Sliding Filament Theory

Steps to Sliding Filament Theory

  1. Nerve impulse creates action potential at neuromuscular junction

    • Action Potential= electrical current= nerve “signal”

    • Action Potential spreads through entire muscle fiber through sarcolemma and T-Tubules

  2. Calcium released from sarcoplasmic reticulum

    • *Increased Calcium in fiber is responsible for initiating contraction within muscle

      • Calcium present= contraction

      • Calcium absent= relaxation

    • Nerve impulse commands sarcoplasm to release calcium for muscle contraction

  3. Calcium binds to troponin on thin filaments

  4. Conformational change of thin filaments, tropomyosin opens binding sites on actin where myosin will attach

  5. Myosin heads bind to actin, forming crossbridge

  6. Thick filament (Myosin) PULLS on thin filament (Actin) toward M-line of sarcomere

    • Myosin head PULLS on Actin

    • Entire sarcomere becomes shorter

    • “ Power Stroke”

  7. ADP and P, released from myosin head

    • Left over from previous crossbridge

  8. ATP binds to myosin head

    • ATP formed by cellular respiration in mitochondria of muscle fiber (by breaking down glucose)

  9. Energy from ATP detaches crossbridge

    • ATP → ADP + P, releases energy from phosphate bonds

    • ADP and P stay attached to myosin head until next crossbridge

  10. Myosin head returns to “cocked position”

    • If Calcium still present in fiber: (AP still signaling muscle)

      • Continuous crossbridges form, pulling on actin closer and closer to M line

      or

    • If Calcium no longer present: (Action Potential has stopped)

      • Last crossbridge breaks while binding sites close= muscle returns to resting state

Sarcomere Anatomy

  • Thin filaments= Actin, tropomyosin and troponin

  • Thick filaments= Myosin

  • Z- discs/ Z line= ends of sarcomere

  • M line= Midline of sarcomere

  • I band= space between myosin of one sarcomere and myosin of next sarcomere

  • A band= length of thick filaments/ myosin

  • H zone= space between actin within 1 sarcomere

Z discs move closer to the M line when a muscle contracts

H zone disappears when fully contracted or gets smaller when partially contracted

I band gets smaller when muscle contracts

A band stays the same

1 motor neuron per muscle fiber

Motor neuron + Muscle fiber= Motor unit

Neuromuscular Junction= Where motor neuron meets sarcolemma of muscle fiber

Proteins like to change shape when stuff binds to them

Changing shapes can allow proteins to bind or unbind with other stuff