The muscle is shaped like a really strong thread. Thousands of tiny, coherent fibers called myofibrils twist to form muscle fibers, which are your actual muscle cells - cells with mitochondria, many nuclei, and a cell membrane called the sarcolemma. Those muscle fibers then become larger bundles, such as tendons, that connect together to form a large, flexible body organ, such as your biceps brachii. Overall, this bundle-of-bundle suspension strengthens muscle tissue. But when you consider how much damage your muscles are causing when you do something very simple, such as lifting a large bag of dog food, it is not surprising that they need little help. That is why every muscle contains a few different types of supporting tissues that support connective tissue - protective straps to keep the spinal cord from rupturing. So that's part of the story structure. But if you want to get into the nitty-gritty-- down-and-dirty - of how you actually go, well, there are rules.
Indeed, only two basic laws, and they are related to protein. And both are true of many of the proteins we are talking about, whether they are enzymes or ion channels or receptors or muscle proteins. And these rules are: One. Proteins tend to change the state in which things bind to them. And two. Changing the shape can allow proteins to bind - or loosen - and other substances. So keep those rules in mind, while we see that muscle fiber contracts and relaxes. Now, remember those tiny myofibrils that accumulate and build up your muscle fibers? They are divided into segments called sarcomeres, consisting of two thin strands of protein - two types of myofilaments called actin and myosin. And it's their annoying star-studded love story that encourages all the body movements you may have dreamed of. The sarcomere consists of both thin strands, which are composed mainly of two strands of light and twisted actin, and thick strands, composed of dense fibers, resembling a myosin.
Each sarcomere is separated by a so-called Z-line at each end, which is simply a border made of tiny alternating threads in the form of a zig zag pattern. Muscle contraction is about the withdrawal of the sarcomere, which brings those Z-lines together. Okay, so now comes the romance. When your muscle cells are at rest, your actin and myosin fibers do not touch, but in reality, they really need. Specifically, that club-owned myosin wants to be close-personal with actin. When this happens - and it will, in the end - is called a smooth filament model of muscle contraction. But in the meantime, as with any good romance, the couple have some obstacles to overcome. That is, actin is blocked by a number of protein regulators - called tropomyosin and troponin - who continue to get in the way. Fortunately, these guards can be bought with low ATP and calcium.
I prefer cash and nayo, but whatever. Remember, ATP is like a cellular type. It contains chemical energy, and your muscles are about to convert the energy of moving chemicals, so they are always hungry for extra ATP. Your muscle cells have many nuclei, but some of them have many mitochondria, whose only purpose in life is to release ATP. And muscle cells also have their own version of the endoplasmic reticulum - a system for transporting and storing cells - but in this case they are special, so they get a special name: the sarcoplasmic reticulum. Its walls are loaded with calcium pumps - which use ATP to conserve large amounts of calcium ions. It is also packed with calcium channels that are linked to energy-sensitive proteins in the cell membrane. It says I want to move my arm. My brain sends energy to do something in a motor neuron until it meets a muscle cell in my arm.
The receptors in that muscle cell are ligand-gated sodium channels, so when the motor neuron releases our old friend acetylcholine at the synapse, the channels open up, and make sodium flow in the cell as potential, i.e. , when strong enough, it causes nearby voltage-gated sodium channels to open. Now, I want to take a moment and point out here that we are still talking about action skills, but not the neuron. This happens in the muscle cell, people. So that action might close to the cell membrane of the muscle, the sarcolemma, which has many tubes running deep inside the cell, called T-Tubules. When the force of action slows down one of these tubes, it eventually produces energy-sensitive proteins that attach to those calcium channels in the sarcoplasmic reticulum of the cell. When those channels open, the calcium stored inside it runs all over the cell, and finally myosin is like, YES! There they are! At this point, the myosin is completely stable, because the guards who were frustrated are facing a
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