Muskelanatomie - Aufbau des Muskels - Skelettmuskulatur im Detail - Aktin, Myosin & Z-Scheiben

Hello friends, my name is Famulus! :) This video is about muscle anatomy. We all know our skeletal muscles. This includes all the muscles that ensure our conscious movement. For example the biceps. But do we also know how muscle is built? Let's start with the biceps brachii muscle that we just mentioned, that is, the biceps. This consists of two muscle heads that are attached to the bone, in this example to the radius, by means of a tendon. In this illustration that I artistically drew, the bone, in this case the radius or, if you want to sound anatomically inclined, the radius, is on the far left.

The tendon connects to this between the radius on the left and the biceps on the right. This is followed by one of the two muscle heads of the biceps. And that's exactly where we continue, because of course it goes further. A muscle head can be divided into bundles of individual adjacent muscle fibers. They are between 0.1 and 1 mm thick, so slightly thicker than a single hair. Typically, a skeletal muscle consists of several bundles of muscle fibers and some connective tissue around them. The connective tissue is shown in white in my artwork and separates the individual muscle fiber bundles from each other, as I hope you can see here and there.

Connective tissue, specifically muscle connective tissue, also includes the so-called fascia, which is always a topic in itself. …But let's get to the muscle! The muscle fiber bundles just listed consist of individual muscle fibers or individual muscle cells. So, something like this. And like every cell, the muscle fiber also has a cell nucleus. Or several at the same time. It is not uncommon for individual muscle fibers to fuse together, which is why the term "cell" is no longer used for muscle fibers. A muscle fiber is between 1 mm and 15 cm long and has a diameter of between 40 and 200 µm and is therefore as thick as a very, very fine hair.

And in case anyone asks: the sarcolemma is the cell membrane of the muscle cell. The muscle fibers, in turn, are made up of myofibrils arranged in parallel, that is, thin sausages on the right side that can no longer be seen with the naked eye. According to current knowledge, myofibrils are also the muscle sections that grow after strength training and therefore lead to muscle hypertrophy, i.e. thickening of the skeletal muscle and thus muscle development. However, we are not there yet, because myofibrils can still divide into individual sarcomeres arranged one after the other. This is done because a striated pattern has been recognized under the electron microscope.

That is why skeletal muscles are also called striated muscles. The myofibrils show dark lines at regular intervals, caused by the so-called Z discs. The letter 'Z' is easy to remember because the Z discs have a zigzag pattern. Or you can simply refer to them as "intermediate" disks. But the real magic happens inside two Z discs. This is where our contractile proteins are found: actin and

myosin

. Both proteins have a thread-like structure, which can be seen quite clearly in the figure below, which is why they are also called filaments. Actin filaments are anchored to the Z discs and are shown in blue in this diagram.

In reality, this scheme is still simplified, since tropo

myosin

can be found around the actin filaments themselves, which is a fibrous structural protein that wraps around the actin filament. Troponin molecules prevent contact between actin and myosin in the resting state. But we'll talk more about this in the video about the muscle contraction itself: Instead, the myosin filaments, shown here in red, at first appear as if they have become independent. They are often drawn floating above or below actin filaments. However, they are attached to relatively short but elastic titin filaments. This protein allows the myosin heads, i.e. these red circles, to move more freely, which is ultimately of great importance for muscle contraction, i.e. the contraction of skeletal muscles.

Oh, and the yellow dots on the Z disks represent the so-called protection protein CapZ, whose function, among other things, is to bind the actin filaments of the Z disks. And now comes a small classification of the sarcomere itself: it is important to know the following sections, which some scientists have declared useful, to be able to find your way around the sarcomere as quickly as possible. We already know about the Z disks. But the Z disks are not the only black lines in the sarcomere, because as you can see in this illustration, there is also a line in the middle.

This is also known as the M line, which simply means "middle line." Both the Z discs and the M line are hypothesized to serve to mechanically stabilize individual actin and myosin filaments. And then there is the H zone, I band and A band. But don't worry, we're getting close to the end! :D Just small

detail

s. The arrows show which part of the sarcomere belongs to the respective section. The H zone is an area completely free of actin in the resting state, which means that the H zone exclusively surrounds the myosin filaments and the M line. The I band, on the other hand, is the complete opposite.

Only the actin filaments and the Z disk are found here. And last but not least, the A band, which to some extent overlaps both areas and therefore contains actin and myosin filaments. Under the electron microscope, the A band is the dark section in the middle and the bright one is the I band. And what happens when the muscle contracts? Then you can clearly see that the I band and the H zone have shifted and the entire sarcomere darkens. ...oh and by the way: we already had the light and dark sections in the muscle fiber. And now we can also explain how transverse striation occurs.

The I band is isotropic, meaning it simply refracts light into polarized light. In contrast, the A band is anisotropic, meaning it refracts twice as much in polarized light. And here you have another great image from the book "Introduction to the science of training" by Hohmann, Lames and Letzelter, which I definitely recommend and that is why I linked it in the video description. *Groans* I don't know why, but if anyone is still watching up to this point, I'd like to thank you so much for your attention! Admittedly, this video was a bit superior in terms of content.

Therefore, I would be interested to know how much you liked this video. Please leave me a comment about it! Oh, by the way: did you know that skeletal muscles are made up of different types of muscle fibers? If not, you should definitely watch this video. If so, give me a like on my Facebook page! :D As always, I'm happy for the thumbs up, constructive criticism and every new subscriber! :)