Muscles consist of fibers that are called myofibrils that are made up of many repeating units called sarcomeres. Each sarcomere is composed of two integral protein filaments: actin and myosin. These proteins are responsible for muscle contraction. The thick filaments are called the myosin while the thin filaments are called the actin. Around two chains of actin, a fibrous protein called tropomyosin is wrapped around them, and another protein called troponin in binded to the actin chains at regular intervals. Tropomyosin is binded to the myosin binding site when muscles are not contracted, so it does not bind to actin. The process of muscle contraction is explained by the sliding filament theory below, in which when the muscle contracts, sarcomeres are shortened when discs are pulled closer together. These proteins slide over each other which need ATP from the respiration.
1. Firstly, when an action potential arrives in a muscle fibre, signals of depolarization are passed down the T-tubules.
2. Then, the sarcoplasmic reticulum is stimulated, and the release of Ca+2 happens.
3. These calcium ions will then bind to troponin and alter its conformation and will pull away tropomyosin from the -myosin binding site on the actin.
4. Then, at this empty site, myosin can bind to actin and will form a cross bridge.
5. Now, the myosin tilts and moves, pulling action towards the centre of sarcomere. Hence, the length of sarcomere decreases.
6. When an ATP molecule attaches to the myosin head and it is hydrolysed, the cross bridge breaks. This process repeats as myosin will bind to another binding site on actin closer to centromere, and will become fully contracted. This continues till the time there is enough ATP and the binding sites are not blocked by troponin and tropomyosin.