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GENERAL MECHANISM OF
 MUSCLE CONTRACTION

 Sequential steps of initiation and
 execution of muscle contraction
MUSCLE CONTRACTION
1. An action potential travels along a motor
   nerve to its endings on muscle fibers.
2. The nerve secretes a small amount of the
   neurotransmitter substance –
   acetylcholine at each ending.
3.       The acetylcholine acts on a local area
     of the muscle fiber membrane to open
     multiple “acetylcholine –gated” cation
     channels through protein molecules
     floating in the membrane.
4. Opening of the acetyl-gated channels
  allows large quantities of sodium ions to
  diffuse to the interior of the muscle fiber
  membrane . This causes a local
  depolarization that in turn leads to opening
  of voltage-gated sodium channels. This
  initiates action potential at the membrane.
5. The action potential travels along the
  muscle fiber membrane in the same way
  that action potentials travel along nerve
  fiber membranes.
6.The action potential depolarizes the
  muscle membrane, and much of the action
  potential electricity flows through the
  center of the muscle fiber. Here it causes
  the sarcoplasmic reticulum to release
  large quantities of calcium ions that have
  been stored within this reticulum.
7. The calcium ions initiate attractive forces
  between the actin and myosin
  filaments, causing them to slide alongside
  each other, which is the contractile
  process.
8. After a fraction of a second, the calcium
  ion are pumped back into the
  sarcoplasmic reticulum until a new muscle
  action potential comes along; this removal
  of calcium ions from the myofibrils causes
  the muscle contraction to cease.
MOLECULAR
MECHANISM OF MUSCLE
   CONTRACTION
Sliding Filament Mechanism of
      Muscle Contraction
                    In contracted
                    state, actin (thin)
                    filaments have
                    been pulled
                    inward among
                    the myosin
                    (thick)
                    filaments, so
                    their ends
                    overlap one
                    another to their
                    maximum extent.
Sliding Filament Mechanism of
       Muscle Contraction
• Muscle contraction occurs by a sliding
  filament mechanism.
• When action potential travels along the
  muscle fiber, sarcoplasmic reticulum
  release large quantities of calcium ions
  that rapidly surround the myofibrils.
• Energy is needed for the contractile
  process to proceed. (ATP to ADP)
Molecular Characteristics of the
    Contractile Filaments
• Myosin filaments is made up of 200 or
  more individual myosin molecules.
Molecular Characteristics of the
    Contractile Filaments
                       arm



                             Cross-bridges



                             hinges

                         body
Molecular Characteristics of the
    Contractile Filaments
• Body- bundled tails of myosin
  molecules.
• Arm- extends the head outward from
  the body.
• Head- globular polypeptide structure.
• Cross-bridges- heads and arms
  together.
• Hinges- flexible point of a cross-bridge.
Molecular Characteristics of the
    Contractile Filaments
• The total length of each myosin filament if
  uniform, almost exactly 1.6 micrometers.
• The distance between cross-bridges and
  body is 0.2 micrometers.
• Myosin filament itself is twisted from the
  previous pair by 120 degrees ensuring that
  cross-bridges are extended in all
  directions around the filament.
Molecular Characteristics of the
    Contractile Filaments
• Myosin molecule is composed of 6 polypeptide
  chains.
  – 2 heavy chains
     • Molecular weight of 200,000 each
     • Double helix
     • Tail
         – Ends: head
  – 4 light chains
     • Molecular weight of 20,000 each
     • Part of myosin head (2 at each head)
     • Help control the function of the head during muscle
       contraction.
Molecular Characteristics of the
    Contractile Filaments
• ATPase activity of the Myosin Head
  – The myosin head functions as an ATPase
    enzyme.
• Actin Filaments are composed of
  Actin, Tropomyosin, and Troponin.
  – The double stranded F-actin protein molecule
    • backbone of the actin filament
    • wounded in a helix
    • (each strand) is composed of polymerized G-actin
      molecules.
Molecular Characteristics of the
    Contractile Filaments
• G-actin molecule has a molecular weight
  of about 42,000.
• One molecule of ADP is attached to each
  of it.
• Each actin filament is about 1 micrometer
  long.
• The bases of the actin filaments is inserted
  strongly into the Z-discs.
Molecular Characteristics of the
    Contractile Filaments
Molecular Characteristics of the
    Contractile Filaments
• Each molecule of tropomyosin found in the
  actin filament has a molecular weight of
  70,000 and a length of 40 nanometers.
• These molecules are wrapped spirally
  around the sides of the F-actin helix.
• In the resting stage, the tropomyosin
  molecules lie on top of the active sites of
  the actin strands.
Molecular Characteristics of the
    Contractile Filaments
• Attached intermittently along the sides of
  the tropomyosin molecule is the troponin
  molecule.
• There are three subunits:
  – Troponin 1: has strong affinity for actin
  – Troponin T: for tropomyosin
  – Troponin C: for calcium ions

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General mechanism of muscle contraction

  • 1. GENERAL MECHANISM OF MUSCLE CONTRACTION Sequential steps of initiation and execution of muscle contraction
  • 2. MUSCLE CONTRACTION 1. An action potential travels along a motor nerve to its endings on muscle fibers.
  • 3. 2. The nerve secretes a small amount of the neurotransmitter substance – acetylcholine at each ending.
  • 4. 3. The acetylcholine acts on a local area of the muscle fiber membrane to open multiple “acetylcholine –gated” cation channels through protein molecules floating in the membrane.
  • 5. 4. Opening of the acetyl-gated channels allows large quantities of sodium ions to diffuse to the interior of the muscle fiber membrane . This causes a local depolarization that in turn leads to opening of voltage-gated sodium channels. This initiates action potential at the membrane.
  • 6. 5. The action potential travels along the muscle fiber membrane in the same way that action potentials travel along nerve fiber membranes.
  • 7. 6.The action potential depolarizes the muscle membrane, and much of the action potential electricity flows through the center of the muscle fiber. Here it causes the sarcoplasmic reticulum to release large quantities of calcium ions that have been stored within this reticulum.
  • 8. 7. The calcium ions initiate attractive forces between the actin and myosin filaments, causing them to slide alongside each other, which is the contractile process.
  • 9. 8. After a fraction of a second, the calcium ion are pumped back into the sarcoplasmic reticulum until a new muscle action potential comes along; this removal of calcium ions from the myofibrils causes the muscle contraction to cease.
  • 11. Sliding Filament Mechanism of Muscle Contraction In contracted state, actin (thin) filaments have been pulled inward among the myosin (thick) filaments, so their ends overlap one another to their maximum extent.
  • 12. Sliding Filament Mechanism of Muscle Contraction • Muscle contraction occurs by a sliding filament mechanism. • When action potential travels along the muscle fiber, sarcoplasmic reticulum release large quantities of calcium ions that rapidly surround the myofibrils. • Energy is needed for the contractile process to proceed. (ATP to ADP)
  • 13. Molecular Characteristics of the Contractile Filaments • Myosin filaments is made up of 200 or more individual myosin molecules.
  • 14. Molecular Characteristics of the Contractile Filaments arm Cross-bridges hinges body
  • 15. Molecular Characteristics of the Contractile Filaments • Body- bundled tails of myosin molecules. • Arm- extends the head outward from the body. • Head- globular polypeptide structure. • Cross-bridges- heads and arms together. • Hinges- flexible point of a cross-bridge.
  • 16. Molecular Characteristics of the Contractile Filaments • The total length of each myosin filament if uniform, almost exactly 1.6 micrometers. • The distance between cross-bridges and body is 0.2 micrometers. • Myosin filament itself is twisted from the previous pair by 120 degrees ensuring that cross-bridges are extended in all directions around the filament.
  • 17. Molecular Characteristics of the Contractile Filaments • Myosin molecule is composed of 6 polypeptide chains. – 2 heavy chains • Molecular weight of 200,000 each • Double helix • Tail – Ends: head – 4 light chains • Molecular weight of 20,000 each • Part of myosin head (2 at each head) • Help control the function of the head during muscle contraction.
  • 18. Molecular Characteristics of the Contractile Filaments • ATPase activity of the Myosin Head – The myosin head functions as an ATPase enzyme. • Actin Filaments are composed of Actin, Tropomyosin, and Troponin. – The double stranded F-actin protein molecule • backbone of the actin filament • wounded in a helix • (each strand) is composed of polymerized G-actin molecules.
  • 19. Molecular Characteristics of the Contractile Filaments • G-actin molecule has a molecular weight of about 42,000. • One molecule of ADP is attached to each of it. • Each actin filament is about 1 micrometer long. • The bases of the actin filaments is inserted strongly into the Z-discs.
  • 20. Molecular Characteristics of the Contractile Filaments
  • 21. Molecular Characteristics of the Contractile Filaments • Each molecule of tropomyosin found in the actin filament has a molecular weight of 70,000 and a length of 40 nanometers. • These molecules are wrapped spirally around the sides of the F-actin helix. • In the resting stage, the tropomyosin molecules lie on top of the active sites of the actin strands.
  • 22. Molecular Characteristics of the Contractile Filaments • Attached intermittently along the sides of the tropomyosin molecule is the troponin molecule. • There are three subunits: – Troponin 1: has strong affinity for actin – Troponin T: for tropomyosin – Troponin C: for calcium ions