Welcome back. We're going to finish talking about the somatic nervous system when we talk about the control of movement. I think one aspect that's interesting about this is how much of it is really focused in the spinal cord versus in the brain. That will become evident in a few minutes. First we're going to talk about the fact that there are actually two types of lower motor neurons, meaning the motor neurons that are innervating the skeletal muscle. There are alpha motor neurons. These are the ones that we discussed in our previous meeting. These are the ones that innervate what we call the extrafusal muscle fibers or cells. These are the fibers of the main part of the muscle. They are the fibers that contract to cause the muscle to produce tension. These are the main muscle cells. The alpha motor neurons control these muscle cells. What enables the muscle to perform work are the alpha motor neurons activating the extra fusal muscle fibers. In muscles we also have another structure called the muscle spindle. This consists of what are called intrafusal fibers. That's what are shown here. These are special muscle cells that shorten, but they don't really generate force of the muscle. Another component of this muscle spindle which is contained in this capsule right here is the afferent neuron that is going to relay information about the stretch of these intrafusal fibers. It's going to be gamma motor neurons that innervate these intrafusal fibers. So you might ask, okay if these intrafusal fibers aren't contributing to the force producing capacity or the ability of the muscle. Why do we have them? The point of this muscle spindle is to have this parallel system of intrafusal and extrafusal fibers, and a parallel system of the alpha and gamma fibers so that whenever the the alpha motor neurons are being stimulated to cause muscle to contract, the gamma motor neurons will also be stimulated. As a result when ever the extrafusal fibers shorten the intrafusal fibers shorten in the same manner. In this way the muscle spindle is like this parallel system that is special because of this afferent neuron sensing the stretch in the fiber. It's this parallel system that let's the body monitor the length of the muscle by monitoring the length of the intrafusal fibers which are activated in the same manner and at the same time as the extrafusal fibers. So this is going to be one aspects of muscle length that is going to be monitored for proprioception is the amount of stretch. This muscle spindle is gonna be critical for monitoring the length of the muscle as it shortens. The more it's stretched, the more firing there will be of this neuron, and then as we contract, the firing will reduce less and less. That will tell the central nervous system that the muscle is getting shorter. ABecause there's less stretch there will be fewer action potentials. That will tell the body about where, for instance, the position of the hand is based on how short the muscle is. These alpha and gamma motor neurons are going to be efferent somatic motor neurons. They're gonna be co-activated so that this muscle spindle will reflect what's happening in the rest of the muscle. We will switch gears just a little bit to now talk about muscle sensory receptors. You've probably guessed we have already talked about one of them. That's the muscle spindle. So an important sensory receptor for muscle is the muscle spindle. You can see how that afferent neuron that we were just talking about, it's coiling around the intrafusal muscles. It's gonna sense stretch. You can remember it because spindle starts with an s and so does stretch. That means that when your muscle is stretched that's when the neuron is firing the most. Then as the muscle shortens it's going to fire less and less. When it's firing a lot, there is a lot of stretch. It can cause a reflex that is leads to contraction of the stretched muscle. So this is going to be a negative feedback loop. Which means that if this muscle is over stretched, then to correct that state it's going to want to cause contraction. For instance, let's say of the bicep. Now, to do that, it's also going to need to worry about what's happening with the tricep because if you want to shorten this muscle, the bicep, then you're also going to need to relax the tricep. So the circuitry, and we'll see the specific circuitry in a minute, is going to cause what we call reciprocal innervation which means we will cause contraction of the stretched muscle, and then we need to cause the opposite action, relaxation, in the antagonistic muscle. So that would be the biceps versus the triceps. In this way that gamma motor neuron is going to activate the intrafusal fibers to cause them to shorten. Alongside the extrafusal fibers and the muscle spindle which is the sensory portion, wich acts in this negative feedback loop to keep the muscle length at a desired value. And we'll see how that happens more in a minute. Our other muscle sensory receptor is called the golgi tendon organ. This is located in the tendon that attaches the muscle to the bone. This is going to respond to a different type of stimulus. We said the muscle's spindle is going to be activated when you stretch the muscle. When you stretch a muscle, the Golgi tendon organ or the tendon itself is not going to be stretched that much, because when you stretch a muscle the muscle is gonna give. So there's not that much tension put on the tendon. The Golgi tendon organ fires very infrequently, when the muscle's at rest, when it's stretched, when the muscle is maximally stretched, the Golgi tendon organ might fire a little bit more. It really fires and remember these are gonna be mecanorreceptors in these sensors, this afferent neuron will respond to tension in the the tendon. When that golgi tendon organ is really going to fire is when your maximally contracting your muscle, because that's when that tendon is pulling against the load. It's kinda caught between the contracting muscle and the load. That's when the golgi tendon organ is going to just fire action potentials like crazy. So that's why we say the Golgi tendon organ is responding to tension in the muscle, not stretch. I always remember it because tendon, of the Golgi tendon organ, starts with a T, just like tension. So, the Golgi tendon organ is sensing tension generated by the muscle, which is basically going to be when it's contracting. Tere is an afferent nerve ending located at the junction of the muscle and the tendon. We're going to have an afferent axon. It's going to be a mechanorecepter stretch-activated ion channels in this Golgi tendon organ. That means that since golgi tendon organ is activated by contraction, then they're going to act to negatively feedback to regulate tension. So that Golgi tendon organ is really firing when you're lifting a really heavy load. It's going to act to reduce contraction, to relax the contracting muscle. Again, it's going to have to act with reciprocal innervation to cause contraction of the antagonistic muscle. It's going to act to fix the problem, to use negative feedback, but it's going to act to relax the muscle that is contracting. We'll see a more concrete example of this in a moment. Let's talk about reflexes. Where we're going to have some sort of sensory input, then the central nervous system is going to decide what to do about that input. And then it's going to cause a response that's going to be controlled by an efferent pathway like the somatic nervous system. We're going to talk about first, the muscle stretch reflex. An example of this would be the knee jerk reflex, when the doctor might use a little hammer as you're dangling your leg over the examining table. When he does that, he is striking the patella tendon under your kneecap, which is then tugging on the kneecap, and then on the quadricep muscle. It's basically causing the quadricep muscle to stretch really quickly. That's going to activate the muscle spindle. We know that when we activate the muscle spindle, that's going to cause contraction of the muscle and relaxation of the antagonistic muscle. That's what we shown here. So, we've activated the spindle, that's going to cause contraction of the quadricep muscle and relaxation of the hamstring muscle. And we're going to do that through the action of an inhibitory interneuron that's sitting between the muscle spindle neuron, afferent neuron, and the neuron for the hamstring. So we have an inhibitory interneuron. It's going to release a neurotransmitter, such as GABA, or glycine, very commonly, in a synapse with the somatic neuron for the hamstring, and those neuron transmitters often cause opening of chloride channels, which is going to cause chloride to rush in to the somatic motor neuron controlling the hamstring. If chloride rushes in, then negative charges rush in and those channel stay open. It's going to be much harder to cause an action potential in that neuron. If you have a reduction in action potential frequency, that's basically relaxation of a muscle. So again, our language here is action potentials. When you want a muscle to contract more, you have a higher frequency of action potentials. If you want it to relax, then you greatly reduce the frequency of action potentials. So when we're talking about antagonistic control, or reciprocal innervation, we're causing one neuron to fire more rapidly to cause one muscle to contract, and then we're causing the other neuron leading to the antagonistic muscle to fire less frequently, which is going to cause relaxation. That's what's happening here. In this reflex where we have activation of the muscle spindle, activation of the quadracep muscle, and then activation of the inhibitory interneuron, which prevents firing of the neuron that is leading to the hamstring. Now obviously the vast majority of our movements are not completely reflex. One thing to note with this is, that this reflex that we're talking about really involves only the spinal cord. These are the neurons, and this is only about involving the spinal cord. However, we can use reflexes like this by influencing them from up above, from the brain to do more everyday tasks. So an example of using this muscle stretch reflex would be if you're holding a glass for someone to pour a drink into. Your brain knows that if you're holding this empty glass, and someone's about to pour a drink, you need to keep your arm level, so that when they start to pour the drink you don't just dump it because of the extra load from the drink. And so what the brain can do is say, okay, this muscle is right where it needs to be. I don't want it to get any longer. I don't want it to stretch any more. So, it changes the sensitivity of the muscle spindle so that now, it's not only going to act just if it's completely stretched in a dramatic way, it's going to act and be super-sensitive manner. It's going to cause a reflex to counter if this muscle gets longer at all. And so what will happen is that you've got this new amount of sensitivity set into the system from the brain, affecting the same circuit we've just been talking about. When someone starts to pour a drink, the load gets greater because this is heavier, your arm dips just a little bit. But that's when that muscle spindle says no I don't want to stretch. It causes this same reflex that we just saw with the knee jerk reflex, where it's going to cause contraction of the biceps and relaxation of the triceps to keep this drink level, keep your arm level so you don't spill the drink. So in this way, the basic circuitry is the same as this completely reflexive action of the knee jerk reflex, but we are able to modify That system, by the brain, through the brain, and use it for a more everyday purpose. So let's talk about the Golgi tendon reflex, that's also gonna cause a reflex. Well we said, remember, the Golgi tendon's gonna be really activated when the muscle is really contracted and really pulling dramatically on that tendon. So that means we've got the signal coming from the Golgi tendon organ, coming into the spinal cord. Its cell body's going be sitting here in the dorsal root ganglion of the spinal cord. It's going to act on an excitatory inner neuron in the tricep, to then cause excitation of the somatic motor neuron leading to the tricep to cause contraction of the tricep. And then it's gonna act to activate an inhibitory neuron which will then inhibit the neuron leading to the bicep to cause it to relax. So, it's again, trying to fix the problem of too much tension in the muscle. It's going to do that by relaxing this muscle and contracting the opposing muscle. It's going to act with reciprocal innervation. Again this is going to act to limit the tension within the muscle so that we can protect the muscle and the muscle tendon junction so that they don't damage or just rip apart. We can also have the influence of a stimulus that is not in the muscle itself, or not in the tendon itself. It could be, for instance, a pain stimulus coming from the skin which can also cause a reflexive action that involves the somatic motor system. This is an example called the withdrawal reflex. In this case someone has just stepped on a jack and as a result the reflex acts to withdraw the foot. That means that the quadricep is going to relax and the hamstring is going to contract to do that withdrawal. Now if you withdrew your foot, and didn't make an adjustment in the opposing foot, you would just fall right over. And so this withdrawal reflex not only involves withdrawal of the limb but it also involves the crossed extensor reflex, which causes the opposite action in the opposite leg. Where instead of withdrawing the other leg you extend it, which allows you to support all of your weight on the foot that's not experiencing the pain. This gets even more complicated, because not only do we have reciprocal innervation in each leg, but we also have opposite actions in each leg. That's what this animation is showing. Where now we're activate withdrawal of this leg and extend this opposite leg by activating an afferent pathway this time from a pain receptor. So we're getting reciprocal innervation of each leg and causing the opposite effect in one leg versus the other, to withdraw one leg and extend the other leg. Again, this sort of reflex can be modulated by the brain. If you pick up a hot plate, your reflex is going to be to withdraw your hands. However, if that plate is really precious china or it's got your favorite meal on it, you might be able to tell yourself okay I know it's hot but hold on. If it's too hot you might not be able to do that. But you do have some control in some instances over your reflexes. Again it's just the brain influencing these very circuits that we're looking at. That's going to move us to talking a little bit about central pattern generators, which are these spinal cord-centered circuits. They are responsible for a lot of our repetitive movements that happen in everyday life. These are neural circuits that are coordinate complex patterns of movement. They are independent of sensory input to some extent, but they can be adjusted in response to some sensory feedback. I'll give you an example. Because these are repetitive movements that means that these circuits that are generated are going to be oscillatory. They are going to be flexible due to control by the brain. So let's talk about one of the easiest things to see an example of this would be walking, where you've got two different actions that you're gonna do. Two main actions are involved. One is that you're going to have the swing phase when you walk, which is going to be whenever your foot is off the ground. You're swinging it from being behind you to in front of you. That's the swing phase. Then the stance phase is when you've just stepped on the foot, and then you push to move your body forward. That's called the stance phase. So each leg is going to alternate between being in the swing phase and the stance phase. Again, you're gonna have to have reciprocal innervation of the different muscles to allow for, basically, withdrawal and extension. But when your right leg is in the swing phase, your left leg will be in the stance phase. And so again, like with the withdrawal reflex you're gonna have reciprocal innervation of the limbs as well. It's actually a somewhat simple, or very repetitive motion where you're just repeating the same sets of muscle contraction. But you need to make sure that the left leg is doing the opposite of the right leg at all times. This can be established and based in the spinal cord through these central pattern generators. They just generate this pattern of neural activity that causes you to walk. This can be adjusted by the brain, and that's adjusted in response to sensory feedback and influenced by the brain. An example of that would be if you want to go from walking to running. It's the same general pattern. It's just you're gonna reduce the stance phase and that will allow you to run. So, it's just a little modulation of the main central pattern generator that allows you to switch to running. It's efficient because you get to use the same basic circuits and just modify them to do a different activity such as running versus walking. This has also been pretty well studied in quadrupeds, in animals that have four legs. When, for instance, a horse goes from a trot to a walk to a gallop, they still have a similar motion of basically a swing versus a stance phase with their leg. But the pattern of which leg is in which phase changes depending on their gait. Again it's a modification of one of these central pattern generators that allows for that different pattern of movement in different types of gaits in a quadruped animal. So we've talked about our sensory receptors in muscle that are going to regulate muscle length and velocity of shortening and tension in the muscle. Remember how we can use these in terms of just being able to monitor what's going on with the muscle, in terms of proprioception. But then also in using it as a reflex to protect the muscle or to do everyday activities as well. Then, we do have some reflex actions that are involved with any sort of sensory information. We talked about sensory information coming from muscle, tendons, or skin and causing a movement in an appropriate muscle. And then we talked about central pattern generators and how they can be important in everyday repetitive movement, such as walking, and how it's gonna involve having antagonistic muscles being reciprocally innervated as well as reciprocal innervation of certain limbs as well. So central pattern generators will be involved in many everyday processes that are repetitive in nature.
