Types of Muscle Contraction, its process and Structure of contractile proteins

The human body involves various daily activities like walking, running, typing, etc. That requires the movement of muscles. But the critical question here is how the process of movement of muscles works. Well, the simple answer to this question is muscle contraction. The process of muscle contractions is followed by muscle relaxation. 

Let's see how the structure of contractile proteins is ensued by the muscle contraction mechanism.

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Structure of contractile proteins

The skeletal muscle of the human body comprises muscle fibers with small units known as myofibrils. The myofibrils are made up of three types of proteins that are:  contractile, regulatory, and structural proteins.

The contractile protein is made up of two filaments that are actin, the thinner filament, and myosin the thicker filament. The actin filaments comprise two helical ‘F’ actin (filamentous actin) with each F actin comprising multiple units of ‘G’ actin. Two filaments of regulatory proteins along with ‘F’ actin namely tropomyosin and troponin are also present intermittently. When muscle relaxation is in action, troponin works by covering the binding sites for myosin on actin filaments.

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Each myosin contains multiple units of meromyosin. A meromyosin contains two main parts a globular head that is termed as heavy meromyosin with a short arm and a tail known as light meromyosin. The head and arm are gauged at a regular distance and angle from each other by the surface of the myosin filament which is known as the cross arm.  The head provides support to the binding sites for ATP and active sites for actin. 

Now let's discuss the mechanism of Muscle Contraction.

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How does the Muscle contraction process take place?

The process of muscle contraction involves the sliding of thin filaments over thick filaments. This process of muscle contractions is commenced by a signal sent by the central nervous system via motor neurons. The junction between a motor neuron and sarcolemma is known as the neuromuscular junction.

When a neural signal reaches this junction it releases Acetylcholine and an action potential is produced in the sarcolemma. On spreading through the muscle fiber, it releases calcium ions in the sarcoplasm. Calcium plays a part of the binding troponin on actin filaments and uncovers the active sites of myosin. Myosin then binds the exposed active sites on actin with the help of energy from the hydrolysis of ATP. This attracts the actin filament toward the center. Along with the actin filament, the Z lines attached to it are pulled too and thus the contraction takes place.  Myosin is in an ease-up state.

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As a result, the presence of hydrolysis of ATP at the myosin head carries on which leads to further sliding. The process continues till the calcium ions are sent back to the sarcolemma, the resultant of which would be covering the actin sites again. The Z lines then move back to their old positions, which results in the relaxation of the muscles. The over-accumulation of lactic acid due to repeated activation of the muscles causes muscle fatigue. 

The muscle appears red due to the presence of a pigment called myoglobin. Muscles that contain a good amount of Myoglobin pigment are called red fibers. The red muscles are also rich in mitochondria, which contribute immensely in the production of energy. Contrary to this our body also comprises muscles that lack  Myoglobin pigment and this is why some of them are white in color and are known as the white fibers.

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Types of muscle contractions

Muscle contractions are of three types that have been discussed here below: 

‌Concentric Contractions

The concentric contractions take place when your muscle is actively shortened. When you use your muscles to lift something heavier, your muscles tighten up which generates tension. 

This type of contraction takes place only when the load is less than your muscle's maximum capacity. The muscles cannot do their job without shortening the fibers in order to move an object physically. 

For example: when you squat down to lift a heavy item the muscles of your arm will contract to have a grip on the weight but the muscle of your legs will tighten up as you stand with added weight.

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Eccentric Contractions

This type of contraction takes place when your muscles are actively lengthened during normal activity. 

For example, these muscles are active when you are walking because your  quadriceps muscles become active on your heels touching the ground or when your knee is bending, or when your legs straighten to take strides. 

These contractions also occur when you are bending down to place something heavy as muscle has to stay tight to manage weight. But in case of shifting a heavy object, the muscle tightens into a different position.

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Isometric Contraction

This contraction occurs when your muscle is actively held at a set length in a single position and the joint attached to it doesn't move. This contraction doesn’t provide strength to the muscle group.  But it strengthens the muscle for that specific movement. Apart from lengthening and shortening as it will in some activities, your body holds it at a specific length once activated.

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