How do our muscles contract thus producing the movement of our bodies?

To answer this question we must first know about the microscopic structure of the muscle fibers.

Muscle fibers are composed by structures called myofibrils, each myofibril is in turn formed by two kinds of filaments: thin filaments and thick ones.

Thin filaments: formed by the actin molecule plus the regulatory proteins: troponin and tropomyosin.

Thick filaments: formed by the myosin molecule plus structural proteins.

Now, the interaction between thin and thick filaments is what will produce the contraction. Through a cycle known as cross-bridge cycle the thin filament will move along the thick one producing the force that makes movement possible.

How do filaments interact with each other?

We can divide the cross-bridge cycle into four stages. But before we dive into this stages it is important to highlight the role of the troponin and tropomyosin molecules whose action is needed for the cycle to start.

For contraction to happen calcium is needed, this calcium will come from a structure called sarcoplasmic reticulum and it will be released after a nervous signal.

The calcium will then bind to the troponin molecules an this ones in turn will act upon the tropomyosin molecules. The tropomyosin molecules at rest are bonded to the actin molecule blocking the binding site for the myosin molecule thus blocking the interaction between thin and thick filaments, but when the troponin acts upon the tropomyosin this one is released from the actin molecule exposing the binding sites for the myosin heads.

After this happens then we enter to the first stage of the cross-bridge cycle:

Attachment:

The myosin head is tightly bound to the actin molecule.

There is absence of ATP.

This stage ends with the binding of ATP with the myosin molecule.

Release:

The binding of ATP – Myosin produces a conformational change in the binding site (on the myosin) for the actin which causes its dissociation from the myosin molecule.

Bending:

ATP is hydrolysed by the myosin head.

Because of the hydrolysis the myosin head assumes its pre-power stroke position.

ADP and Pi remain bound to the myosin head as a result of ATP hydrolysis.

Force Generation:

The myosin head binds weakly to its new binding site on the actin molecule.

Then Pi is released from the myosin producing two effects: The binding affinity between the myosin head and its new attachment site increases. The myosin head generates a force as it returns to its original unbent position.

As a result of (2) the thin filament moves along the thick filament.

Finally ADP is released from the myosin head and we return to the attachment stage of the cross-bridge cycle.





Below is an infographic that sums up muscle contraction.

Source: Histology a Text and Atlas – With Correlated Cell and Molecular Biology, 7E.