Muscle Contraction

The everyday routine of the normal person is jam-packed with a variety of activities such as walking, running, writing and typing, among others. We all know that the muscles in our bodies assist us in doing all of these functions, but how exactly do they do so? Muscle contraction and relaxation, to put it another way, is the fundamental concept of movement. Let’s have a look at the structure of contractile proteins, followed by the mechanism of muscle contraction.

Contractile Proteins 

Skeletal muscle is made up of muscle fibres that are subdivided into smaller units known as myofibrils. The contractile, regulatory, and structural proteins that make up each myofibril are distinguished from one another by the fact that they have three different functions.

Actin (thin filament) and myosin (long filament) are examples of contractile proteins (thick filament). Each actin filament is composed of two helical “F” actin (filamentous actin), and each helical “F” actin (filamentous actin) is composed of numerous units of ‘G’ actin (filamentous actin). In addition to the ‘F’ actin, two filaments of the regulatory proteins tropomyosin and troponin are present at regular intervals along the length of the cell. During muscular relaxation, troponin binds to myosin binding sites on actin filaments, preventing myosin from binding.

Muscles Contraction

Each myosin is formed of numerous units of meromyosin, which has two main parts: a globular head known as heavy meromyosin with a short arm and a tail known as light meromyosin. Heavy meromyosin has a short arm and a tail known as light meromyosin does not. The cross arm is comprised of the head and arms that extend from the surface of the myosin filament at a regular distance and angle from one another and are known as the cross arm. The head contains ATP-binding sites as well as actin-binding active sites. As a next step, let us attempt to comprehend the mechanics of muscle contraction.

Because of this sliding movement, the thin filaments slide across the thick filaments during contraction. Muscle contraction is triggered by a signal transmitted by the central nervous system to the motor neuron. The neuromuscular junction is the point at which a motor neuron and a sarcolemma come together. It is at this junction that an action potential is created, which results in the release of acetylcholine from the neuron that has reached it. Ca2+ is released into the sarcoplasm as a result of this spreading across the muscle fibre. Calcium then attaches to troponin on actin filaments, allowing myosin to bind to the active sites on the filaments. Myosin attaches to the exposed active site on actin, which is powered by the energy released during the hydrolysis of ATP. The actin is drawn towards the centre as a result of this. The Z lines that are linked to them are likewise pulled, resulting in contraction. Myosin is in a state of relaxation.

Therefore, the hydrolysis of ATP at the myosin head proceeds, causing the myosin head to slide even more. Until the calcium ions are pushed back into the sarcolemma, which results in the actin sites being covered once more, the process is repeated. The Z lines revert to their original locations at the end of the animation. This has the effect of calming you down. Muscle tiredness develops as a result of repeated activation of the muscles, which results in the accumulation of lactic acid in the muscles.

Myoglobin, a pigment found in muscles, gives them their characteristic red colour. Red fibres are muscles that contain a lot of myoglobin. They also include a large number of mitochondria, which are thought to be responsible for energy production. White fibres are muscle fibres that are devoid of myoglobin and appear white in colour.

Actin and myosin as a sliding mechanism 

1. 1.The ATP molecule binds to the myosin head, causing the cross-bridge between actin and myosin to be broken.
2. 2.By changing their location and swivelling, the myosin heads move closer to the next actin binding site, which is caused by ATP hydrolysis.
3. 3.The myosin heads connect to the new actin sites and revert to their old shape after a period of time.
4. 4.As a result of this reorientation, the actin is dragged along the myosin in a sliding mechanism.
5. The myosin heads propel the actin filaments in a manner similar to that of a rowing or propelling a row boat.

What Is a Sarcomere ?

When muscle cells are examined under a microscope, it is possible to observe that they have a striped pattern on them (striations). It is composed of a succession of fundamental units known as sarcomeres that are piled one on top of the other throughout muscle tissue to create this pattern  Sarcomeres can be found in large numbers within a single muscle cell, perhaps as many as thousands. Muscle cells include a large number of sarcomeres, which are highly stereotyped and repeated throughout the body. The proteins contained within them can alter in length, resulting in changes in the overall length of a muscle. A single sarcomere is made up of multiple parallel actin (thin) and myosin (thick) filaments that run parallel to one another. Sacroiliac junction (SJ) shortening has been studied extensively, and our current understanding is based on the interaction of myosin and actin proteins. What causes this shortening to occur? Something to do with the sliding interaction between actin and myosin is at the root of the problem.

CONCLUSION

It is the sliding motion of the thin actin and thick myosin filaments that causes the contraction of the muscle. On the basis of current evidence, it is hypothesised that this mechanism is driven by cross-bridges that protrude from the myosin filaments and cyclically engage with the actin filaments throughout the process of ATP hydrolysis