FORCED TO WORK...
The sarcomere is the smallest contractile unit of the striated muscle. Contractile elements are capable of developing or producing force also addressed as strength. Since "strength" as a muscular fitness component, I will use the term force rather than strength while addressing the product of sarcomere work.
The sarcomere is more complex than what I will present in this blog post, since it is my intent to simplify the sarcomere as much as possible for an easy read. That said, I will make sure to touch upon important functional pointers.
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The sarcomere (simplified) includes mainly three components called myosin, actin, and tropomyosin. The sliding filaments theory describe sarcomere force production as the outcome of relative movement between myosin and actin, while the myosin is said to slide back and forth compared to the actin.
Much prior to that, several processes and reactions must occur to cause the sarcomere to produce force. During the non-contraction phase of the sarcomere's force production cycle, myosin and actin are physically disconnected. Physical connection between the two is crucial to the force production of a sarcomere. The actin filament has areas that act similar to "handles" called binding sites. These binding sites are where myosin can physically to actin to create force.
At first, these binding sites are not physically positioned is a way that would allow myosin to connect with the actin. Tropomyosin is covering (physically) the binding sites of the actin filament, physically preventing any chances of myosin and actin connecting.
Thus, tropomyosin needs to be moved enough to expose the actin binding sites by changing its 3D conformation (how it is arranged within a 3D space). To achieve this, calcium is released into the sarcomere and binds to the tropomyosin, causing it to rotate just enough to expose the actin binding sites.
The influence of calcium on tropomyosin is physical, not chemical; Thus, it is said the calcium acts as a biophysical weight using its weight as an element to cause physical imbalance to the tropomyosin filament, making it rotate away from the binding site. Now the binding site are available for possible binding with myosin.
Simplifying myosin, it has a tail and a round (globular) head that acts like an "arm". In order for myosin and actin to bind, the arm of the myosin (globular end) must be extended and connect with the actin at its binding site, creating a physical connection termed a "cross bridge". The globular arm creates a physical connection between the two, resembling a bridge crossing from the myosin to the actin.
The next mandatory process explains why muscle work, sarcomeric work increases energy investments. Basically, the sarcomere, and for this reason the muscle, will not conduct any work (will not produce force) if not paid first. The muscle has the harshest payment demands there are: 1) Must be paid in "cash"; 2) Must be paid in full; 3) Must be paid before any work is done. No tab can be opened, no "I will pay you later", no payments in installments, no credit cards allowed, no checks, and no loans.
Thus, payment in the form of an ATP molecule (adenosine tri-phosphate) must be "paid" to the arm of the myosin filament. Nearby the globular head of the myosin are, there is a catalytic site that specializes in hydrolyzing ATP and extracted its energy as payment. In the hydrolization of ATP, a water molecule is used to split ATP into ADP and Pi, releasing energy (this is commonly the case in catabolic reactions meant to split or "breakdown" molecules).
Upon energy "payment" (ATP) the myosin arm extends towards an actin binding site and creates a physical connection (cross bridge). The energy payment "covers" the extension of the arm, creating the cross bridge, and the next process where the arm moves forward and backwards, creating a sliding-like motion. This sliding-like motion is the result of relative motion between the myosin and actin.
The relative movement of these two filaments produces the force of a sarcomere. As long as the filaments are "sliding" the sarcomere is producing force. Once the energy payment has been "used up", the filaments no longer slide and the sarcomere is ready to go back to its initial state of non-contraction (non-force production). Additional energy payment is required for the myosin to let go of the actin and return to its original state. Thus, upon a second energy payment in the form of ATP hydrolysis, myosin disconnects from actin and retracts back towards its shaft/tail.
At this point, calcium disconnects causing tropomyosin to rotate back and recover the binding sites of actin, eliminating any option of force production of the sarcomere. A single muscle cell will have about 100,000 sarcomeres creating force. Thus, a muscle's total force developed is the accumulation of the forces developed in each sarcomere. The more contracting sarcomeres in a muscle, the greater the potential force production.
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Simply put, hundreds of sarcomeres per single muscle cell contracting and demanding 2 ATP molecules for every force production cycle, multiplied by the number of such cycles, could amount to an extensive energy consumption, explaining the energetic costs of existence, physical activity, exercise, and sports.
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