PHYSIOLOGY OF MUSCLE CONTRACTION

Muscle movement plays an important role in day to day life where the contraction and relaxation of muscle is significant. The current slide has been developed with the focus on different phases during muscle contraction and the physiological change involved on it.

1. PHYSIOLOGY OF MUSCLE CONTRACTION BY : ANAND SAGAR TIWARI

2. Basic concepts The muscle which looks with alternate light and dark band under microscope, found on the skeletal bone and work in a voluntary manner is known as skeletal muscle. There are 656 skeletal muscles altogether. The fine transparent tubular sheath which envelops the fibres of skeletal muscles is known as Sarcolemma.

3. Contd… Each single muscle fiber remains surrounded by a connective tissue layer of reticular fibres known as endomysium . Several such endomysium remain surrounded by perimysium and several such perimysium in turn remain surrounded with epimysium . Cytoplasm of striated muscle cells is known as sarcoplasm.

4. Contd Sarcomere is a contractile unit in a striated muscle fiber. Myofibril: [Myo means muscle and fibrilla means little fiber]. The thread like structure extending longitudinally within the muscle fibres consisting mainly of thick filaments(myosin) and thin filaments (actin) is called myofibril.

5. Contd… Examined under microscope alternate light band (I- band) and dark band (A-band) is observed. I band is bisected by a Z-line. A band present in the centre is called H-Zone. The thin and thick filaments are called as myofilaments.

6. Importance of muscle movement Movement is the basic property of living systems. Animals move by contracting muscles. Muscle contraction is one of the key processes of animal life. Movement of muscle is the prerequisite for vital activities like digestion, reproduction, excretion and circulation.

7. Cori’s Lactic acid cycle The Cori cycle is also known as Lactic acid cycle named after its discoverers Carl Ferdinand Cori and Gerty Cori. In this metabolic pathway lactate produced by anaerobic glycolysis in the muscles move to the liver. In liver by gluconeogenesis lactate gets converted to glucose. This again returns to the muscle and gets metabolized.

8. Molecular changes during muscle contraction Binding Ca 2+ ions to troponin. Troponin- Ca 2+ complex removes tropomyosin blockage of actin sites. Heads of myosin- ATP complex from cross-bridges to actin filament. Hydrolysis of ATP induces conformational changes in the heads of myosin. 1 mole of Actin + 3 moles of myosin– Actomyosin.

9. Molecular changes during muscle relaxation Ca 2+ ions sequestered from actin filament by sarcoplasmic reticulum. Calcium ion return to the sarcoplasmic reticulum. Calcium released from troponin- calcium ion complex. Troponin permits tropomyosin return to blocking position. Myosin-action cross-bridges break. ATP-myosin complex reformed in heads of thick filament.

10. Major events of excitation A nerve impulse reaches the end of motor neuron, release of acetylcholine takes place. Diffuse rapidly across the gap of neuromuscular junction. Ach(Acetylcholine) binds to Ach receptor on the motor end plate of the muscle fiber. Stimulation Ach receptors initiates an impulse that travels along the sarcolemma , through the T –tubules and to the sacs of sarcoplasmic reticulum.

11. Contd… Calcium ion is then released into sarcoplasm when it binds to troponin molecules in the thin myofilaments. Tropomyosin molecules in the thin myofilaments shift exposing actin’s active sites. Myosin cross bridges of the thick myofilaments bind to actin and use their energy to pull the thin myofilaments toward the centre of each sarcomere.

12. Contd… This cycle repeats itself many times/sec, as long as ATP is available. As the thin filaments slide part the thick filaments, the entire muscle fiber shortens. After the impulse is over, the SR begin active pumping of calcium ion into its sacs. Calcium ion stripped from troponin molecules in the third myofilaments, tropomyosin returns to its position, blocking actin’s active sites.

13. Myosin cross bridges are prevented from binding to actin and thus can no longer sustain the contraction. Since the thick and thin myofilaments are no longer connected, the muscle fibres may return to its longer, resting length.

14. Physical changes during muscle contraction The physical changes observed during muscle contraction are : Heat production Electricity generation Volume changes Changes in optical properties Sound production