The muscular system consists of a set of more than 650 muscles which give shape and support to the human body. Many of them can be controlled at will, which allows us to exert sufficient force on the skeleton to move. For some authors, the muscular apparatus is composed only of tissues that can be moved at will, while for others, the involuntary muscles (heart and viscera, for example) are also included in this conglomerate.
Either way, muscles allow us to move from movement to life itself because, without going any further, the heart muscle tissue (myocardium) pumps 70 milliliters of blood with each beat, that is, say whole body blood in just over a minute. Throughout our lives, this titanic tissue can contract about 2 billion times.
Whether it is pumping blood or making conscious movement, every muscle in our body has a specific, essential and irreplaceable function. Today we come to talk to you about sarcomere, The anatomical and functional unit of striated muscles.
Types of muscles
The basic properties of all muscle tissue are contractility, excitability, extensibility and elasticity. This allows the muscles to receive and respond to stimuli, stretch, contract and return to their original state so that no damage occurs. According to these qualities, the muscular system allows the production of bodily movements (with the joints), the contraction of blood vessels, the heart and the production of peristaltic movements, the maintenance of posture and mechanical protection, among others.
In addition to these common features, it should be noted that there are 3 essential types of musculature. We define them succinctly:
- Smooth muscles: involuntary contraction. It is the most primitive type and constitutes the lining of the viscera, in addition to appearing on the walls of blood and lymphatic vessels.
- Striated muscle tissue: is the most abundant and has its origin and its insertion in the bones. These are the voluntary muscles.
- Heart muscle tissue: It is found exclusively in the wall of the heart. It is not under voluntary control because it works automatically.
This initial distinction is essential, because the functional unit that belongs to us here (the sarcomere) is only present in the striated muscles. Now yes, let’s 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. These are a series of repeating units that give rise to morphological structures called myofibrils, and are perhaps the most ordered macromolecular structures of all eukaryotic cell typology. We are going to introduce a lot of terms quickly, so don’t despair, we’ll go through parts.
The cells that make up striated muscle are called myofibers and are long, cylindrical structures surrounded by a plasma membrane called a sarcolemma.. They are very long cell bodies, ranging from several millimeters to over a meter (10 and 100 microns in diameter) and have peripheral nuclei in the cytoplasm, which gives the cell a large space for contractile machines.
If we go into specificity, we will see that muscle myofibers contain in their sarcoplasm (cell cytoplasm) several hundred or thousands of myofibrils, a lower level of morphological order. In turn, each myofibril contains myofilaments, at the rate of approximately 1,500 myosin filaments and 3,000 actin filaments. To give you a simple idea, we are talking about an electricity “cable” (myofibers) which, if cut crosswise, contains thousands of much smaller cables inside (myofibrils).
It is in this scale that we find the sarcomeres because, as we have already said, they are the repeated functional unit that makes up the myofibrils.
Characteristics of the sarcomere
In the composition of the sarcomere two biological elements of essential importance that we have already mentioned are distinguished: actin and myosin. Actin is one of the most essential globular proteins in living things, as it is one of the 3 main components of the cytoskeletons (cell skeleton) of cells in 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 such features will be explored on another occasion.
The sarcomere has a very complex structure, because it is composed of a series of “bands” which move in the contractile movement. These are:
- Band A: band composed of thick filaments of myosin and fine actin. Inside are the H and M zones.
- Band I: band composed of fine actin filaments.
- Z discs: here the adjacent actins are attached and continuity is maintained with the next sarcomere.
Thus, the sarcomere can be referred to as the region of a myofibril located between two consecutive Z discs, which is approximately two microns in length. Between the Z disks is a dark section (corresponding to band A) where, during contraction, the thick myosin filaments and fine actin glide over each other, varying the size of the sarcomere.
Besides the typical contractile proteins, actin and myosin, the sarcomere contains two other major groups. We tell you succinctly.
One of the protein accessory groups present in the sarcomere is the regulatory proteins, In charge of initiating and stopping contractile movement. Perhaps the best known of all is tropomyosin, with a coiled structure made up of two long polypeptides. This protein regulates, with tropin, the interactions of actin and myosin during muscle contraction.
Another block is the structural proteins, which allow this very complex cell network to stay in order and not to collapse. Most important of all is titin, the largest known protein, With a molecular mass of 3 to 4 million Daltons (Da). This essential molecule works by connecting the line of the Z disk with the line of the M zone at the sarcomere, aiding in the transmission of force in the Z line and releasing the tension in the region of the I band. It also limits the margin of movement of the sarcomere when requested.
Another essential structural protein is dystrophin or nebulin. The latter binds to muscle actin, regulating the extension of fine filaments. In short, they are proteins that enable band and disc communication to the sarcomere, promoting the efficient production of the complex and efficient contractile movement that characterizes muscles.
It is interesting to know that when the transcription of any of these proteins fails, very serious health problems can occur. For example, some titin gene mutations have been linked to familial hypertrophic cardiomyopathy, A congenital heart disease that affects 0.2% to 0.5% of the general population.
Another of the most notorious diseases with regard to the musculature is Duchenne muscular dystrophy, Caused by a defective dystrophin gene. This is associated with intellectual disability, fatigue, motor problems and general incoordination which usually results in the death of the patient from associated respiratory failure. As surprising as it may sound, something as simple as a faulty protein synthesis can lead to life-threatening conditions.
If you’ve learned anything today, it’s probably that the sarcomere is an extremely complex and organized functional unit, the structure attempts to strike a balance between strong and efficient contraction and biological viability (i.e. -to say that everything follows its place once the movement has occurred).
Between the bands, the discs and the lines, one thing is clear to us: the sarcomeres could encompass a book only with their anatomical organization. in the organization of actin, myosin and other associated proteins is the key to the movement of living beings.
- Aranya-Suárez, M. and Patten, SB (2011). Musculoskeletal disorders, psychopathology and pain. Musculoskeletal disorders Psychopathology, 1.
- Band, A., Zone, H., Band, I., & Discs, Z. sarcomere: Structure and Parts, Functions, and Histology.
- Bonjorn, M., Rosines, MD, Sanjuan, A. and Forcada, P. (2009). Injuries to the soft parts due to friction. Biomechanics, 17 (2), 21-26.
- Duchenne muscular dystrophy, Medlineplus.gov. Collected January 10 at https://medlineplus.gov/spanish/ency/article/000705.htm#:~:text=La%20distrofia%20muscular%20de%20Duchenne,una%20prote%C3%ADna%20en%20los% 20m% C3% BAsculos).
- Gómez Díaz, I. (2013). Titin in the genetic diagnosis of familial heart disease.
- Marrero, RCM, Rull, IM and Cunillera, MP (2005). Clinical biomechanics of tissues and joints of the musculoskeletal system. Masson.
- Martin-Dantas, IH, da Silva-Borges, EG, Gastélum-Cuadras, G., Lourenço-Fernandes, M., and Ramos-Coelho, R. (2019). Concentrations and relative mobility of titin isoforms after three different flexibility trainings. Chihuahua Technoscience, 13 (1), 15-23.
- Mora, IS (2000). Muscular system.
- Roses Cabrera, RA (2006). Study of the mechanical properties of the titin protein.