Sarcomere – within myofibrils are smaller structures called filaments: Thick and thin. These filaments are directly involved in the contractile process. There are two thin filaments for every thick filament in the regions of filament overlap. They do not extend the entire length of the muscle fibre, instead they are arranged in compartments called Sarcomeres which are the basic functional unit of a myofibril.
Thin filaments: contain Actin, Troponin, and Tropomyosin.
Thick filament: contains about 300 Myosin molecules . The myosin tails form the shaft of the thick filament and the heads project outward toward the surrounding thin filaments. Thick and thin filaments overlap one another to a greater or lesser extent depending on whether the muscle is contracted, relaxed, or stretched. This overlap pattern creates the striations that are seen in muscle. Z-discs: narrow, plate-shaped regions of dense protein material that separate one sacromere from the next. Therefore, a sarcomere extends from one Z-disc to the next Z-disc. A-band: the darker middle part of the sacromere which extends the entire length of the thick filament. At the end regions of the A-band are the “zones of overlap” where thick and thin filaments lie side by side. I-band: a lighter, less dense area that contains the rest of the thin filaments but NO thick filaments. A Z-disc passes through the centre of each I-band.
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