Sarcomere: parts, functions and associated diseases.

The sarcomeres are one of the fundamental parts of the muscular system; let's see how they work.

The muscular system comprises a set of more than 650 muscles that give shape and support to the human body. Many of these can be controlled at will, allowing us to exert sufficient force on the skeleton to move. For some authors, the muscular apparatus is composed only of those tissues that can move at will, while for others, involuntary muscles (heart and viscera, for example) are also included in this conglomerate.

Be that as it may, muscles allow us to move and even to live, since, without going any further, the muscular tissue of the heart (myocardium) pumps 70 milliliters of Blood in each heartbeat, that is to say, all of the body's blood in little more than a minute. Over the course of our lifetime, this titanic tissue may contract some 2 billion times.

Whether pumping blood or performing a conscious movement, each and every muscle in our body has a specific, essential and irreplaceable function. Today we come to talk to you about the sarcomerethe anatomical and functional unit of the striated musculature.

Types of muscles

The basic properties of all muscle tissue are contractility, excitability, extensibility and elasticity.. This allows muscles to receive and respond to stimuli, stretch, contract and return to their original state so that no damage occurs. Based on these qualities, the muscular system makes possible the production of body movements (together with the joints), the contraction of blood vessels, the heart and production of peristaltic movements, the maintenance of posture and mechanical protection, among many other things.

In addition to these common characteristics, it is necessary to point out that there are 3 essential types of musculature. We define them briefly:

• Smooth musculature: of involuntary contraction. It is the most primitive type and constitutes the lining of the viscera, as well as being present in the walls of blood and lymphatic vessels.
• Striated muscle tissue: it is the most abundant and has its origin and insertion in the bones. They are the voluntary muscles.
• Cardiac muscular tissue: it is found exclusively in the wall of the heart. It is not under voluntary control, as it functions automatically.

Making this initial distinction is essential, since the functional unit that concerns us here (the sarcomere) is only present in striated muscle. Now, let's take a look at its properties.

What is a sarcomere?

The sarcomere is defined as the functional and anatomical unit of the striated muscle, i.e., the voluntary. They are a series of repeated units that give rise to morphological structures called myofibrils, and are perhaps the most ordered macromolecular structures in all eukaryotic cell typology. We are going to introduce many terms quickly, so do not despair, as we will go in parts.

The cells that form striated muscle are called myofibrils, and are long cylindrical structures surrounded by a plasma membrane known as sarcolemma.. They are very long cell bodies, ranging from several millimeters to more than a meter (10 and 100 µm in diameter) and have peripheral nuclei in the cytoplasm, which gives the cell a large amount of space for the contractile machinery.

If we advance in specificity, we will see that muscle myofibers contain in their sarcoplasm (cell cytoplasm) several hundreds or thousands of myofibrils, a lower level of morphological arrangement. In turn, each myofibril contains myofilaments, in the proportion of about 1,500 myosin filaments and 3,000 actin filaments. To give you a simple idea, we are talking about a "wire" of electricity (myofiber) that, if cut transversely, contains thousands of much smaller wires inside (myofibril).

It is on this scale where we find the sarcomeres because, as we have said before, they are the functional repeated unit that makes up the myofibrils.

Characteristics of the sarcomere

In the composition of the sarcomere two biological elements of essential importance stand out, which we have already mentioned: actin and myosin.. Actin is one of the most essential globular proteins in living beings, since it is one of the 3 main components of the cytoskeletons (cellular skeleton) of the cells of eukaryotic organisms.

On the other hand, myosin is another protein which, together with actin, allows muscle contraction, since it represents up to 70% of the total proteins present in this tissue. It is also involved in cell division and vesicle transport, although these functionalities will be explored on another occasion.

The sarcomere presents a very complex structure, since it is composed of a series of "bands" that move in contractile motion.. These are the following:

• Band A: band composed of thick myosin and thin actin filaments. In its interior are the H and M zones.
• Band I: band composed of thin actin filaments.
• Z-discs: here the adjacent actins are bound together and continuity with the subsequent sarcomere is maintained.

The sarcomere can thus be referred to as the region of a myofibril between two consecutive Z-discs, which is approximately two microns in length. Between the Z-discs is a dark section (corresponding to the A-band) where, upon contraction, the thick myosin filaments and thin actin filaments slide over each other, varying the size of the sarcomere.

A question of proteins

Apart from the typical contractile proteins, actin and myosin, the sarcomere contains two other major groups. We will tell you about them briefly.

One of the accessory protein groups present in the sarcomere are the regulatory proteins, which are responsible for initiating and stopping the contractile process.which are responsible for initiating and stopping contractile movement. Perhaps the best known of all is tropomyosin, with a coiled structure formed by two long polypeptides. This protein regulates, together with tropin, the interactions of actin and myosin during muscle contraction.

We also observe in another block the structural proteins, which allow this very complex cellular framework to remain in order and not collapse. The most important of these is titin, the largest protein known to man, with a molecular mass ofwith a molecular mass of 3 to 4 million Daltons (Da). This essential molecule acts by connecting the Z-disk line with the M-zone line in the sarcomere, contributing to the transmission of force in the Z-line and releasing tension in the I-band region. It also limits the range of sarcomere motion when the sarcomere is tensioned.

Another essential structural protein is dystrophin or nebulin. The latter binds to muscle actin, regulating the extension of the thin filaments. In short, they are proteins that allow the communication of bands and discs in the sarcomere, promoting the efficient and complex contractile movement that characterizes muscles.

Related pathologies

It is interesting to know that, when the transcription of any of these proteins fails, very severe health disorders can occur. For example, Some titin gene mutations have been associated with familial hypertrophic cardiomyopathy, a congenital heart disease affecting 0.2% to 0.5% of the general population.a congenital heart disease that affects 0.2% to 0.5% of the general population.

Another of the most talked-about diseases of the musculature is Duchenne muscular dystrophycaused by a defective gene for dystrophin. It is associated with intellectual disability, fatigue, motor problems and a general incoordination that usually ends with the death of the patient due to associated respiratory failure. Surprising as it may seem, something as simple as a defect in the synthesis of a protein can translate into fatal pathologies.

Summary

If you have learned anything today, it is probably that the sarcomere is an extremely complex and organized functional unit, whose structure tries to find the balance between a strong and efficient contraction and biological viability (i.e. that everything remains in place once the movement has taken place).

Between bands, discs and lines, one thing is clear: the sarcomeres could cover a book with their anatomical organization alone. In the organization of actin, myosin and other associated proteins lies the key to movement in living beings.

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