Believe it or not, it takes a lot to allow even the simplest of muscle movements. In order to contract, many microscopic structures are uniquely suited to do a specific job. The epimysium is wrapped around the entire cell and is made of dense irregular tissue. Inside the epimysium is the perimysium, made of fibrous connective tissue. The perimysium wraps around fascicles- bundles of muscle fibers grouped together. Inside the muscle fibers is the sarcolemma, which serves as a sort of cell membrane. Directly inside the sarcolemma is the sarcoplasmic reticulum. The sarcoplasmic reticulum houses transverse tubules, or “T-tubules.” These allow chemical changes in polarity to quickly enter the cell. The sarcoplasmic reticulum also allows calcium to diffuse through it to the cytoplasm. The muscle fiber also contains mitochondria, which house ATP. ATP is vital for energy in the muscle. Also inside the sarcoplasm is glycosomes; granules of glycogen that provide glucose during muscular activity. It also has myoglobin, which stores red pigment which stores oxygen. Myosin is a rod like structure with a “head.” When actin bonds with this head, it creates a crossbridge.

The first step to creating a contraction is to go into labor. Haha just kidding, it’s  to generate an action potential. After this action potential, the electrochemical gradient is disrupted causing depolarization. This causes calcium to be released which then bonds to the troponin. The troponin then pulls the tropomyosin off of the actin sites. Once the actin sites are revealed, they bond to the myosin heads to create a crossbridge. Contraction then begins with repeated cycles of the crossbridge binding. You could think of contraction like shaking someone’s hand. The myosin and the actin are the two hands, and when they connect, one pulls the other (that’s the contraction). In order to shade hands both peoples fingers must be outstretched, not in a ball (just like the actin sites must be out, not covered by the tropomyosin). This is all powered by ATP (but so is everything else in our bodies.) After the contraction, the electrochemical gradient must be restored; repolarization. The sarcoplasmic reticulum now re-absorbs the calcium is released and the tropomyosin re-covers the actin sites. The body needs this time to recover so that it can collect enough ATP to keep contracting the muscle. You could think of this like shooting a gun. After each shot, you have to reload. Unless you are a semi-automatic machine gun. You ever seen one of those things shoot? They’re incredible. Alas, if humans were guns we would not be semi-automatic machine guns, we would be muskets. French Revolution muskets. Cause the French are classy. Overall, if muscles could not contract, then human’s would not be able to even perform simple movements. If someone is unable to move it is safe to say that homeostasis has been disrupted.

Visual image of the muscle contraction process


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