Sarcomere: parts, functions and associated diseases

The muscular system comprises a set of more than 650 muscles that shape and support the human body. Many of these can be controlled at will, allowing us to exert enough 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, the involuntary muscles (heart and viscera, for example), are also included within this conglomerate.

Be that as it may, the muscles allow us from movement to life itself because, without going any further, the muscle tissue of the The heart (myocardium) pumps 70 milliliters of blood with each beat, that is, all of the body's blood in just over a heartbeat. minute. Throughout our entire lives, this titanic tissue can contract some 2,000 million 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 about the sarcomere, the anatomical and functional unit of striated musculature.

• Related article: "Muscular system: what it is, parts and functions"

muscle types

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

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

• Smooth muscle: involuntary contraction. It is the most primitive type and constitutes the lining of the viscera, in addition to appearing 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 muscle tissue: Found exclusively in the heart wall. It is not under voluntary control, as it works automatically.

Making this initial distinction is essential, since the functional unit that concerns us here (the sarcomere) is only present in striated musculature. Now yes, let's see its properties.

What is a sarcomere?

The sarcomere is defined as the functional and anatomical unit of striated muscle, that is, 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 the entire typology eukaryotic cell. We are going to introduce many terms quickly, so do not despair, as we will go in parts.

The cells that make up striated muscle are called myofibers, and they are long cylindrical structures surrounded by a plasma membrane known as the sarcolemma.. They are very long cell bodies, they can range from several millimeters to more than a meter (10 and 100 µm in diameter) and have a few peripheral nuclei in the cytoplasm, giving the cell plenty of room for machinery contractible.

If we go further in specificity, we will see that muscle myofibers contain several hundred or thousands of myofibrils in their sarcoplasm (cell cytoplasm), a lower level of morphological ordering. 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 an electricity "cable" (myofiber) that, if cut across, it 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 repeating unit that makes up the myofibrils.

Sarcomere Characteristics

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

On the other hand, myosin is another protein that, 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 such functionalities will be explored on another occasion.

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

• Band A: band composed of thick myosin filaments and thin actin filaments. Inside are zones H and M.
• Band I: band composed of thin actin filaments.
• Z discs: Here adjacent actins are attached and continuity with the succeeding sarcomere is maintained.

Thus, the region of a myofibril located between two consecutive Z discs can be called the sarcomere, which means an approximate length of two microns. Between the Z discs there is a dark section (corresponding to the A band) where, when contracted, the thick myosin filaments and thin actin filaments slide past each other, varying the size of the sarcomere.

• You may be interested in: "Neuromuscular junction: the bridge between neuron and muscle"

protein question

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

One of the protein accessory groups present in the sarcomere are regulatory proteins., responsible for the initiation and stop of the contractile movement. Perhaps the best known of all is tropomyosin, with a coiled structure made up of 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 highly complex cellular network to remain in order and not collapse. The most important of them all is titin, the largest protein known, with a molecular mass of 3 to 4 million Daltons (Da). This essential molecule works by connecting the line of the Z disc with the line of the M zone in the sarcomere, contributing to the transmission of force in the Z line and releasing tension in the region of the i band It also limits the range of movement of the sarcomere when it is stressed.

Another of the essential structural proteins is dystrophin or nebulin. The latter binds to muscle actin, regulating the extension of the fine filaments. In summary, they are proteins that allow the communication of bands and discs in the sarcomere, encouraging the the highly complex and effective contractile movement that characterizes muscles can be produced efficiently.

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 that affects 0.2% to 0.5% of the general population.

Another of the most notorious diseases as far as the muscles are concerned is Duchenne muscular dystrophy, caused by a defective gene for dystrophin. This is associated with intellectual disability, fatigue, motor problems, and general incoordination that usually ends with the death of the patient due to associated respiratory failure. Although it may seem surprising, something as simple as a defect in the synthesis of a protein can translate into deadly pathologies.

• You may be interested in: "Duchenne muscular dystrophy: what it is, causes and symptoms"

Summary

If you have learned something today, it is surely that the sarcomere is an extremely complex and organized functional unit, whose structure tries to Finding the balance between strong and efficient contraction and biological viability (that is, everything still in place once the contraction has occurred). motion).

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

Bibliographic references:

• Araña-Suárez, M., & Patten, S. b. (2011). Musculoskeletal disorders, psychopathology and pain. Musculoskeletal Disorders Psychopathology, 1.
• Banda, A., Zona, H., Banda, I., & Discos, Z. Sarcomere: Structure and Parts, Functions and Histology.
• Bonjorn, M., Rosines, M. D., Sanjuan, A., & Forcada, P. (2009). Friction soft tissue injuries. Biomechanics, 17(2), 21-26.
• Duchenne muscular dystrophy, Medlineplus.gov. Collected on January 10 in https://medlineplus.gov/spanish/ency/article/000705.htm#:~:text=La%20distrofia%20muscular%20de%20Duchenne, a%20prote%C3%DNA%20in%20the%20m%C3%Bascules).
• Gomez Diaz, I. (2013). Titin in the genetic diagnosis of familial heart disease.
• Marrero, R. c. M., Rull, I. M., & Cunillera, M. Q. (2005). Clinical biomechanics of the tissues and joints of the locomotor system. Masson.
• Martin-Dantas, E. H., da Silva-Borges, E. G., Gastélum-Cuadras, G., Lourenço-Fernandes, M., & Ramos-Coelho, R. (2019). Concentrations and relative mobility of titin isoforms after three different flexibility training sessions. Technoscience Chihuahua, 13(1), 15-23.
• Mora, I. S. (2000). Muscular system.
• Rosas Cabrera, R.A. (2006). Study of the mechanical properties of the protein titin.