What happens when a muscle contracts?

Muscle contraction is the process by which muscle cells shorten, generating force and movement. It involves a complex series of events occurring at the molecular and cellular levels. Here is a general overview of what happens when a muscle contracts:

1. Action Potential:

- When a muscle receives a signal from the nervous system, an electrical impulse known as an action potential is generated.

- The action potential travels along the muscle fiber's surface membrane, causing it to depolarize.

2. Calcium Release:

- The depolarization of the muscle fiber membrane triggers the release of calcium ions (Ca2+) from the sarcoplasmic reticulum (SR), which is the muscle cell's internal store of calcium.

3. Calcium-Troponin Interaction:

- Calcium ions bind to troponin, a regulatory protein located on the thin filaments (actin filaments) within the muscle fiber.

4. Myosin-Actin Interaction:

- The binding of calcium to troponin causes a conformational change that exposes a binding site on actin for the head of myosin, a motor protein.

- Myosin heads bind to the exposed binding site on actin, forming crossbridges between the thick filaments (myosin filaments) and thin filaments.

5. Power Stroke:

- Myosin heads undergo a conformational change, which generates force by pulling the thin filaments toward the center of the sarcomere (the repeating unit of the muscle fiber).

- This sliding of thin filaments past the thick filaments causes muscle contraction.

6. ATP Hydrolysis:

- The energy required for the power stroke comes from the hydrolysis of adenosine triphosphate (ATP) by myosin.

- Each myosin head contains an ATPase enzyme that breaks down ATP into adenosine diphosphate (ADP) and inorganic phosphate (Pi), releasing energy.

7. Muscle Relaxation:

- When the electrical stimulus ends, the calcium ions are pumped back into the SR by calcium pumps.

- As calcium levels decrease, the troponin-tropomyosin complex reverts to its original position, blocking the binding site on actin.

- Myosin heads detach from actin, and the muscle fiber relaxes.

The coordinated contraction and relaxation of muscle fibers allow for movement, including skeletal muscle contractions for voluntary movements, cardiac muscle contractions for heart function, and smooth muscle contractions for involuntary movements such as digestion and blood vessel regulation.