Muscular system: what is it, parts and functions

Humans and the rest of the living beings on Earth navigate in a three-dimensional environment and, therefore, we require a powerful system skeletal and muscular that allows us to respond to external stimuli and overcome the force of gravity, among many other things.

It is interesting to know that it is estimated that there are more than 650 types of muscles in the human body (according to the established definition) and that, a process as seemingly simple as smiling, involves a total of 20 facial muscle structures to be carried out.

It is clear that we cannot live without a correct muscular system due to the psychomotor importance it harbors and, therefore, diseases such as Duchenne dystrophy (which progressively weaken this apparatus) have a long-term survival rate of about a 10%. Without proper muscle tissue function, vital tasks such as breathing and blood circulation can be seriously compromised.

With these data we wanted to demonstrate the very important functionality of this device, but there are many more terms and particularities enclosed in each of our muscles and tendons. Stay with us, because in the following lines we tell you everything you need to know about the muscular system.

• Related article: "Neuromuscular junction: the bridge between neuron and muscle"

What is the muscular system?

From a physiological point of view, the muscular system can be defined as the set of muscles (more than 600) that exist in the human body. This device stabilizes the body position in three-dimensional space, produces the movement of bone structures, regulates the volume of the organs, mobilizes substances within our body and plays an essential role in the production of heat, among many others things.

In the first place, we must emphasize that there is a clear disagreement between various bibliographic sources, since certain experts include within this system all tissue of nature muscle, while other sources include in this definition only muscles that can be controlled voluntarily by a living organism (the skeletal musculature). For informative reasons, we focus our attention on the broadest and most friendly definition of the term.

The muscle: the functional unit

According to clinical sources, a muscle can be defined as a tissue made up of muscle fibers endowed with a contractile capacity.

In general, we can summarize that the basic and general physiological properties of all muscle tissue are contractibility, excitability, extensibility and elasticity. All this allows the biomechanical action of living beings and other processes as basic as the heartbeat or respiration.

Types of muscle groups

As we have said, if we take into account the muscular tissues that are part of the organs, we can observe 3 different types of them. We tell you about them in the following lines.

1. Smooth muscle

It is described as visceral or involuntary. It forms the walls of the hollow viscera, such as the stomach and bladder, and those of various tubular ducts, such as the different units of the circulatory system, the respiratory system, the digestive tube, the respiratory system and the reproductive organs, for example.

Generally, smooth muscles cannot be controlled at will of the individualThey are automatically at the mercy of the nervous system. They contract slowly, are more extensible and able to stay in a state for a longer time, that is, they do not "tire" as easily as striated muscles.

2. Striated or skeletal muscle

According to certain authors, only this type of muscles would form the muscular system per se. Unlike smooth muscles, the striated is the one that shapes the body and is attached to the bones, mainly in the legs, arms, abdomen, chest, neck and face. These muscles are called "striated" because they are made up of alternating bands of light and dark tones.

These muscle fibers are innervated by the cranial or spinal nerves and, as we have said previously, are limited to the voluntary government of the individual. They can contract quickly and energetically, but they "tire" quickly and therefore they must rest between periods of great effort.

3. Heart muscle

As its name suggests, the heart muscle is located in the heart. As you can imagine Due to its functionality it is not under the control of individual will, which is why it is considered automatic.. This type of muscle tissue has four basic properties:

• Rhythm, or what is the same, the ability to generate periodic impulses.

• Conductivity, the ability to transmit these impulses throughout the myocardial tissue.

• Irritability, the property to respond to stimuli.

• Contractibility, the property to contract in response to a specific stimulus.

• You may be interested in: "Neuromuscular diseases: what are they, how are they treated, and examples"

Muscle nomenclature

It is interesting to know that muscles can be classified into a very extensive string of terminology according to multiple parameters. For example, based on its function, a muscle can be a supinator, pronator, adductor, levator, and tensor. Based on their shape, they are classified in terms such as triangular, square, pyramidal, rhomboid, and trapezoid.

Other classificatory criteria that we are not going to investigate are according to size, bone relationship, fiber direction, number of insertions or topographic relationship. To give you a general idea, there are more than 30 muscle types based on each and every one of the parameters described.

The mechanism of muscle contraction

Everything we've explained so far is really interesting, but how does it translate from a functional point of view? In the following lines we explain, in broad strokes, the mechanism of muscle contraction.

First, an action potential travels down a motor nerve to the end of the nerve, which leads to muscle fibers. The signal comes from the brain which, using electrical impulses carried through neurons and releasing of neurotransmitters (synapses), communicates the signal to the muscle that it must perform the action for which it is prepared.

At each end of the nerve, it releases a very special neurotransmitter: acetylcholine. Summing up a lot, we will say that acetylcholine allows the opening of sodium channels to muscle fibers and that, Indeed, the concentration of this sudden ion causes an action potential that stimulates muscle cells to release calcium. The release of calcium allows the reconfiguration of the fibers of actin and myosin, globular proteins essential for this tissue that, with the expenditure of ATP, generate the typical movement of muscle contraction.

Once this has been described, we must also note that there are two main types of contractions:

• Isometric: the contracted muscle fibers do not vary almost in length, but the muscle tone is intensified and there is no displacement.
• Isotonic: here the length of the muscle fiber is shortened by contraction, but the muscle tone remains almost constant and displacement occurs.

Functions of the muscular system

We have already covered the main structures of the muscular system and its method of action but, in these last lines, we see of special interest to elucidate the functionality of the muscular system. Among them, we find the following:

• The production of movement of the body by interacting with the joints.
• Contraction of the heart, blood vessels and the production of peristaltic movements (which allow the transport of food through the digestive system).
• Provide joint stability and maintain the postural shape of living beings in a three-dimensional space.
• Contribution of body heat when necessary, since energy expenditure is transformed into metabolic heat.
• Protection and physical resistance. Skeletal muscles correspond to 40% of the weight of an adult individual, which is why they act as protectors of organs and systems.
• It stimulates the correct flow in blood and lymphatic vessels, with all that this entails for the homeostasis of the organism.

In general, if we consider the muscular system in its strictest sense (only the skeletal or striated musculature), we could say that its main function is solely the production of voluntary movements in the body in response to some type of environmental stimulus. On the other hand, if we take into account all tissue of a muscular nature, its functionality extends to the own conception of life, because without the muscles of the heart the blood would not beat and, simply, we would exist.

Resume

Compressing the particularities of the muscular system in a few lines is a real challenge, because each small beam, each fiber and each globular protein included here provides enough information to write a book on its own alone. Anyway, we have given you a general picture about this system, from the typology of its tissues to its functionality, going through the mechanism of action at the chemical level.

If we want you to have an idea, this is the following: in its most closed sense, the system Muscle is one that is composed of voluntary muscles, that is, of a nature skeletal. In a broader sense, this apparatus is made up of more than 600 types of muscles, ranging from the tissues that are attached to the bones to which form the various organs essential for the survival.

Bibliographic references:

• Of education, t. R. D. O., & lomce, f. S. (2020). The muscular system. Structure and functions. Particular characteristics of the evolutionary period corresponding to the stage. Considerations to keep in mind in physical education class. Summary Syllabus of Secondary Physical Education Oppositions (LOMCE) Volume II: Access to the body of Secondary Education teachers.
• Diniz, G. P. (2003). Muscular system.
• Bones, muscles and joints. RadysChildren.org.
• Mora, I. S. (2000). Muscular system.
• Schoenau, E. (2006). The muscular system is the driver of skeletal development. Annales Nestlé (Spanish Ed.), 64 (2): pp. 55 - 62.