's actually this reciprocal stepping where your left leg will automatically move forward as your right leg moves back and then four leg of the animals that also happens in a diagonal direction between the four limbs and the hind limbs as well and all of this does not involve the higher centers of the brain there's purely these reflexes within the spinal cord that's able to tell when one extends and one flexes and then the amount of extension and the direction of the extension is all sensed purely within that limb and that instantly causes a reflex arc within the limb itself animals also have the scratch reflex you know if there's a little irritant some kind of parasites on their skin then they're able to actually identify that location with a position sense and then have a turn for a scratching movement with their back leg to actually try to get rid of that parasite and then we have some other reflexes here through the spinal cord including muscle spasm around the the muscles of a broken bone we also have muscle spasms if we have peritonitis a lot of inflammation irritating those sensory receptors and there's a reflex arc to our abdominal muscles causing abdominal spasm muscle cramps start off with the local irritating factor so some kind of metabolic abnormality of the muscle or some severe cold or over exercise results in a reflexive contraction of the muscle and that contraction can actually stimulate the same sensory receptors so we get even more intense of a contraction and that results in a cramp through positive feedback this also occurs in our autonomic nervous system and there's a list here of some examples for instance a change in vascular tone sweating intestinal reflexes and evacuation reflexes of emptying a full bladder like we went over in chapter 26 and then we have this mass reflex where we have a sensation coming into the spinal cord and if there's a lack of inhibitory neurons to that particular sensation usually in someone with a severed spinal cord then you can have a massive discharge of stimuli to all of the motor neurons so we can get a major portion of muscle spasm emptying of your colon bladder hypertension from vasoconstriction and then also profuse sweating and that's almost like a seizure of the spinal cord because epilepsy is just massive stimulation or excitation of all the neurons in your brain then lastly here we talk about spinal shock which is an interesting phenomenon where actually when the spinal cord gets severed you have a sudden dysfunction of all of your spinal reflexes even if they remain intact and that can take a few hours to a few weeks to actually recover and that essentially finishes up our chapter if you'd like to support the channel there is a link in the description for patreon and you can get access to these chapters in the downloadable audio format as well otherwise feel free to leave a comment and we'll see in the next video