Sarcomere: Structure and Parts, Functions and Histology

A sarcomere it is the fundamental functional unit of striated muscle, that is, of skeletal and cardiac muscle. Skeletal muscle is the type of muscle that is used in voluntary movement and the heart muscle is the muscle that is part of the heart.

To say that the sarcomere is the functional unit means that all the components necessary for contraction are contained in each sarcomere. In fact, striated muscle is composed of millions of small sarcomeres that shorten, individually, with each muscle contraction.

Micrograph of a sarcomere (above) and its representation (below)

Here lies the main purpose of the sarcomere. The sarcomeres are able to initiate large movements by contracting in unison. Its unique structure allows these small units to coordinate the contractions of the muscles.

In fact, the contractile properties of muscle are a defining characteristic of animals, since the movement of animals is remarkably smooth and complex. The locomotion requires a change in the length of the muscle as it flexes, which requires a molecular structure that allows muscle shortening.

Structure and parts

If the skeletal muscle tissue is examined closely, a striped appearance called striation is observed. These"stripes"represent a pattern of alternating bands, light and dark, corresponding to different protein filaments. That is, these stripes are formed by interlaced protein fibers that make up each sarcomere.

Myofibrils

Muscle fibers are composed of hundreds to thousands of contractile organelles called myofibrils; These myofibrils are arranged in parallel to form muscle tissue. However, the myofibrils themselves are essentially polymers, that is, repetitive units of sarcomeres.

Myofibrils are fibrous and long structures, and are made of two types of protein filaments that are stacked one on top of the other.

Myosin and actin

Myosin is a thick fiber with a globular head, and actin is a thinner filament that interacts with myosin during the process of muscle contraction.

A given myofibril contains approximately 10,000 sarcomeres, each of which is approximately 3 micrometers in length. Although each sarcomere is small, several aggregate sarcomeres span the length of the muscle fiber.

Myofilaments

Each sarcomere consists of thick, thin beams of the proteins mentioned above, which together are called myofilaments.

By expanding a portion of the myofilaments, you can identify the molecules that make them up. The thick filaments are made of myosin, while the fine filaments are made of actin.

Actin and myosin are the contractile proteins that cause muscle shortening when they interact with each other. In addition, the thin filaments contain other proteins with regulatory function called troponin and tropomyosin, which regulate the interaction between contractile proteins.

Functions

The main function of the sarcomere is to allow a muscle cell to contract. For this, the sarcomere must be shortened in response to a nervous impulse.

The thick and thin filaments do not shorten, but slide around each other, which causes the sarcomere to shorten while the filaments retain the same length. This process is known as the sliding filament model of muscle contraction.

The sliding of the filament generates muscular tension, which is undoubtedly the main contribution of the sarcomere. This action gives the muscles their physical strength.

A quick analogy to this is the way a long ladder can be extended or folded depending on our needs, without physically shortening its metal parts.

Myosin involvement

Fortunately, recent research offers a good idea of ​​how this slippage works. The theory of the sliding filament has been modified to include how myosin is able to pull actin to shorten the length of the sarcomere.

In this theory, the globular head of myosin is located near actin in an area called the S1 region. This region is rich in segments with hinges that can bend and thus facilitate contraction.

The flexion of S1 may be the key to understanding how myosin is able to"walk"along the actin filaments. This is achieved by binding cycles of the S1 myosin fragment, its contraction and its final release.

Union of myosin and actiba

When myosin and actin come together, they form extensions called"crossed bridges". These crossed bridges can be formed and break with the presence (or absence) of ATP, which is the energy molecule that makes contraction possible.

When ATP binds to the actin filament, it moves it to a position that exposes its myosin binding site. This allows the globular head of the myosin to attach to this site to form the cross bridge.

This union causes the phosphate group of ATP to dissociate, and thus myosin initiates its function. Then, the myosin enters a state of lower energy where the sarcomere can be shortened.

To break the cross bridge and allow again the binding of myosin to the actin in the next cycle, the binding of another ATP molecule to the myosin is necessary. That is, the ATP molecule is necessary for both contraction and relaxation.

Histology

The histological sections of the muscle show the anatomical characteristics of the sarcomeres. Thick filaments, composed of myosin, are visible and are represented as the A band of a sarcomere.

Thin filaments, composed of actin, bind to a protein on the Z disk (or Z line) called alpha-actinin, and are present along the entire length of band I and a part of band A.

The region where the thick and thin filaments overlap has a dense appearance, since there is little space between the filaments. This area where the thin and thick filaments overlap is very important for muscle contraction, since it is the place where the movement of the filament begins.

The thin filaments do not extend completely in bands A, leaving a central region of band A that only contains thick filaments. This central region of band A seems slightly lighter than the rest of band A, and is called zone H.

The center of zone H has a vertical line called line M, where accessory proteins hold together the thick filaments.

The main components of the histology of a sarcomere are summarized below:

Band A

Thick filament zone, composed of myosin proteins.

Zone H

Central zone of band A, without actin proteins superimposed when the muscle is relaxed.

Band I

Zone of thin filaments, composed of actin proteins (without myosin).

Z disks

These are the boundaries between adjacent sarcomeres, formed by actin-binding proteins perpendicular to the sarcomere.

Line M

Central zone formed by accessory proteins. They are located in the center of the thick filament of myosin, perpendicular to the sarcomere.

As mentioned earlier, shrinkage occurs when the thick filaments slide along the fine filaments in rapid succession to shorten the myofibrils. However, a crucial distinction to remember is that the myofilaments themselves do not contract; it is the sliding action that gives them their power to shorten or lengthen.

References

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