Sliding Filament Theory

Anatomy and PhysiologyMuscles

Sliding Filament Theory

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  1. Muscle Contraction

    Slide 1 - Muscle Contraction

    • Sarcomeres shorten, distance between z-lines reduced
    • Thick and thin myofilaments overlap more during contraction
    • 3 stages:
    • Neural stimulation
    • Muscle cell contraction
    • Muscle cell relaxation
  2. Motor Unit

    Slide 2 - Motor Unit

    • Stimulation of a muscle by a nerve impulse (motor nerve) is required before a muscle can shorten
    • Neuromuscular junction: point of contact b/w nerve ending and the muscle fiber it innervates.
    • Motor unit: motor neuron + muscle cell
  3. Neuromuscular Junction

    Slide 3 - Neuromuscular Junction

  4. Slide 4

    • Sarcolemma
    • (cell membrane)
  5. Neural Stimulation

    Slide 5 - Neural Stimulation

    • Motor neuron releases neurotransmitters to stimulate a contraction
    • Acetylcholine (Ach) binds to receptors located on sarcolemma (muscle cell membrane)
    • Changes transport proteins found in sarcolemma
    • Alters transport of ions
    • Normally, more Sodium (Na+) ions outside muscle cell, while Potassium (K+) higher inside
    • Sodium/Potassium pumps maintain this unequal concentration
    •  Excitable condition
  6. Slide 6

    • When stimulated, ion channels open, depolarizing the cell
    • Na+ flows in, K+ out
    •  sarcoplasmic reticulum releases stored calcium
    • Ca2+ travels to sarcomere, initiating muscle contraction phase
  7. Muscle Contraction Animation

    Slide 7 - Muscle Contraction Animation

    • Click here to view animation
  8. Sliding filament theory

    Slide 9 - Sliding filament theory

    • In the absence of Calcium ions…
    • Troponin (purple M&M) sits on Tropomyosin filament (gray)
    • This “protein complex” blocks access to the myosin head’s binding site on actin.
    • Troponin
    • Tropomyosin
  9. Slide 10

    • When Calcium (small specks) is released by the Sarcoplasmic reticulum
    • it diffuses into the muscles
    • binds to the troponin/tropomyosin complex
    • shifting both the troponin and tropomyosin filament
  10. Slide 11

    • This reveals the binding sites on actin… myosin now has access!
    • (But it still can’t reach)
  11. Slide 12

    • Myosin splits ATP and moves into a high-energy state, extending the “arm” of the myosin.
    • The head of myosin can now bind to actin
    • Forms a cross-bridge between the thick and thin filaments.
  12. The ATP energy stored by myosin is released

    Slide 13 - The ATP energy stored by myosin is released

    • The myosin molecule relaxes, bending back to its original place…
    • Causes rotation of the head, pulling on the actin…
    • This leads to the sliding of the filaments.
    • This cycle continues until Ca2+ ions gone (and stimulus stops)
  13. As long as Calcium is present, though…

    Slide 14 - As long as Calcium is present, though…

    • New ATP binds to Actin/myosin cross bridge, causing myosin to disconnect from actin.
    • Splitting of ATP (turning into ADP + P) leads to re-energizing (extending)/ repositioning of the cross bridge.
  14. What it REALLY looks like…

    Slide 15 - What it REALLY looks like…

  15. Relaxation Phase

    Slide 16 - Relaxation Phase

    • Complete contraction of muscle cell requires several cycles of neural stimulation and contraction phases
    • Ca2+ ions transported back to sarcoplasmic reticulum (req. ATP)
    • When the calcium level decreases
    • troponin and tropomyosin move back into the blocking position
    • thin filament (actin) slides back to the resting state (when ATP binds to myosin head)
  16. Slide 17

    • Relaxation phase occurs when no more neural stimulations are stimulating the sarcolemma to release Calcium…
    • Na+/K+ pump returns ions to resting state
    • Muscle cell remains in contracted, but pliable state
    • Must be “stretched” back into position
  17. Review of the Role of ATP

    Slide 18 - Review of the Role of ATP

    • ATP transfers its energy to the myosin cross bridge, which in turn energizes the power stroke.
    • ATP disconnects the myosin cross bridge from the binding site on actin.
    • ATP fuels the pump that actively transports calcium ions back into the sarcoplasmic reticulum.
  18. Rigor Mortis

    Slide 19 - Rigor Mortis

    • In death…
    • Calcium leaks out of sarcoplasmic reticulum into sarcomere
    • Causes muscle tension = rigor mortis
    • Muscle cell structures start breaking down, causing muscle to loosen (unless body becomes dehydrated)
  19. Creatine phosphate

    Slide 20 - Creatine phosphate

    • Stores energy in muscle cells
    • Collects energy from ATP, stores for long periods of time
    • Transfers back to ATP when needed
  20. Glycogen

    Slide 21 - Glycogen

    • Stored form of glucose
    • Energy reserve for muscle action
    • Continuous supply needed to produce ATP
  21. Myoglobin

    Slide 22 - Myoglobin

    • Red pigment that stores oxygen for muscle cells
    • “Grabs” oxygen from hemoglobin in blood
    • High affinity for oxygen
    • Allows cells to produce large amounts of ATP