Muscle fiber: what it is, its parts and functions

The locomotor system refers to the set of organs and structures that allow us to move in three-dimensional space and maintain posture despite the force of gravity. Without it we would surely be like a worm or a small nemertino, stuck to the ground and performing movements in the horizontal plane slowly and expensive, with a flattened body and basic morphology. Can you imagine what life would be like for a human being without muscles and skeletons?

The locomotor system includes the osteoarticular system (bones, joints and ligaments) and the muscular system (muscles and tendons). This real work of biomechanics allows us to interact with the environment and in turn to support the various organs of the body without collapsing. Something as simple as getting out of bed would be impossible without the bones and muscles involved.

Today we are reducing significantly. We have already dealt with the skeletal system, isolated parts of the skeleton, human musculature, face and many other thematic fronts most associated with the musculoskeletal system. In this case, we are approaching a tissue level, much more basic, but just as important as the most complex system of living things: stay with us if you want to know everything about muscle fiber.

    What are Muscles?

    Muscle fibers, as the name suggests, make up muscles. So, to understand them, we have to take a little trip through the muscular system in general and the types of muscles that we can observe. We don’t linger.

    The muscular system refers, in general, to all the muscles that can be voluntarily contracted by the body. Other authors argue that heart muscles or those that promote peristaltic movements in the intestines should also be included in this group, but these are usually left out, as their action is independent of individual desire.

    If we only count the muscles associated with the bones that voluntarily respond to commands from the brain, the muscular system looks like it is made up of around 650 muscle units. If we also take into account the involuntary muscles, this figure would easily increase above 800. In any case, there are 3 types of muscles in our body:

    • Skeletal muscles: are those that form the muscles themselves, as they are attached to the bones and consciously contract. They are called striated, because under the microscope the muscle fibers that compose them are observed.
    • Smooth muscles: they look smooth and are automatically controlled by the nervous system. They are found in the walls of the blood and lymphatic vessels, digestive tract, respiratory tract, bladder, bile ducts, and uterus.
    • Heart muscle: corresponds to the muscle fibers that line the heart. He is an involuntary type, and thanks to him there is beating and pumping of blood.

    About 40% of the weight of an adult human is skeletal muscle tissue. In contrast, only 10% (at most) are smooth muscles. There are many more skeletal muscles than smooth muscles, but they are all essential for the maintenance of the individual over time.

    By following these lines, we get a little idea of ​​what the muscular apparatus is and what types of muscles make up (or stay away from) it. Now yes, we are ready to dissect all the muscle fiber.

    What is a muscle fiber?

    Muscle fiber (or skeletal myocyte) is a multinucleate or syncytium. The latter term refers to a cell body that has multiple nuclei, due to the fusion of multiple cells. Since most cells of eukaryotic multicellular organisms have a single nucleus and a well-defined cytoplasm, the syncytium is a special structure worth mentioning.

    Continuing with the classical definition, we can say that a muscle fiber is the type of cell that makes up the tissue of skeletal or striated muscles, that is, the one that attaches to bones and causes conscious movement in humans. The main characteristic of this cell body will therefore be contractility: the ability to shorten its own length by initiating work to do so.

    From there, things get complicated. It is best to imagine the cross section of a muscle as a large cable in which many small cables have been stored. We explained ourselves in the following lines.

    The organization of muscle fibers

    If you are doing the cross section of a circular muscle, the first thing you will find in the outermost part is the epimysis., A layer of connective tissue that is in direct contact with the external environment. If you look closely, you will see that in the large circle that is the cross section, there are other smaller circles grouped together. These are the fascicles, which are surrounded by another layer, known as the perimeter.

    Inside the fascicle we find the muscle fibers themselves, arranged in a bundle. Review what we’ve learned so far:

    Muscle cut (epimesis)> various fascicles (perimisis)> Muscle fibers

    Using an analogy, it is as if several smaller but also large cables (fascicles) were inserted into the sheath of a large diameter cable (muscle) and inside these are really the conductive elements. (muscle fibers). Is it a little clearer?

      The anatomy of the muscle fiber

      The complexity didn’t stop here, we’ve already described where the muscle fiber is, but not what it’s made up of. As a cell it must have organelles, cytoplasm and nucleus, Right? This is true, but in this case the myofibrils occupy a large part of the cell space, completely changing the typical arrangement of their structures.

      Let’s start with the basics: muscle fiber has a plasma membrane, like other cells in living things. It is a semi-permeable and lipid membrane, however, it stretches in the form of trabeculae inside the cell. This membrane is known as the sarcolemma.

      Like any other cell, muscle fiber also needs a cytoplasm in which other substances are lodged, and in this case, it is known as sarcoplasm. It is a solution phase based on water, ions and diffusible small molecules, which surrounds fixed macromolecular structures, the myofibrils.

      Like any cell body, muscle fibers also need energy. Therefore, mitochondria appear in myofibrils, well compacted and in contact with each other. The mitochondria are located practically attached to the myofibrils, as they must provide all the energy necessary for the process of contraction., Which is not really little. The sarcoplasmic reticulum also surrounds the myofibrils, as it stores the calcium needed to start the muscle contraction cascade reaction.

      The sarcoplasm (remember it is analogous to the cytoplasm) of a muscle fiber contains a huge amount of myofibrils: we are talking about several hundred or even thousands of them. Each myofibril, on its own, contains approximately 1,500 myosin and 3,000 actin filaments. These biopolymers are responsible for the contraction of the myofibril, and therefore of the muscle fiber, to reach the whole muscle.

      Finally, it is essential to note that this type of cell is part of a stable tissue with very little nucleus rotation. Therefore, the rate of replacement of muscle fibers does not exceed 1-2% per week, a very low figure compared to replacement rates of the outermost layer of the epidermis, for example.

      There are slow twitch and fast twitch fibers, which will determine the functionality and efficiency of muscle tissue depending on the task at hand. We will explore this physiological diversity on future occasions.

      summary

      What did you think? It is very curious to know that at the microscopic level, some of the cells in our body have undergone process of drastic changes in order to acquire specialized functionalities. Muscle fiber is a clear example of this: is the product of several cells, has several nuclei, is separated from the middle by a sarcolemma and in its sarcoplasm houses thousands of myofibrils, So that its contraction can occur.

      Thanks to these physiological specializations, many cells are able to perform tasks optimally. specialized inconceivable without them. Without muscle fiber, the movement and permanence of the human being as we know him today in the three-dimensional environment would be completely impossible.

      Bibliographical references:

      • González Montesinos, JL, Martínez González, J., Mora Vicente, J., Salt Chamorro, G., and Álvarez Fernández, I. (2004). Back pain and muscle imbalances.
      • Marrero, RCM, Rull, IM and Cunillera, MP (2005). Clinical biomechanics of the tissues and joints of the musculoskeletal system. Masson.
      • Mora, IS (1989). Muscular system.
      • Organization of skeletal muscles: fibers. Collected February 22 from elsevier.com/es-es/connect/medicina/edu-organizacion-del-musculo-esqueletico-las-fibras
      • Sanabria, NS and Patiño, AMO (2013). Biomechanics of the shoulder and the physiological basis of Codman exercises. CES Journal of Medicine, 27 (2), 205-217.

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