Health and medicine
The muscle stretch reflex is a crucial nervous system response that protects muscles from rapid stretching. The reflex involves afferent and efferent parts, both of which are necessary for it to work. This reflex is the reason for the knee-jerk reaction when the doctor taps just below the kneecap. Created by Matthew Barry Jensen.
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- When I was at the doctor's office, they were very confused because I had absolutely no reflex to getting hit with the mallet, and I was wondering why there was no reaction? The doctor couldn't answer any of my questions.(12 votes)
- They can not give you a clear answer if you have only! hyporeflexia o areflexia. Usually, if you have a problem you should have more signs or symptoms associated with hyporeflexia. Another, reason the test was not done properly (you were not relaxed or physician did not stimulated the muscle spindle to cause the stretch reflex.(10 votes)
- The muscle spindle and the accompanying muscle stretch reflex seem like odd things to be present in our bodies. Why were these two selected for during our evolution? What exactly was happening to our ancestors so often that they needed to detect rapid muscle stretching and counteract it immediately, in order to stay alive?(8 votes)
- This is a great question, so I had to look up the answer. Here is one:
"Although the mechanism of the myotatic reflex is well known, the utility of this reflex
response in identifying deficits of posture and movement remains elusive. Although the role of the stretch reflex in adult gait remains speculative and has not been specifically
addressed in classical texts such as that of Inman et al.8, recent studies suggest that primary afferent reflex pathways may play a role in walking, running, and error correction
Link to the article: http://www.physther.org/content/70/3/188.full.pdf(4 votes)
- When I received my medical training, years ago, these reflexes were called Deep Tendon Reflexes, or DTR's. Is the term Deep Tendon Reflexes now considered obsolete, having been replaced by the term Muscle Stretch Reflexes?(8 votes)
- Can this method of reflexes also work in the face?(2 votes)
- The facial muscle are controlled by cranial nerves, meaning they come directly from the brain. They do have reflexes, however. For example, have you ever noticed that you will close your eye and move away from an object that is about to poke you? In the brain, there is something called the lateral geniculate nucleus (LGN). This acts as the "reflex" center for your eye. As the optic nerve sends signal to your occipital lobe, the signal passes through your LGN and is analyzed. The LGN can initiate closure of the eye lids and can tell your brain to make you move away from an object before you can even see it (before the signal even gets to your occipital lobe!) Cool, right?!(12 votes)
- Why do pain signals travel through small-diamter axons without myelin? Wouldn't you want it to be a rapid transmission to occur so you could respond even faster or would that overcompensate?(5 votes)
- thise great question, I think because the receptor for pain (nociceptor) need more intent stimulies than the propioceptor, if the pain signals travel through some-diamter axons with myelin it's may be make brain feel pain in every move our body.(4 votes)
- I find it interesting that for the leg, there needs to be one set of lower motor neurons that are excited, and then there is also an inhibitory neural circuit that ensures that the opposite muscles relax.
My question is: is this the case even for conscious movements too? For example, if I flex my bicep, is that conscious motor movement exciting the bicep AND inhibiting the tricep?
Thank you!(5 votes)
- I imagine that it is, since moving one muscle require the opposite muscle to relax. Think of it this way: if both the bicep and tricep are excited, then which way is the arm supposed to move? One of the muscles must be inhibited to prevent the arm from getting "locked" in position.(1 vote)
- If somebody's spinal cord was severed above both the afferent and efferent neurons for this reflex, would the reflex still happen because the connection between the neurons is not interrupted?(2 votes)
- This is a complex question. The neurons that carry afferent and efferent action potentials synapse with other neurons in the spinal cord at a particular vertebral level. In the case of the knee reflex it is lumbar 4,5,6. These other neurons carry the signals to and from our brains so we can feel that someone has tapped our knee with a reflex hammer and so our brain can diminish the motor reaction at the level of that reflex. If the spinal cord is severed below the level of the reflex neurons entering the cord, there would be no effect as the signal or action potentials are uninterrupted so the knee would kick out normally. If the damage was at the level of the spinal cord where the neurons synapse, then they would be damaged and there would be no reflex. The patient could not feel or voluntarily move the limb as nothing is going to or from the brain, so the limb is paralysed. Finally, and here is the complex part, if the cord is damaged above the region that the neurons enter and exit the spinal cord, say thoracic 12 and remember lumbar 4,5,6 is below that area, then the femoral nerve is not damaged, so there is a reflex, but it is a wild hyperreflex and the patient kicks out repeatedly in clonus. This hyperreflexia indicates there is no input from the brain. But since the afferent and efferent neurons are not damaged, we observe a hyper reflex. The action potentials are not going to the brain so again, the patient can not feel or voluntary move the limb. To see this, search youtube for Neuro exams and then search on the words hyperreflexia and clonus. To read more, try this,https://www.ncbi.nlm.nih.gov/books/NBK396/(3 votes)
- Is there really a way to improve reflexes in sports? What happens when that occurs?(0 votes)
- I was recently reading a book about the nervous system, and it talked about this. It used potty training as an example. At birth, babies use the bathroom whenever their bladder is sufficiently full. However, they are slowly trained to use the bathroom only when the social circumstances allow it. This requires use of higher level thinking (i.e. the cerebral cortex). I assume improving reflexes in activities such as sports works in the same way. When first beginning to train, precise movement and thinking is required. However, once an action is done over and over again, it slowly becomes a reflex, and higher thinking is no longer required. Using this knowledge, one would assume that turning or jumping or kicking faster than usual, over and over again, would cause that action to become integrated into the system (i.e. your body).(6 votes)
- In the video you say that hyporeflexia can be caused by abnormalities in either the afferent neuron (somatosensory) or the efferent neuron (motor nueron). I was wondering how a doctor would be able to differentiate between the two to determine the cause a diminished muscle stretch reflex.
I also was wondering if a doctor knew there was an abnormality in the efferent part of the reflex, how would they know if the abnormality was in the motor neuron or the muscle cells themselves?(2 votes)
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.