Muscle fiber: what it is, 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 gravitational force. Without it, we would surely be like an earthworm or a small nemertean, glued 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 encompasses 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 in turn support the different organs of the body without them collapsing. Something as simple as getting out of bed would be impossible without the bones and muscles involved.

Today we are drastically down the scale. We have already covered the bone system, isolated parts of the skeleton, the human musculature, the facial and many other thematic fronts more associated with the locomotor system. In this case, we are approaching a tissue level, much more basic, but just as important as the more complex system of living beings: stay with us if you want to know everything about

muscle fiber.

• Related article: "Locomotor system: what is it, parts and characteristics"

What are muscles?

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

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

If we only count the muscles associated with bones that respond voluntarily to brain commands, we would say that the muscular system is made up of about 650 muscle units. If we also take involuntary muscles into account, this figure would easily rise above 800. Be that as it may, in our body there are 3 types of muscles:

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

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

After these lines, we get a slight idea of ​​what the muscular apparatus is and what types of muscles make it up (or are left out). Now we are ready to fully dissect the muscle fiber.

What is a muscle fiber?

The muscle fiber (or skeletal myocyte) is a multinucleated cell or syncytium. This last term refers to a cell body that has several nuclei, due to the fusion of several cells. Since most cells in eukaryotic multicellular organisms have a single nucleus and a well-defined cytoplasm, the syncytium is a special structure worthy of mention.

Continuing with the classical definition, we can say that a muscle fiber is the cell type that makes up the skeletal or striated muscle tissue, that is, one that 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, triggering work in 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 ourselves in the following lines.

The organization of muscle fibers

If you make the cross section of a circular muscle, the first thing you will find in the outermost part is the epimysium, 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 several more cables were inserted into the sheath of a large diameter cable (muscle). small, but also large (fascicles) and within these is where the conductive elements would really be (fibers muscular). So has it been a little clearer?

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The anatomy of the muscle fiber

The complexity did not end there, as we have described where the muscle fiber is located, but not what it is composed of. As a cell that it is, it must present organelles, cytoplasm and nucleus, truth? Yes, but in this case, the myofibrils occupy a large part of the cell space, completely changing the typical arrangement of their structures.

We start 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 within the cell. This membrane is known as the sarcolemma.

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

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

The sarcoplasm (remember that it is the analogue to the cytoplasm) of a muscle fiber has a huge amount of myofibrils inside: we are talking about several hundred or even thousands of them. Each myofibril, by itself, contains about 1,500 myosin and 3,000 actin filaments. These biopolymers are responsible for the contraction of the myofibril, and therefore of the muscle fiber, until reaching the entire muscle.

Finally, it is essential to emphasize that this cell type is part of a stable tissue with very little nucleus rotation. Therefore, the rate of muscle fiber turnover does not exceed 1-2% per week, a very high figure. low 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 effectiveness of muscle tissue depending on what task is to be performed. We will explore this physiological diversity on future occasions.

Summary

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

Thanks to these physiological specializations, many cells are capable of 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.

Bibliographic references:

• González Montesinos, J. L., Martínez González, J., Mora Vicente, J., Salto Chamorro, G., & Álvarez Fernández, E. (2004). Back pain and muscle 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: fibers. Collected on February 22 at elsevier.com/es-es/connect/medicina/edu-organizacion-del-musculo-esqueletico-las-fibras
• Sanabria, N. S., & Patiño, A. M. OR. (2013). Biomechanics of the shoulder and physiological bases of the Codman exercises. CES Medicine Magazine, 27 (2), 205-217.