What is the correct order of steps in muscle contraction?

Muscle contraction involves several steps that occur in a specific sequence. Here is the correct order of events during muscle contraction:

1. Action potential generation and transmission:

- A nerve impulse (action potential) is generated in the motor neuron.

- The action potential travels along the motor neuron axon toward the neuromuscular junction.

- At the neuromuscular junction, the action potential causes the release of the neurotransmitter acetylcholine (ACh) into the synaptic cleft.

2. ACh binding and muscle membrane depolarization:

- ACh molecules bind to receptors on the muscle cell membrane, causing the membrane to become locally depolarized.

- This depolarization is an increase in the electrical potential inside the muscle cell, making the inside less negative compared to the outside.

3. Muscle fiber depolarization (action potential propagation):

- If the depolarization reaches a threshold level (about -55 mV), an action potential is generated on the muscle cell membrane.

- The muscle action potential spreads rapidly over the entire muscle fiber surface through the membrane system called transverse tubules (T-tubules).

4. Sarcoplasmic reticulum (SR) calcium release:

- The action potential also travels along the T-tubules and triggers the release of calcium ions (Ca2+) from the sarcoplasmic reticulum.

- Calcium is the main signaling molecule that initiates muscle contraction.

5. Calcium binding to troponin:

- Calcium ions bind to troponin, a protein complex on the thin filament (actin).

- This binding changes the conformation of troponin, which allows another protein called myosin to bind to the actin filament.

6. Cross-bridge formation:

- Myosin heads (myosin motor domain) project out from the thick filaments (myosin) and bind to specific sites on the actin filaments, forming cross-bridges.

7. Power stroke:

- Each myosin head goes through a conformational change, called the power stroke, pulling the thin filament toward the center of the sarcomere.

- The thin filament slides past the thick filament, causing muscle contraction.

8. Muscle relaxation:

- After the action potential ends, the SR pumps calcium ions back into its storage areas.

- Without calcium, troponin reverts to its original conformation, causing myosin to detach from actin and the cross-bridges to break.

- The muscle fiber returns to its relaxed state.

9. ATP consumption:

- The power stroke requires energy in the form of ATP.

- ATP is hydrolyzed to ADP and inorganic phosphate (Pi), providing the energy needed for myosin to bind, release, and move the actin filaments during muscle contraction.

This sequence of events occurs repeatedly during muscle contraction, allowing for controlled muscle movements.