Muscle contraction Steps
Step 2. Contraction
The motor neuron terminal releases the neurotransmitter acetylcholine (ACh).
Step 10. Contraction
The muscle fiber shortens and contracts.
Step 5. Contraction
The muscle impulse reaches the sarcoplasmic reticulum, and calcium channels open.
Step 2. Powerstroke
The power stroke. ADP and Pi are released and the myosin head pivots and bends, changing to its bent low-energy state. As a result it pulls the actin filament toward the M line.
Step 4. Contraction
The sarcolemma is stimulated, and a muscle impulse travels over the surface of the muscle fiber and deep into the fiber through the transverse tubules.
Step 9. Contraction
Thin (actin) filaments are pulled toward the center of the sarcomere by myosin cross-bridges.
Step 7. Contraction
Tropomyosin molecules move and expose specific sites on actin.
Step 5. Relaxation
Troponin and tropomyosin molecules inhibit the interaction between myosin and actin filaments.
Step 4. Relaxation
Breakdown of ATP "cocks" the cross-bridges.
Step 2. Relaxation
Calcium ions are actively transported into the sarcoplasmic reticulum.
Step 6. Contraction
Calcium ions diffuse from the sarcoplasmic reticulum into the sarcoplasm and bind to troponin molecules causing it to pull
Step 4. Powerstroke
Cocking of the myosin head. As ATP is hydrolyzed to ADP and Pi, the myosin head returns to its prestroke high-energy, or "cocked," position.
Step 3. Powerstroke
Cross bridge detachment. After ATP attaches to myosin, the link between myosin and actin weakens, and the myosin head detaches.
Step 1. Powerstroke
Cross bridge formation. Energized myosin head attaches to an actin myofilament, forming a cross bridge.
Step 6. Relaxation
Muscle fiber remains relaxed, yet ready until stimulated again.
Step 8. Contraction
Actin and myosin form cross bridges.
Step 1. Contraction
A nerve impulse travels down a motor neuron axon.
Step 3. Contraction
ACh binds to ACh receptors (on the sarcolemma of the muscle fiber).
Step 3. Relaxation
ATP breaks linkages between actin and myosin filaments without breakdown of the ATP itself.
Step 1. Relaxation
Acetylcholinesterase decomposes acetylcholine, and the muscle fiber membrane is no longer stimulated.