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.