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19 myology 3 - Coggle Diagram

- - - - Depolarization of muscle membrane (sarcolemma) by increasing its
        permeability to Na+
        
        Generation of AP
        in the muscle fiber.
        
        Inward spread of AP
        along T-system.
        
        Spread of depolarization to
        terminal cistern
        with release of Ca2+
        in the myofibrils.
        
        Incr in conc of Ca2+ in
        ICF by 2000 times i.e. from 10-7
        moles/L to 2 x 10-4 moles/L.
        
        Role of calcium
        
        Ca2+ binds to troponin C
        to saturation point
        causing tropomyosin to move laterally.
        This exposes the binding sites
        for myosin heads on actin.
        
        Activates prosthetic group of
        myosin filament
        which acts as enzyme (ATPase)
        catalyzing the breakdown of ATP
        to produce energy for:
        (a) contraction of actomyosin complex;
        (b) and activating sliding filament system.
        
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  - - - Once Ca2+ concentration decr in ICF
        sufficiently to 10-7 moles/L,
        chemical interaction betw
        myosin and actin ceases
        and muscle relaxes.
- - - - Therefore, the process by which
        depolarization of the muscle fiber
        initiates contraction is called Excitation contraction coupling.
- - - - The sliding of filaments
        is brought about by
        interaction between the
        Cross-bridges of myosin
        and actin molecules.
        
        The force of contraction
        is developed by the
        cross-bridges in the overlap
        region between actin and myosin.
    - - In resting muscle, troponin ‘I’ is
        lightly bound to actin and tropomyosin
        covers the actin sites
        where myosin heads bind to actin.
        Therefore, the “troponin-tropomyosin” complex constitutes a Relaxing Protein which
        inhibits the interaction between actin and myosin.
        
        Ca2+ released from the terminal cisterns by the action potential binds to troponin ‘C’ and causes:
        
        the binding of troponin ‘I’ to actin is weakened,
        this permits tropomyosin to move laterally,
        uncovering the binding sites for
        myosin heads on actin filaments, and
        
        hydrolysis of ATP by ATPase activity
        in the myosin heads to produce energy.
        
        7 myosin binding sites on actin filaments are uncovered for each molecule of troponin
        that binds a Ca2+
        
        The cross-bridges of the myosin molecules
        (which serve as the cross linkages)
        link to the actin at 90 degree angle,
        produce movement of
        myosin on actin by swiveling
        (revolving i.e. turns freely on a point),
        and then disconnect and reconnect
        at the next linking site.
      - Mechanism of sliding of thin filaments
        over thick filaments
        
        The sliding during muscle contraction is produced by breaking and re- forming of cross-bridges
        of cross-linkages between actin and myosin,
        
        repeating the process in serial fashion for 5-6 times.
        
        The width of ‘A’ bands is constant,
        whereas the ‘Z’ lines move closer
        together when the muscle contracts
        and farther apart when it is stretched.
        As the muscle shortens,
        the actin filament from the opposite
        end of the sarcomere approach each other;
        when the shortening is marked,
        these filaments apparently overlap.