Muscle fiber: what it is, parts and functions.

Summary of the functional and anatomical characteristics of the muscle fibers of the human body.

The locomotor apparatus refers to the set of organs and structures that allow us to move in three-dimensional space and maintain our posture despite the gravitational force. Without it, we would probably be like an earthworm or a small nemertean, stuck to the ground and performing movements in the horizontal plane in a slow and costly way, with a flattened body and basic morphology. Can you imagine what human life would be like without muscles and skeleton?

The locomotor system includes the osteoarticular system (bones, joints and ligaments) and the muscular system (muscles and tendons). This true work of art of biomechanics allows us to interact with the environment and at the same time support the different 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 scale down drastically. We have already covered the skeletal system, parts of the skeleton in isolation, the musculature of the human, the facial and many more thematic fronts associated with the locomotor system. In this case, we approach a tissue level, much more basic, but just as important as the most complex system of the living beings: stay with us if you want to know all about the muscle fiber.

What are muscles?

Muscle fibers, as their name suggests, make up the muscles. So, to understand them, we must take a little trip through the muscular system in general and the types of musculature that can be observed. We will not delay.

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

If we count only the muscles associated with bones that respond voluntarily to brain commands, we would say that the muscular system is composed of about 650 muscle units. If we also take into account the involuntary muscles, this figure would easily increase to over 800. In any case, there are 3 types of muscles in our body:

• Skeletal muscles: these are the ones that form the musculature itself, as they are attached to bones and contract consciously. They are called striated, because the muscle fibers that compose them are observed under the microscope.
• Smooth muscles: they have a smooth appearance and are automatically controlled by the nervous system. They are found in the walls of Blood and lymphatic vessels, the digestive tract, respiratory tract, bladder, bile ducts and uterus.
• Cardiac muscle: corresponds to the muscle fibers that line the heart. It is involuntary, and it is thanks to it that the heart beats and pumps blood.

Approximately 40% of the weight of an adult human being corresponds to skeletal muscle tissue.. On the other hand, only 10% (at most) is smooth muscle. There are many more skeletal muscles than smooth muscles, but all are essential to maintain the individual over time.

After these lines, we have a slight idea of what the muscular apparatus is and what types of muscles compose it (or are left out). Now we are ready to dissect the muscle fiber in its entirety.

What is a muscle fiber?

The muscle fiber (or skeletal myocyte) is a multinucleated cell or syncytium. The latter term refers to a cell body that has several nuclei, due to the fusion of several cells. As most cells in eukaryotic multicellular organisms have a single nucleus and a well-demarcated cytoplasm, the syncytium is a special structure worth mentioning.

Continuing with the classical definition, we can say that a muscle fiber is the cell type that makes up the tissue of the skeletal or striated musculature, i.e., that which is attached to the bones and causes conscious movements in humans. The main characteristic of this cell body will therefore be contractility: the ability to shorten its own length by triggering a work by doing so.

From here, things get a bit complex. It is best to imagine the cross section of a muscle as a large cable in which many other small cables have been stored. We explain in the following lines.

The organization of muscle fibers

If you make a cross section of a circular muscle, the first thing you will find in the outermost part is the epimysium, a layer of connective tissue.a layer of connective tissue that is in direct contact with the external environment. If you look more closely, you will see that within the large circumference that is the cross section, there are other smaller circumferences grouped together. These are the fascicles, which are surrounded by another layer, known as the perimysium.

Within the fascicle we find the muscle fibers themselves, arranged in a bundle.. Reviewing what we have learned so far:

Muscle cut (epimysium) > various fascicles (perimysium) > Muscle fibers.

Making an analogy, it is as if inside the sheath of a large diameter cable (muscle) various smaller but also large cables (fascicles) were introduced and inside these is where the conductive elements (muscle fibers) would actually be. Is this a little clearer?

The anatomy of the muscle fiber

The complexity did not end there, because we have described where the muscle fiber is located, but not what it is composed 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 cellular space, completely changing the typical arrangement of its structures.

Let us begin with the basics: the muscle fiber has a plasma membrane, like the rest of the cells of living beings. It is a semi-permeable and lipidic membrane, however, it extends in the form of trabeculae inside the cell. This membrane is known as sarcolemma..

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

Like all cell bodies, muscle fibers also need energy. Therefore, mitochondria appear between the myofibrils, closely packed and in contact with each other. The mitochondria are located practically glued to the myofibrils, since they need to provide all the energy necessary for the contraction process, which is not exactly little.which is not exactly little. The sarcoplasmic reticulum also surrounds the myofibrils, since it stores the calcium necessary to start the cascade reaction of muscular contraction.

The sarcoplasm (let us remember that it is the analogue of the cytoplasm) of a muscle fiber presents an enormous quantity of myofibrils in its interior: we are talking about several hundreds or even thousands of them. Each myofibril alone contains about 1,500 myosin filaments and 3,000 actin filaments. These biopolymers are responsible for the contraction of the myofibril, and therefore of the muscle fiber, until it reaches the entire muscle.

Finally, it is essential to emphasize that this cell type is part of a stable tissue with very little rotation of nuclei.. For this reason, the turnover rate of muscle fibers does not exceed 1-2% per week, a very low figure when compared to the turnover rates of the most superficial layer of the epidermis, for example.

There are slow twitch and fast twitch fibers, which will determine the functionality and efficiency of the muscle tissue depending on the task to be performed. 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 gone through processes of drastic changes in order to acquire specialized functionality. The muscle fiber is a clear example of this: is the product of several cells, has several nuclei, is separated from the medium by a sarcolemma and within its sarcoplasm houses thousands of myofibrilsin order to allow its contraction to take place.

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

Bibliographical references:

• González Montesinos, J. L., Martínez González, J., Mora Vicente, J., Salto Chamorro, G., & Álvarez Fernández, E. (2004). Back pain and muscular imbalances.
• Marrero, R. C. M., Rull, I. M., & Cunillera, M. P. (2005). Clinical biomechanics of the tissues and joints of the locomotor system. Masson.
• Mora, I. S. (1989). Muscular system.
• Organization of skeletal muscle: the fibers. Retrieved February 22 from elsevier.com/en-eng/connect/medicine/edu-organization-of-skeletal-muscle-the-fibers.
• Sanabria, N. S., & Patiño, A. M. O. (2013). Biomechanics of the shoulder and physiological basis of Codman exercises. Revista CES Medicina, 27(2), 205-217.