17.2 Alpha Gamma Co Activation

[@starbase606]

I transcribed this video. Let’s consider a typical muscle. It has extrafusal fibers, and then it has these muscle spindles that have interfusal fibers. Let’s consider that they are a resting length. And then, along comes a contraction, and the extrafusal fibers are going to shorten. If they shorten, and the interfusal fiber is not shortened, it’s going to become lax. It’s much longer than this. Now, what happens if a stretch comes along? If I now stretch this, the interfusal fiber couldn’t care less. It cannot signal that stretch. (1:00) That’s not good. The nervous system is not going to tolerate that kind of slop. What happens instead is that when… This (crossing out the lax fiber diagram) does not happen. Instead what we get is called Alpha Gamma Co-activation. That means nothing to you, but it is going to mean something to you in one second. What happens is that as the extrafusal fibers contract, so does the interfusal fiber. It contracts because the polar regions contract. If a stretch comes along now, this interfusal fiber is now going to be informative. It will be stretched and it will give us information. Great! How did we get that? We got that through Alpha-Gamma Co-activation. (2:00) Alpha and Gamma refer to two different types of motor neurons. Alpha motor neurons and gamma motor neurons. The Alpha motor neurons innervate extrafusal fibers. They’re the ones that we talked about. Those of the motor units. Then the gamma motor neurons innervate interfusal fibers in the same muscle. Ok? When I say that they innervate interfusal fibers, I mean that, if this is the interfusal fiber and these two red areas are the polar regions, and this is the equatorial region, if the red regions are contracted because of gamma motor neuron activation, then this equatorial region will be stretched. Ok? (3:00) So they will be contracted. So, that at all times information is given, the same information is given to alpha motor neurons and gamma motor neurons. What that does is it enables your feedback system, how much is my muscle being stretched by an external load. It enables that system to always be online. (3:29) What is the stimulus that is going to excite the stretch reflex? That is going to excite the 1a reflex. The stretch, the stimulus is a load. In other words, I am standing here, I’m holding a cup, and someone pours water into the cup. That is a load. If I don’t oppose that increased weight in the cup, my arm is going to drop, the water is going to pour out. I have to be able to sense that. I sense that through the lengthening of my muscles. (4:00) Another example where that happens is if you are walking along and your foot strikes something that is going to lengthen muscles that are involved in flexing your foot. The flexors will be lengthened and that will elicit/engage a stumbling reflex. So,Alpha Gamma co activation is the rule. (4:28) And now what we are going to consider is the possibility where we start with a gamma motor neuron activation. This is called the Gamma Loop. Let’s just imagine that a gamma motor neuron was zapped. This is excited (drawing arrow pointing to polar region of gamma muscle spindle). We are going to excite this cell. The result of that excitation is that the interfusal fiber, (5:00) the polar regions, are contracted. And that stretches the equatorial region. Now, if we have a 1a afferent that is wrapped up here, now that 1a afferent is stretched. The 1a afferent is now excited. That’s excited. What does that 1a afferent do? It comes into the spinal chord where it talks to a motor neuron. That motor neuron goes back out to innervate an extrafusal fiber in the same muscle. Now, what have you done? By stimulating the gamma motor neuron, you have ended up with a contraction, a physical contraction of that muscle. Alright? That’s the gamma loop. (6:00) I wouldn’t be talking about that if it didn’t occur. It does occur. It occurs primarily because, primarily from certain sources, the primary one that is responsible for exciting gamma motor neurons comes from the cerebellum, not directly, but indirectly. The cerebellum is very important in setting the gamma motor neuron tone. It can contract these. If you don’t’ have that, what would be the result? The result would be less contraction of the extrafusal fibers and less muscle tone. One of the consequences of cerebellar disorders is floppiness. (7:00) Floppy muscles, muscles that have a lower than typical tone. Why might we have a way for engaging this gamma loop? When you engage the gamma loop, you’re essentially sensitizing the stretch reflex. Let’s say that in normal circumstances, I need 5 grams of weight in order to respond to a stretch, I need a load of 5 grams to elicit stretch reflex. If I’m walking on a tight rope, I may need to be a little more sensitive to perturbations [A perturbation is a small change in the movement, quality, or behavior of something] in my muscles. If I’m walking on a tightrope, I want to be much more sensitive to, if my legs are starting to slip, (8:00) or if my foot starts to extend or flex because I’m falling off, I may want to be sensitive to 1 gram. This type of modulation allows me to change the sensitivity to the 1a reflex. In general, the 1a reflex is going to be more, there is going to be more sensitivity in situations of finely controlled motor units. One example, we are recording in the summertime, but in the wintertime in Chicago, you engage, you heighten your sensitivity of your 1a reflex all the time if you walk on ice. If we think about the gain of the 1a reflex, we can think about the output over the input. (9:00) This is the contraction over the stretch. [Text on screen: The gain of the stretch reflex is equal to the output contraction divided by the input stretch] If this were a perfect world, if the 1a reflex was absolutely 100% accurate, it would be 1.0, every stretch would be opposed by a perfect contraction that undid that stretch. The gain of the 1a reflex is nowhere near 1.0 under normal circumstances. But under specific circumstances it can be closer to that. The final point about stretch reflex is where is it the most important? It is most important in the thing that we terrestrial animals have to deal with all the time. All animals, marine animals have to deal with gravity, too. (10:00) But gravity is a much more problematic force in our lives than in the lives of aquatic animals. How do we deal with gravity? We use what are called: physiological extensors. Physiological extensors are muscles are muscles that oppose gravity. Typically you think about a flexor as something that increases the joint angle, and an extensor is one that increases joint angle. Physiological extensor, physiological flexor aren’t defined by joint angle. They are defined by what the do with respect to gravity. A physiological extensor opposes gravity. A physiological flexor works with gravity. In the arms, the biceps, which is a joint flexor is a physiological extensor. It opposes gravity. Another example is in the jaw. (11:00) The jaw closing closes the joint angle but it is opposing gravity so it is a physiological extensor. The 1a reflex is strongest in all of the physiological extensors. These, the muscles in your legs that keep you upright, the muscles in your back, muscles in your neck, and also jaw closers. So, when you are standing there at rest, your jaw is closed because you have this activity in your physiological extensors. That is going to be very sensitive to any stretch. You continue to oppose gravity well. In the next video, I am going to give you an assignment that is going to allow you to work through a series of clinical tests and understand what the outcome of those tests should be. KZbin 17.2 Alpha Gamma Co Activation kzbin.info/www/bejne/b4q3e6J3pphgaJo Medical Neurobiology Professor Peggy Mason, PhD, Department of Neurobiology, University of Chicago Transcribed by Eric Cooper

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[@shailkumarjain]

Thank you Sir. Could you explain if how gamma loop sensitivity develop ? My brain is ready for 5 gram load as example says, now i encounter a situation where i need to respond with in 1 gram load so this will happen automatically? How 1a efferent will be more sensitive ? The information is not yet stored that 1 gram should elicit response. Could you please throw some light ? Thank you..

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Another comment here because I had to watch this until the end because it was so interesting.... So you're saying, that the intrafusal fibre contraction at the polar regions is controlled task-specific? How is this done? Or... Where AND how is it done?

[@jwise7439]

Are their any pathologies where the intrafusal fiber stays slacked as extrafusal fibers contract (dysfunctional alpha gamma co-activation)?

[@ishtaiwemoftah6602]

No, its only hypothetical, because in reality, when the brain signals the alpha MNs to initiate muscle contraction, it sends parallel impulses to the gamma neurons to cause the intrafusal fibers to contract.

[@jwise7439]

@@ishtaiwemoftah6602 Thank you for responding. What would be the mechanism of a muscle spasm in regards to alpha gamma co-activation?

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So the gamma loop only occurs when the we need a more specific feedback? And its stimulated by the cerebellum?

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polar regions contract , it will be stretched and gives information alpha Neron motors infusal muscle fibres ..... contracted gamma Neron activation ..... different information is given ..... stimulus is low ....... increased weight may lengthen the muscles in flexing reflexes .....stumbling reflexes ... gamma motor neuron activation ..... excite the cell intra fusal region .... stretches the apron is excited ..... talks to motor neuron extradural muscle ...... visual communication contraction ....... primarily in sources .... cerebrum important in contracting ...less contraction ....and less tone ....... chief medical superintendent , railways , .......account p t usha ........railways , state bank of India ...........

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