What is the role of actin and myosin in muscle contraction?

Actin and myosin are proteins that interact to cause muscle contraction, enabling movement and force generation.

In more detail, muscle contraction is a complex process that involves several steps and is primarily driven by the proteins actin and myosin. These proteins are found in the muscle fibres, which are the basic units of a muscle. The muscle fibres are made up of smaller units called myofibrils, which are further divided into sarcomeres. Sarcomeres are the functional units of muscle contraction and are composed of thick and thin filaments, which are primarily made up of myosin and actin respectively.

The process of muscle contraction begins when a nerve impulse reaches the muscle fibre. This triggers the release of calcium ions, which bind to a protein called troponin on the actin filaments. The binding of calcium to troponin causes a shift in the position of another protein, tropomyosin, which normally blocks the binding sites for myosin on the actin filaments. When tropomyosin moves, these sites are exposed, allowing myosin to bind to actin.

The myosin heads, which are attached to the thick filaments, have ATPase activity. This means they can break down ATP (adenosine triphosphate), the main energy currency of the cell, into ADP (adenosine diphosphate) and a phosphate group. This breakdown of ATP provides the energy for the myosin heads to change shape and pull the actin filaments towards the centre of the sarcomere. This pulling action is what causes the muscle to contract.

After the power stroke, the myosin heads release the actin filaments and return to their original position, ready to bind to another actin molecule and repeat the process. This cycle of binding, power stroke, and release is known as the cross-bridge cycle and is the fundamental mechanism of muscle contraction.

In summary, actin and myosin play crucial roles in muscle contraction. The interaction between these two proteins, driven by the energy from ATP, allows muscles to contract and generate force, enabling movement.