Muscle Contraction Mechanism — NEET Biology
📚 Practice ConceptMuscle fibre is the anatomical unit of muscle. Each muscle fibre has many parallelly arranged myofibrils. Each myofibril contains many serially arranged units called sarcomere which are the functional units. Each sarcomere has a central 'A' band made of thick myosin filaments, and two half 'I' bands made of thin actin filaments on either side of it marked by 'Z' lines. Actin and myosin are polymerised proteins with contractility. The active sites for myosin on resting actin filament are masked by a protein-troponin. Myosin head contains ATPase and has ATP binding sites and active sites for actin. A motor neuron carries signal to the muscle fibre which generates an action potential in it. This causes the release of Ca²⁺ from sarcoplasmic reticulum. Ca²⁺ activates actin. Myosin binds to the myosin head to form a cross bridge. These cross bridges pull the actin filaments causing them to slide over the myosin filaments and thereby causing contraction. Ca²⁺ are then returned to sarcoplasmic reticulum which inactivate the actin. Cross bridges are broken and the muscles relax.
Arrange the following steps of muscle contraction in the correct sequence: 1. Release of Ca²⁺ ions into the sarcoplasm from sarcoplasmic reticulum 2. Formation of cross-bridges between actin and myosin using ATP energy 3. Neural signal releases acetylcholine at neuromuscular junction 4. Troponin undergoes conformational change, exposing myosin-binding sites on actin 5. Sliding of actin filaments towards the centre of A band, shortening of sarcomere
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