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CONTRACTION OF SKELETLAL MUSCLE: SLIDING FILAMENT THEORY. PROPOSED BY: H.E. HUXLEY & J. HANSEN. Introduction. When a muscle cell contracts, the thin filaments slide past the thick filaments and the sarcomere shortens. Molecules Involved. 1. Myosin - thick protein filament
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PROPOSED BY: H.E. HUXLEY & J.
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
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
(troponin moves the tropomyosin out
of the way)
2. Myosin binds to actin to form a cross bridge.
4. ATP binds to the myosin head, causing actin to disconnect from the cross bridge.
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.
*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.
-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.
-Potential energy stored in cross bridges is
converted to chemical energy (ATP)
-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.
-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.
*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*
Sliding Filament Animation