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CLIL SCIENCE Muscle tissue and muscle contraction Rosalie Crawford - Veronica Revel Fondazione Liceo Linguistico Courmayeur
How does it happen? ? How do our muscles work? ?
Skeletal, cardiac and smooth muscle
Skeletal muscle fibre
The structure of skeletal muscle • A group of myofibrils (from myo, the prefix for muscle), each of which is surrounded by the specialized endoplasmic reticulum of muscle cells, the sarcoplasmic reticulum. • The sacs of the sarcoplasmic reticulum contain calcium ions (Ca 2+) , which, when released, trigger muscle contraction. • Myofibrils are composed of units called sarcomeres, which have two types of filaments running parallel. • The sarcomere is composed of two types of filaments running parallel to one another. The thicker filaments are composed of the protein myosin while thinner ones are primarily actin.
What causes a muscle contraction? • Muscle contractions occur as the sarcomeres present in muscle fibers shorten. When muscle fibres are triggered by neuromuscular junctions, a process begins in which thick and thin filaments overlap causing the sarcomeres to shorten over a brief period of time. The thick filaments are composed of about 200 mysosin fibres that look similar to golf clubs. Along the long filament, the myosin proteins line up in opposite directions with their “heads” protruding out from the filament. The thin filaments, composed of actin, tropomyosin, and troponin, form a spiral helix with myosin binding sites on the actin.
The cycle of muscle contraction
Muscle contraction • When a muscle contraction is triggered, • 1) calcium ions and energy-supplying ADP cause rearrangement of the thin filaments, • 2) the myosin heads bind to the actin, • 3) the myosin heads pull the thin filaments towards the center of the sarcomere. • Once contraction is complete, ATP binds to myosin, the myosin-actin bond is broken, and the myosin head “re-cocks” to the starting position until the cycle repeats. This cycle occurs simultaneously in many muscle fibers present in the entire muscle tissue causing contractions in the muscles of your eye, to the movement of your arms and legs.
Molecular mechanism of muscular contraction
Isometric & isotonic contractions • Skeletal muscles undergo two types of contractions: isometric and isotonic. Because isometric contractions occur when the muscle does not shorten, as when pushing against a wall, no work is done and no power is produced. In isometric contractions, myofibrils slide over each other causing the sarcomeres to shorten and external work is performed.