Fascicle Arrangements - Video Tutorials & Practice Problems
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Fascicle Arrangements
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One way to classify skeletal muscles is by their fasical arrangements. But also just knowing the fasical arrangements of a particular muscle is gonna let you know something about how that muscle works. So let's recall that fascicles are groups of muscle fibers that are bound together by paray. And importantly, the muscle fibers run the entire length of a muscle fascicle in a skeletal muscle, there is never a muscle fiber that's joined end to end to another muscle fiber. So as we think about the fascicles and the muscle fibers, we're gonna have these two relationships that are pretty important. And that's gonna be that the length of the fibers in a rough sense are gonna be proportional to the length of the contraction for that muscle. Now, that makes intuitive sense, right? If you have a muscle with really long fibers, they can shorten a lot. So a muscle with really long fibers can get a lot shorter a muscle with short fibers. Well, they can't shorten as much in a total sense. So that muscle can't shorten as much. Our other relationship is going to be that the total number of fibers in the muscle is gonna be roughly proportional to the strength of the contraction. Now, that also makes sense, right? You got more muscle fibers, they can contract with more total force. You get a stronger contraction in your muscle. What we're gonna look at here is that how the fascicles are, are arranged in a muscle can be arranged in ways that either optimize the length of contraction or they optimize the strength of contraction. Now, to see how that works, we have a anat topical model here with some uh key muscles shown. And we have six different fasical arrangements that we're gonna go through and look at the muscles that have those arrangements. Now, first up is gonna be what probably the simplest one is the parallel fasical arrangement in the parallel arrangement. The fibers run the length of the muscle all in parallel. And our example here is the sartorius. And you can see that the sartorius is this long thin muscle and it's actually the longest muscle in your body. You can see it here, it starts at the hip, it sort of wraps around the thigh and inserts on the tibia. So because it's long and thin like that, well, the fibers run in parallel, the fascicles run in parallel, the fibers are really, really long. That means that this muscle can shorten a lot, but it doesn't have a lot of total fibers. So that means it can't contract with as much force as some other muscles all right, as sort of a subset of the parallel arrangement, we're gonna have the fusiform arrangement. And fusiform means that it's gonna be spindle shaped. And by spindle, we mean that it is wider in the middle than it is on the end. So here we have the biceps brachii, you can see the biceps rachi is kind of narrow on each end. It gets nice and fat in the middle though. So that's that spindle shape. And we've pulled that out, of course, from the upper arm there by getting a little wider in the middle. That's a way that the muscle can pack in a little bit more force while keeping this parallel arrangement. All right, our next arrangement is going to be the convergent fasical arrangement in a convergent arrangement. The origin of the muscle is going to be larger than the insertion. And our example for this is gonna be the pectoralis major. So that's this sort of major muscle of the chest here. So we have it pulled out and the right edge here, you can see it gets really wide. That's all the origin of this muscle and all the fascicles come in in one direction and they all basically run the whole length of the muscle and they converge at this small insertion point which for this muscle is on the humerus. So because you're getting so many fascicles running into a single point, that's a lot of muscle fibers, this can contract with a lot of force, but the fascicles are all still pretty long. So that also means it's got a pretty decent amount of contraction that it can do. So that seems like it gets the best of both worlds. But the drawback to a convergent muscle like this is that the origin has to be really big. It takes up a lot of space in the body to have a convergent muscle like that. All right, our next capac arrangement is gonna be the circular arrangement. Now, this is also sometimes just called a sphincter and a circular arrangement or a sphincter is something that wraps an opening and I'm sure that everybody is aware of the anal sphincter. But our example here is gonna be the orbicularis oculi. So you can see here this is a circular fasical arrangement and it surrounds your eye. So when you close your eye like that, what you're doing is you're squeezing down on the sphincter and that circular muscle is getting smaller and it closes that opening around your eye. All right, those first three fasical arrangements. In all cases, the fibers are the fibers and the fascicles are still basically running the entire length of the muscle that's gonna be different for our next three. So our next three are all versions of the pennate classical arrangement. And that word pennate comes from the Latin meaning feather. And you can see here we have a feather here. That's also the root for the word pen, right? People used to write with feathers as a pen. So pennate and pen come from the same Latin root, meaning feather. So what you see the structure of a feather though it has this shaft running down the middle and then the soft parts of the feather are actually kind of short running off the sides. So a pennate muscle does the same thing. So our first type of pennate muscle is just called a pennate muscle or sometimes called a unipennate muscle. And these ones, the short fibers run at an angle from one side of a tendon. So our example here is the extensor digitorum longus, that's a muscle down here in the calf. But you can see as we pulled it out here, this tendon runs along the side of the muscle and the muscle fibers are actually kind of short, they're running off at an angle. Now, why would you do that? Well, short muscle fibers means that it can't contract much in sort of total length, but you're able to pack in a lot more fibers. So you're able to contract with more force again at the expense of the total length of contraction. All right, that's pennant or unipennate where it comes off of one side of the tendon. Well, our next one is going to be bi pennant. So you might be able to see where this is going uni to buy fibers here extend from both sides of a central tendon. And our example here is the rectus femoris. You can see this is that major muscle of the quadricep running right down the middle of the thigh there. And you can see here there's a tendon running down the middle of the muscle. And these short fibers, relatively short fibers running off the angles off of both sides of that tendon. Again, short fibers means that this muscle can't contract a lot in terms of total length, but we fit more fibers in. So it's able to contract with more force. All right, our final one multi pennant. Again, you might be able to see where this is going with that prefix multi, this one, the fibers extend from a branched tendon. And our example here is going to be the deltoid muscle. So you can see this muscle here. It's sort of one of those major muscles of the shoulder, it runs over the shoulder there. And you can see here how there are multiple tendons running through this muscle and multiple tendons all having these short fibers coming off of it at an angle. Again, short fibers, we're sacrificing the length of the total contraction. But we're fitting in a lot more fibers, it's able to contract with a lot more strength. Ok. Again, remembering these for different muscles can again, sometimes just feel like another thing on the list that you need to memorize. But if you understand how these work it's going to tell you something about the muscle. Like, always we have examples to follow and practice problems after that. I'll see you there.
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example
Fascicle Arrangements Example 1
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2m
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Our example tells us that below is an image of the muscles of the back in a cadaver. And it says that the infraspinatus is highlighted. So let's take a look. This is gonna be the infraspinatus and the infraspinatus is a muscle that runs runs along the scapula. The origin is along this medial border of the scapula there and the insertion is on the proximal end of the humerus up right near the shoulder. All right. So it says based on the image, what type of vascular arrangement is the infraspinatus appear to have? Well, as I look at this, the first thing I wanna rule out is whether or not it's a pennate muscle. So remember, pennate muscles have a tendon that runs either along the side or down the middle with short muscle fibers coming off of it. So as I look here, I don't see anything like that. There's no tendon along the side, there's no tendon up the middle with short muscle fibers coming off at an angle. So I can rule that out that leaves me with parallel convergent or circular. Well, it ain't circular. So when I look at this, I look at the origin. And you can see this is a pretty big origin and the insertion is pretty small, that shape is indicative of a convergent bicycle arrangement. And the question it asks next is, is this fasical arrangement optimized for fourth generation or for range of movement? Well, remember fourth generation, we get that from having a lot of muscle fibers, we get range of movement by having long muscle fibers. So which is true for a convergent muscle. Well, the fact that the origin is so big and it all converges to the small point means that you have a lot of muscle fibers coming into the insertion that gives it a fair amount of force. But these muscle fibers and these fascicles all basically run the entire length of the muscle, long fibers give you greater range of motion. So, is this optimized for fourth generation or range of movement? I'm gonna say both a convergent muscle has a lot of fibers that are long, that's good for power and movement. The drawback, they take up a lot of space because this origin needs to be so big. All right, more practice problems to follow. I'll see you there.
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Problem
Problem
Which fascicle arrangement is most well suited to a muscle that needs to generate a lot of force over a small distance?
A
Unipennate.
B
Multipennate.
C
Circular.
D
Fusiform.
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Problem
Problem
Four muscles are shown below. Based on their fascicle arrangement, which muscle doesn't belong in the group?
A
Rectus Abdominis.
B
Interossei.
C
Bicep Brachii.
D
Sternocleidomastoid.
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