Biological basis of behavior: Nervous system
Muscle stretch reflex
Voiceover: In this video, I'm gonna talk about the muscle stretch reflex. The nervous system performs many reflexes. And a reflex is a response to a stimulus that doesn't require the involvement of consciousness. All reflexes have two parts. The first part is called the afferent part, afferent. And that involves bringing information about a stimulus in to the central nervous system. So there'll be some sort of receptor somewhere in the body that can detect the stimulus. And then there'll be some sort of neuron that brings that information in to the central nervous system. The other part of a reflex is called the efferent part of the reflex. Efferent, which carries information away from the central nervous system to cause a response somewhere in the periphery. So there will be some sort of neuron that'll carry that information away from the central nervous system out into the periphery to cause some sort of response. Now, some reflexes, like the muscle stretch reflex that I'm just about to describe happen on the same side so that the afferent part of the reflex brings information in from one side of the body. And the efferent part of the reflex brings information back to that same side of the body to cause the response. Other reflexes, particularly those up in the brain stem, have an afferent limb that comes in on one side, and then efferent responses that come out to both sides. So there's some variety for how the information travels in reflexes, depending on the reflex. One of the simplest reflexes that's a good example and that happens to be one of the most medically useful is called the muscle stretch reflex, muscle stretch reflex. If a skeletal muscle, like in this drawing, here's a skeletal muscle in the arm. If this is rapidly stretched, the muscle stretch relfex will cause it to contract very quickly after it stretched presumably as a protective response to prevent injury to a muscle from being stretched too rapidly. But let's go over the one that happens around the knee. And that's also called the knee-jerk. Because most of us are probably familiar with this one. Because a lot of us, when we're in our doctor's office, have had the experience where we're sitting in a chair. So here, I've drawn a person. And we're looking at their right side. And here's their trunk and their leg. And if you're in the clinic, often your doctor will take a little, rubber hammer. And they'll take that little, rubber hammer. And they'll hit you right below the kneecap. And to your surprise, when they hit you below the kneecap with the little, rubber hammer, your leg will often kick out without you telling your leg to kick out. There's this involuntary response of the leg kicking out to the stimulus of the rubber hammer hitting you just below the kneecap. So why does this happen? Well, the place that your doctor's hitting you at the little, rubber hammer is not actually in the kneecap, itself. But it's in the tendon that's just below the kneecap. So let me draw that here in orange. And that tendon hooks onto the bones in the lower leg. And connected to the kneecap on the other side from the tendon is a large group of muscles in the front of the thigh. And when your doctor hits you in that tendon, it actually stretches this large group of muscles. Because for just a moment, the little, rubber hammer bends this tendon, and that pulls on the kneecap like this. And that pulls on this muscle, and it stretches it. Now, it doesn't stretch it very far. But it does stretch it rapidly. And there are receptors in skeletal muscle that can detect muscle stretch. I'll just write a big "R" here to represent one of the receptors. And there are lots of these receptors spread out throughout all of the skeletal muscle in the body. And these receptors are called muscle spindles. Muscle spindles, and here's a drawing of a muscle spindle. So here's a skeletal muscle. And they've magnified this little receptor. And we won't go into the details. But there are these specialized, little fibers inside the muscle spindle that gets stretched when the rest of the muscle gets stretched. And then there are neuron axons that are wrapped around these special fibers that can detect that stretch of these fibers and send that information back into the central nervous system. So that these axons that are leaving the muscle spindle here will travel back through nerves of the peripheral nervous system. And then they'll enter either the spinal cord or the brain stem. And these are somatosensory neurons that tend to have their somas and ganglia close to the spinal cord or the brain stem. And since these are neurons carrying information into the central nervous system, we can call them afferent neurons. And they make up the afferent part of the muscle stretch reflex. Let me just write that out. That for the muscle stretch reflex, the afferent part or the somatosensory neurons, somatosensory neurons inside the central nervous system. Like here in the spinal cord, these somatosensory neurons carrying that muscle stretch information, that information about the stimulus, are gonna form an excitatory synapse. So let me just draw a little plus sign here to represent an excitatory synapse with another neuron, whose soma is in the central nervous system. And this neuron is gonna send an axon out through nerves of the peripheral nervous system back to the same muscle that was stretched. And it's gonna synapse on and excite skeletal muscle cells in that same muscle. Causing the muscle to contract, causing the response. And the neurons that synapse on and control skeletal muscle cells are lower motor neurons. Lower motor neurons. And for the muscle stretch reflex, the lower motor neurons make up the efferent part of the reflex that causes the response of contraction of the muscle that was stretched. In the video where we went over the motor unit, we talked about lower motor neuron signs that can appear with abnormalities of the lower motor neurons. And we talked about hyporeflexia, meaning a decrease in the muscle stretch reflexes. And I think you could see why that would happen. If the lower motor neuron is not able to communicate with the muscle, then it can't tell it to contract in response to the stimulus of muscle stretch. But it turns out that you can also have diminished muscle stretch reflexes if there's a problem with these somatosensory neurons, bringing information about the muscle stretch back to the lower motor neurons. So that if there's a problem with either the lower motor neurons or the somatosensory neurons, you can have a diminished muscle stretch reflex. And it turns out this is true for all reflexes. If there's a problem with either the afferent part of the reflex, bringing stimulus information into the central nervous system, or if there's a problem with the efferent part of the reflex carrying response information out to the periphery. A problem with either the afferent or efferent part of a reflex can cause a diminished or a lost reflex. Because both parts have to be working for the reflex to occur. Now, one important thing to notice about reflexes is how all of this just occurs down here. In this case, in the spinal cord. Or it could occur in the brain stem if it was a brain stem reflex. But the higher parts of the nervous system, the cerebrum, where a lot of the higher functions of the nervous system like cognition, emotion and consciousness, they don't have to get involved for a reflex like this to occur. And this is the reason we say reflexes are responses to stimuli that don't require the involvement of consciousness. Because the wiring tends to occur at these lower parts of the central nervous system and peripheral nervous system without having to involve the higher parts of the nervous system way up in the cerebrum. Now, this is all you need for the muscle stretch reflex. But there is another part to it that isn't necessary. But does add something, because it turns out that while this muscle is contracting, in response to the stretch of the muscle, the muscle on the opposite side, in this example, the muscle on the back of the thigh that bends the knee when it contracts. This muscle actually relaxes. So while the muscle on the front of the thigh is contracting, the muscle on the back is relaxing. The way this occurs is that this same somatosensory neuron that's exciting the lower motor neuron back to the muscle that was stretched is also sending axon terminals to other neurons. And it's gonna excite those neurons. So I'll draw a little plus sign. But these neurons are actually inhibitory neurons. So they're gonna form a synapse that's inhibitory. So I'll draw a little minus sign to represent that they're inhibitory. And what they inhibit are lower motor neurons to the muscles on the opposite side. So these lower motor neurons would normally be exciting the muscles here in the back of the thigh that would cause the knee to bend. But when they're being inhibited by this other neuron, these lower motor neurons aren't exciting that muscle in the back of the thigh, so it relaxes. Now, this isn't necessary for the reflex to occur. You just need the afferent and the efferent part of the reflex for it to occur. But because this muscle, when it's contracting, isn't fighting against this muscle in the back of the thigh since it's relaxing, that does increase the response. So there's more straightening of the leg at the knee and kicking outward. And a lot of the reflexes in the nervous system have some similarities to this sort of setup. Where you can almost think of a balance between responses that the nervous system can choose from. And that the reflex tips the balance in favor of a response in one direction.