CONTRACTION OF SKELETLAL MUSCLE: SLIDING FILAMENT THEORY

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CONTRACTION OF SKELETLAL MUSCLESLIDING FILAMENT
THEORY

• PROPOSED BY H.E. HUXLEY J.
• HANSEN

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Introduction

• When a muscle cell contracts, the thin filaments
  slide past the thick filaments and the sarcomere
  shortens

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Molecules Involved

• 1. Myosin - thick protein filament
• 2. Actin - thin protein filament
• 3. Tropomyosin - covers binding sites on the
  actin to prevent cross bridges from forming when
  the muscle is not stimulated

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Molecules Involved

• 4. Troponin - exposes binding sites on the actin
  molecule during muscle stimulation
• 5. ATP - provides the energy needed for
  contraction
• 6. Calcium ions - enables actin and myosin to
  bind together to form cross bridges

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Sequence of Events at a Cross Bridge Cycle

• The influx of calcium triggers the exposure of
  binding sites on actin.
• (troponin moves the tropomyosin out
• of the way)
• 2. Myosin binds to actin to form a cross bridge.

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Sequence of Events at a Cross Bridge Cycle

• The cross bridge produces a power stroke
  (flexion) that causes the sliding of the thin
  filaments toward the center of the sarcomere.
• 4. ATP binds to the myosin head, causing actin
  to disconnect from the cross bridge.

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Sequence of Events at a Cross Bridge Cycle

• 5. ATP is broken down and the energy released
  enables the cross bridge to be repositioned.
• 6. Calcium ions are transported back into the
  sarcoplasmic reticulum and the tropomyosin moves
  back to cover the actin binding sites.

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Sequence of Events at a Cross Bridge Cycle

• In contraction of a typical sarcomere, step 1
  occurs then steps 2 - 5 are repeated over and
  over again before step 6 occurs. This allows the
  thin filaments to slide all the way inward.
  Steps 2 - 5 may repeat as long as both ATP and
  calcium ions are available. Multiple cross
  bridge cycling is coordinated sequentially to
  prevent cross bridges from being connected or
  disconnected at the same time.

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SUMMARY REQUIREMENTS FOR MUSCLE CONTRACTION

• STIMULATION
• -A nerve impulse stimulates the neuromuscular
  junction and acetylcholine is released,
  initiating an action potential which triggers the
  release of calcium ions from the sarcoplasmic
  reticulum - the calcium ions enable actin and
  myosin to bind together via cross bridges by
  causing troponin to move tropomyosin out of the
  way.

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SUMMARY REQUIREMENTS FOR MUSCLE CONTRACTION

• ENERGY
• -Potential energy stored in cross bridges is
• converted to chemical energy (ATP)

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SUMMARY REQUIREMENTS FOR MUSCLE CONTRACTION

• CONTRACTION
• -Energized by ATP, each cross bridge attaches
  and detaches several times during a contraction,
  acting much like tiny oars to generate tension
  and pull the thin filaments toward the center of
  the sarcomere. As this event occurs
  simultaneously in sarcomeres throughout the cell,
  the muscle cell shortens to about two thirds of
  its normal length.

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SUMMARY REQUIREMENTS FOR MUSCLE CONTRACTION

• RELAXATION
• -When the action potential ends, calcium ions
  are immediately reabsorbed into the sarcoplasmic
  reticulum storage areas, actin and myosin
  filaments separate and the muscle cell relaxes
  and returns to its original length. This whole
  series of events takes just a few thousandths of
  a second.

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SUMMARY REQUIREMENTS FOR MUSCLE CONTRACTION

• While the action potential is occurring, the
  acetylcholine, which started the process, is
  broken down by enzymes present in the sarcolemma.
  In this way, a single nerve impulse produces
  only one contraction, preventing the continued
  contraction of a muscle cell in the absence of
  additional nerve impulses

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Animation

• Sliding Filament Animation

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The End