Muscle

In biology, muscles are existing structures or tissues in humans and most animals that have the ability to generate movement by contracting and relaxing. The tissue that makes up muscle is called muscle tissue and is made up of specialized cells called myocytes that have the property of increasing or decreasing their length when stimulated by electrical impulses from the nervous system.

There are three types of muscle tissue:

• Striated muscle fabric that constitutes the voluntary muscles.
• Heart muscle tissue that forms the heart.
• Smooth muscle tissue: which is mainly found on the wall of the digestive system, bronchus, blood vessels, urinary bladder and uterus.

In the human body and in all vertebrates, striated muscles are attached to the skeleton by tendons and are responsible for the execution of voluntary body movements. Cardiac muscle and smooth muscle contract automatically by impulses received from the autonomic nervous system.

The functional and structural unit of skeletal muscle is the muscle fiber or myocyte, several muscle fibers are grouped to form a fascicle, several fascicles come together and form the complete muscle that is surrounded by a connective tissue membrane called fascia. The human body contains approximately 650 skeletal muscles.

Etymology

The word muscle comes from the Latin diminutive musculus, formed by mus (mouse) and the diminutive ending -culus, because at the time After contraction, the Romans compared it to a small mouse, due to the shape it acquires during this process.

Properties of muscle tissue

Contraction and relaxation of the heart muscle.

Muscle tissue is made up of cells called myocytes and has four main properties that differentiate it from other tissues:

• Electrical excitability. Muscle tissue receives electrical impulses from the nervous system and responds to them by generating movement.
• Contractibility. It is defined as the shortening capacity that causes a tension called contraction force. If the pressure produced exceeds the resistance, a movement will be different depending on where the muscle is located.
• Extensibility. It is the ability of the muscle to extend without any harm. This property can be seen in the muscle layer of the stomach that is dissited considerably when the stomach is filled with food during the digestion process.
• Elasticity. It refers to the ability of muscle tissue to return to its original length after the contraction process or after its stretching.

If muscle tissue is compared to other tissues such as bone tissue, the importance of these four properties can be easily understood. Bone tissue is not electrically excitable, nor does it have the ability to contract or change its shape. It is not extensible, if it is elongated it breaks causing a fracture.

Types of muscle tissue

Striated or skeletal muscle tissue

Microphotography of skeletal muscle fibers.

It is responsible for the movement of the axial and appendicular skeletons, and for maintaining posture or body position. Thanks to the striated muscle we can carry out voluntary movements, move the trunk and extremities, walk, jump, run, lift objects, chew and move our eyes in all directions. Each skeletal or skeletal muscle is attached to bone by fibrous processes called tendons and is surrounded by a membrane called an aponeurosis.

The fundamental unit that constitutes skeletal muscle is the muscle fiber. Each of them is actually a very long cylindrical cell that has numerous nuclei located on its periphery. A group of fibers are grouped to form a fascicle, several fascicles unite and originate the complete muscle.

Smooth Muscle Tissue

Diagram in which the contraction mechanism of smooth muscle fiber is observed.

Unlike skeletal smooth muscle tissue, it does not participate in voluntary movements. It is found in the wall of internal hollow structures, including the wall of the digestive tract, gallbladder, blood vessels, airways, bronchi, ureters, urinary bladder, and uterus. There is also smooth muscle in the skin associated with hair follicles and in the eye where it has the function of contracting and dilating the pupil and allowing focusing by varying the shape of the lens. It gets its name because if a tissue sample is observed under a microscope, no striations are visible, which is why it is called smooth. The smooth muscles of the body perform important functions and automatically contract or relax in response to nerve stimuli generated by the autonomic nervous system.

Smooth muscle fibers are shorter than skeletal fibers and have a single nucleus, they have internal filaments that are of two types: thick and thin. These filaments do not have a compact distribution, so there are no visible striations. Smooth fiber contraction is based on the same principles as in skeletal muscle, but has some particular properties, it is slower onset but longer in duration than in skeletal muscle, and the fibers can be stretched or shortened to a much greater degree. greater without losing its contractile capacity.

The smooth muscle of the uterus is what makes the birth process possible, during which the uterus contracts periodically with increasing intensity that reaches its maximum during the expulsive period.

Cardiac striated tissue

The heart muscle forms the myocardial and represents 75% of the total volume of the heart.

It is of a modified striate nature and of involuntary control. It is present only in the heart and generates the movements by which this organ propels blood through the circulatory system. 75% of the total volume of the heart is muscle. Cardiac muscle tissue has some special characteristics. The cells that make it up are branched and have structures called intercalary discs that join the ends of two adjacent myocytes, in such a way that the organ contracts synchronously. The regulation of the force and speed of contraction is involuntary and is carried out through the autonomic nervous system. The sympathetic nervous system has a positive action by increasing the frequency of contractions, while the stimulation of the parasympathetic nervous system has the opposite action.

Muscle contraction

Contractility is the property of muscle fibers to shorten and thicken. This is possible because each cell contains numerous filaments that are made up of two different proteins called actin and myosin, both types have a different appearance, the actin filaments are thin and light in color, while the myosin filaments are dark in color and thick. They alternate with each other, imbricated as when the fingers of the hands are intertwined.

According to the sliding filament model, at rest the muscle fiber presents a moderate degree of overlap between the actin and myosin filaments, in a state of contraction the overlap increases, while if muscle elongation occurs the overlap decreases and can become null.

Contraction mechanism in the striated muscle.

Types of contraction

Types of contraction

Isometric or static contraction does not generate movement, but there is tension in muscle and energy spending.

Isotonic or dynamic contraction generates movement.

Skeletal muscles are attached to bones at their ends by tendons. Therefore, there is a resistance that the muscle must overcome in order to shorten. When the resistance is higher than the tension established in the activated muscle, it cannot be shortened and movement does not occur, while when the resistance is lower than the tension generated, a shortening occurs that will be faster the lower the tension. burden. The term contraction is used here to designate the development of tension in the muscle, but it does not necessarily imply that it is shortened, since this depends on the external resistance that exists. Based on the above, there may be several types of contractions, depending on whether or not they generate movement:

• Isometric or static contraction. In this type of contraction the muscle tension does not exceed the resistance to defeat. The muscle does not decrease its length and does not generate movement, although there is an energy expenditure.

• Isotonic or dynamic contraction. Unlike the former muscle is shortened or lengthened. Isotonic contractions are the most common in everyday activity and in most sports, as muscle tensions usually cause shortening or lengthening of the muscle fibers of a particular muscle. It can be two types: concentric or eccentric.

• • Concentric iconic. There is an approximation between the joint segments, leading to positive work. The force applied is greater than the resistance to defeat. There's a shortening of the muscle.

• • Isotonic eccentric. In this type of contraction, there is a separation of the joint segments, leading to negative work. The force applied is less than the resistance to defeat. There's a muscle lengthening.

Other types of contractions can be defined that are really nothing more than the combination of the three basic ones mentioned above:

• Auxotonic contraction. It combines isotonic contraction with isometric in different proportions. Example of this contraction can be the lifting of weights in a bank.

• Isocenetic contraction. It is a type of dynamic contraction with fixed speed and resistance to conquer of variable type. It is a combination of three types of contraction; firstly eccentric contraction, then a minimum isometric time and a final time of concentric work.

Skeletal muscle fiber

Miofibrilla with sarcomer defined by two Z estrias.

The striated muscle fiber is an elongated cylindrical cell. It measures 50 microns in diameter and can reach a length of several centimeters. It is the result of the fusion of several cells, which is why it has numerous nuclei located on its periphery (multinucleated cell). It is surrounded by a membrane called the sarcolemma, while the inner region (cytoplasm) is called the sarcoplasm.

Sarcoplasm contains numerous longitudinal structures (myofibrils) that are compactly arranged and are formed by alternating two types of filaments: thick filaments made up of myosin protein molecules and thin filaments made up of myosin protein molecules. actin. Both types of filaments alternate with each other, forming a perfectly ordered structure that is responsible for muscle contraction. The myofibril is regularly cut by dark-colored striations called Z striations. The region between two successive Z striations is called the sarcomere. The sarcomere is the basic unit of muscle contraction. Each muscle fiber contains a large number of sarcomeres arranged in an ordered array with perfect regularity.

Structure of a skeletal muscle fiber.

Types of Skeletal Muscle Fibers

Cross-section of skeletal muscle enlarged 400 times.

There are two types of skeletal muscle fibers that are differentiated by their functional activity and some aspects of their structure: type I muscle fibers, also called red or slow-twitch muscle fibers, and type II muscle fibers, also called white or fast-twitch. Within a muscle there are usually fibers of both types, although depending on the type of movement usually performed, those of one of them predominate. Red fibers predominate in postural muscles (trunk muscles) whose activity is continuous and white fibers in movement-related muscles (limb muscles) that need to contract more rapidly.

• Type I. Also called slow or red contraction fibers, they owe their color to the abundance of mioglobin, they are of small diameter, they are irrigated by a lot of blood vessels and have in their interior numerous mitochondria but very little glucogen. They operate mainly for activities that require low-intensity but long-term contractions, such as maintaining body posture. The abundance of mitochondria and the oxygen storage capacity conferred upon it by mioglobin, determine that the energy needed for its processes is obtained mainly by aerobic, through the Krebs cycle. They are fibers that are not easily fatigued, because they get a lot of energy per unit of consumed matter.
• Type II. Also quick or white contraction calls. They have characteristics opposed to type I fibers, have little mioglobin, the diameter is greater, are not vascularized, contain few mitochondria and much glucogen. The organism uses them mainly for short-lasting exercises in time, but of high intensity. They are very sensitive to fatigue.
• Type IIa. They have intermediate features between type I and type II. Depending on the type of training performed by a person, type IIa fibers can be converted into type I fibers, if prolonged force exercises predominate, or type II fibers if the training predominates exercises that require intense but short-lived muscle activity (between 30 seconds and 2 minutes).

Energy systems

Energetic systems are understood to be the metabolic pathways that the (internal) organism uses to obtain energy. This energy is used to perform work, for example muscle contraction (External or internal).

• ATP-PC or anaerobic anaerobic system. It uses ATP and phosphocreatin muscle reserves, does not require oxygen and does not generate lactic acid. It produces a lot of energy in a short time, but reserves are very limited.
• Anaerobic glycolysis. It degrades glucose by obtaining energy without the presence of oxygen, producing lactic acid as a waste substance.
• Aerobic or oxidative system. It is the main system, it is done in the mitochondria of the cell and produces a lot of energy. Requires oxygen presence and produces as CO waste products₂ and H₂Or. It is the energy method that predominates in long-lasting and low-intensity activities.

General Structure of Striated Muscle

Section of a muscle in which the situation of endomisio, perimisio and epimisio can be observed.

Individual skeletal muscle fibers are covered by a layer of connective tissue called the endomysium. Several fibers are grouped together and form a larger structure called the muscle fascicle that is covered by the perimysium. Finally, several united fascicles form the complete muscle, which is covered by the epimysium. Most muscles are attached to bone by tendons, which are made of solid, fibrous tissue with some degree of elasticity. Typically there are 2 tendons, one at each end of the muscle.

The shape of the muscles is highly variable depending on their function and location, most are elongated or fusiform like the biceps brachii, others are flat like the rectus abdominis muscle, some are fan-shaped like the pectoralis major. Certain muscles have special shapes with an opening in the center to fit into a cavity, including the orbicularis oculi and orbicularis oculi.

Muscle functions

They are different depending on the type of muscle: striated, cardiac or smooth.

• Striated muscles: They promote body movements, generate heat, serve as protection of internal organs, make it possible to maintain body posture.
• Single muscles: Peristaltic movements of the intestine and stomach are made possible during digestion, dilating the pupil, contracting the urinary bladder during urination, increasing or decreasing the caliber of the blood vessels, producing the piloersection and increasing or diminishing the caliber of the bronchus.
• Heart muscle: Protests blood through the circulatory system to reach all the tissues in the body.

Muscular strength

In biology, strength is defined as the ability of muscles to contract against resistance. It is necessary for most daily activities, since all movements are caused by a force, both those necessary to move and those intended to move more or less heavy objects. Three different aspects are included within the concept of force:

• Maximum strength. It refers to the ability to reach as much force as possible at a given time. For example when lifting a load, the greater the weight that is possible to lift, the greater the maximum force.
• Force-resistance. It is the ability to maintain strength as long as possible or repeat it many times. For example, the number of consecutive times a heavy object can be raised.
• Explosive force. This concept refers to the ability to reach certain strength in the shortest possible time period, at shorter time greater explosive force.

The strength that a given individual can achieve is related to several factors, one of which is the cross section of the muscle. A muscle can generate 3 to 4 kg of force per cm² of cross section. Therefore, the muscles with the greatest section are the ones that develop the greatest strength, although the increase in the size of the muscle is not always accompanied by an increase in the strength that it can develop.

Chemical composition of muscle tissue

Distrophine is a muscle protein.

Skeletal muscle is made up of 75% water and 20% protein. The remaining 5% corresponds to other substances such as fats, glycogen, sodium, potassium, calcium and phosphorus.

• Water, which represents 75% of the muscle weight.
• Proteins correspond to 20% of muscle tissue, can be distinguished among others:
  • Miosina. It represents about 55% of muscle protein.
  • Actina. It corresponds to 25% of the muscle protein.
  • Mioglobin. Mioglobin is a structurally very similar muscle hemoprotein to hemoglobin. It is formed by a polypeptide chain of 153 amino acids and by a hemo group containing an iron atom. The function of mioglobin is to store oxygen.
  • Tropomiosine.
  • Troponin complex.
  • Distrofin. It is a muscle structural protein, it is encoded by the DMD gene located on the X chromosome. It has the function of joining the membrane of muscle cells and maintaining the cell structure during the contraction process. The absence of dystrophine or its alteration causes severe damage to muscle tissue. Duchenne's muscular dystrophy is caused by a mutation in the gene that encodes this protein.
• Carbon dioxide. The main substance of this group present in the muscle is glucogen. The muscle contains about 1% of glucogen that is used as a form of glucose storage. When the muscle performs an increased activity, it mobilizes its glucogen reserves that transforms into glucose. From glucose the muscle cell produces ATP which is the source of energy that makes the contraction possible.
• Lipids. The amount of fats that contains muscle tissue varies with food and is different according to the animal species.
• Inorganic compounds. Among the most important inorganic salts are sodium, with whose ions the excitability and contraction. Potassium, whose ions retard muscle fatigue. Calcium ion and phosphorus.

Muscular atrophy and hypertrophy

Muscle hypertrophy.

• Muscle atrophy. It consists of loss of muscle mass. It can be due to multiple causes, one of the most frequent varieties is muscle atrophy due to disuse that takes place in people who remain totally or partially immobilized for prolonged periods. In muscle atrophy by disuse, the decrease in contraction activity causes decreased muscle proteins and reduced cross-section of fibers. However, the number of fibers remains stable, so if the activity is resumed, the muscle recovers its initial properties after a longer period.

• Muscle hypertrophy. The term hypertrophy designates the size increase of an organ. When a muscle or group of muscles is subjected to repetition exercises against resistance, especially isometric contraction exercises, the muscle responds by increasing its size and strength, a phenomenon known as active hypertrophy. This process is mainly due to the increase in the size of the muscle fibers, since the number of them remains virtually unchanged. In order for the phenomenon to persist, it is necessary for the exercise to be performed continuously, otherwise the fibers end up returning to their initial size and the hypertrophy disappears. The use of doping substances to artificially cause muscle hypertrophy in athletes is a dangerous practice that can cause muscle and tendinous injuries. On the other hand, some of the products used unlawfully for this purpose, including anabolizers, can cause damage to internal organs such as the liver and kidney.

Muscular diseases

Musculature diseases and disorders are varied and have different etiologies.

• Muscle dystrophies. They are a heterogeneous group of hereditary disorders that have weakness and muscle atrophy, in some cases severe. Among the most common diseases included in this group are Duchenne's muscular dystrophy, Steinert's myotonic dystrophy and Becker's muscular dystrophy.
• inflammatory myopathies: include dermatomiositis and polymyositis. More recently, myositis with inclusion bodies has been included in this group.
• Miastenia gravis: causes muscle weakness for loss of acetylcholine receptors.
• Tumors, such as demoid tumor, rhabdomioma, leiomioma and rhabdomyosarcoma.