Introduction Muscle Tissue

Muscle tissue is part of the four primary tissues of the body, and its functions are: to assist in locomotion and movement of the limbs (musculoskeletal system), motility of some organs (smooth muscle of the gastrointestinal tract), control of the secretion of some glands (Smooth muscle ), chewing (masseter and temporalis muscle, for example), contractile activity (heart) and feeding (most of the meat is muscle tissue).

Muscle tissue divides into three major groups: Skeletal Muscle Tissue (the large majority of muscles), Cardiac Muscle Tissue (heart muscle) and Smooth Muscle (a muscle that surrounds some organs and tissues). Muscles are different in the size and number of muscle fibers, organization of contractile elements, and type of innervation.

Muscle Tissue Types

Even with its differences, muscle tissue has some characteristics that are common to all types, such as common embryonic origin, large amount of mitochondria, to supply its high energy requirement; contact elements, derived from filamentous proteins mainly actin and myosin.

Another characteristic of this tissue is its high vascularization, which is necessary to maintain a good amount of energetic substrates (Glucose and lipids, for example) and oxygen, which is essential for an adequate aerobic muscle contraction. In the absence of good oxygenation for the muscle, intense exercise, for example, starts to contract in an anaerobic way, generating lactic acid that is aggressive to the muscle fibers, fatigue, and causing pain.

Muscle tissue also has a high innervation, especially skeletal muscle tissue, as in this case, each muscle fiber has its innervation.

Muscle cells have characteristic names such as Sarcolemma (Plasmalemma), is the plasma membrane of muscle cells, although it can also be used to designate the basal lamina and reticular fibers; Sarcoplasm, used to designate the cytoplasm; Sarcosomes, used to designate the mitochondria; sarcoplasmic reticulum, used to designate the smooth endoplasmic reticulum (REL) of skeletal and cardiac muscles, as their REL are an essential reservoir of Ca² for the contraction of these cells.

Embryonic Origin

Although the three types of musculature originate in the embryonic layer of the mesoderm, each of the three tissues arises from a different subdivision of the mesoderm.

Skeletal muscle tissue arises from somatic mesoderm (parietal leaflet), cardiac muscle tissue arises from splanchnopleuric mesoderm (visceral leaflet), smooth muscle arises from splanchnopleuric some parts of somatic mesoderm.

Skeletal Striated Muscle Tissue (Skeletal Muscle)

It receives its name due to the streaks seen under the light microscope, which are derived from Myofibrils and are the primary muscle part of the Skeletal Muscle system, responsible for locomotion and limb movement. They are fabrics of voluntary control.

Muscle tissue cells have a very elongated cylindrical shape and bundles, reaching up to 40 cm in width and 10 to 100 µm in diameter in a single pile; they are multinucleated and contain, inside, numerous myofibrils, which are muscle fibers arranged in small longitudinal cylinders.

The nuclei of the cells are arranged on the periphery, and although in the microscopic view, some may appear to be in the middle of the cell this is nothing more than a cut artifact, where a sarcolemma was cut that was arranged differently on the lamina. The large presence of mitochondria is due to the fact that this tissue needs a lot of energy to perform its contractile activities.

There is also an organelle that deserves special attention, which is the sarcoplasmic reticulum, which stores and regulates the flow of Ca²; this organelle is actually an extension of the smooth endoplasmic reticulum that forms cisternae surrounding the myofibrils.

Connective Tissue Wrap

Each muscle fiber is surrounded by a sheath of dense connective tissue, which forms the structure known as the endomysium. In the basal lamina of the endomysium there are small, unnucleated, fibrocyte-like cells known as satellite cells. Satellite cells are source cells for new muscle fibers in case of muscle damage.

The endomysium are grouped in sets delimited by connective tissue, this group is called the perimysium and this group also contains vessels, nerves, collagen and fibroblasts. The set of perimysium forms the muscle itself, which is surrounded by another layer of connective tissue, which we call epimysium.

The function of the connective tissue that surrounds the muscle fibers is: to unite the common muscle fibers of a specific type of muscle, differentiating it from another muscle in the region, allowing the force of contraction.
A single beam can act on every muscle, supporting the muscle fibers because it is in the connective tissue that an extensive network of blood capillaries, nerves and lymphatic vessels is immersed and this helps in the gradual transition of some muscles to tendon.

Organization of Muscle Fibers

The myofibrils of muscle fibers can be seen under an optical microscope, however, their details can only be seen at the level of electron microscopy.

Myofibrils are composed of Bands (I, A and H) and lines (Z), the region between two Z lines is called Sarcomere. The I band is lighter because it is formed by thin filaments (Actin, Troponin and Tropomyosin), the A band is darker because it is formed by thick filaments (Myosin) and the H band is intermediate because it is formed by thin and thick filaments.

Muscle Tissue Types

Myofibrils are composed of four types of proteins: myosin, Actin, troponin and Tropomyosin. These four proteins are arranged in two filaments: the thin (Actin, troponin and Tropomyosin) and the thick (Myosin).

In the thin filament we have Actin, which is a long structure (5nm), formed by two filaments, twisted in the shape of a helix, of actin polymers (F or fibrous Actin). Actin polymers are made up of around 200 small actin monomers (G or globular Actin).

Tropomyosin is a long, thin protein, made up of two polypeptide chains that wind onto F-actin filaments. Troponin is a complex divided into three parts (TnT, TnC and TnI), each with a specific function. The TnT portion binds to Tropomyosin, the TnC portion binds to Ca² F ions and the Tnl portion enables the interaction between F actin and myosin to trigger muscle contraction.

In the thick filament, it receives this name because the myosin molecule is very thick (10 nm in diameter by 1.5 µm in length) and with a molecular mass equivalent to 500 kDa. Myosin is divided into two filaments, the heavy filament (Meromyosin heavy) produces globular regions that interact with myosin, in this segment there is a site where the energy molecule ATP is broken down into ADP by the ATPase enzyme; and the light filament (Light Meromyosin), consisting of two chains of fibers twisted into a helix. These segments were defined because they appear after being cleaved by the trypsin enzyme.

Innervation

Skeletal muscle is highly vascularized by motor (efferent) nerves of the peripheral nervous system, which originate from the spinal cord. Each nerve fiber can innervate from one to more than 150 muscle fibers, a process known as endplate.

Motor nerves reach the perimysium where they branch into portions devoid of myelin sheath. These small branches enter the surface of the muscle fiber at a structure called the myoneural junction or motor plate.

When the impulse from the motor nerve arrives at the myoneural junction, the presynaptic vesicles are stimulated to release acetylcholine (Ach) at the myoneural junction. Ach binds to postsynaptic receptors in the sarcolemma of the muscle fiber, making it more permeable to Na, which depolarizes the cell. Depolarization propagates not only through the fiber membrane, but also inside it, thanks to the invaginations that the sarcolemma makes in the cells, transverse or T tubules, reaching deep regions and making intimate interactions with the sarcoplasmic reticulum, the so-called triads. Thus, the sarcolemma, which is depolarizing, manages to penetrate and take the depolarization to the interior of the fibers.

In the myoneural cleft and basement membrane there is an enzyme called acetylcholinesterase that hydrolyzes excess Ach from the cleft and removes those Ach that have bound to sarcolemma receptors, this prevents the sarcolemma from being depolarized for a long period of time.

Muscle Tissue Function

The intensity of muscle contraction is not dependent on which nerve sends its impulse, as the nerve always sends the same impulse to the muscle, regardless of whether it generates a strong or weak muscle contraction. The muscle fiber also does not have the ability to contract with greater or lesser intensity, because the intensity of its contraction is always the same. What really modulates the force of muscle contraction is the amount of muscle fibers that are stimulated, so the total contraction of the muscle will be less if fewer fibers are stimulated to contract or greater if more fibers are stimulated to contract.

In addition to the efferent fibers, the muscle also has afferent, that is, sensory, nerve activity. Amid the muscle fibers there are modified muscle fibers called intrafusal fibers, these modified fibers contain receptors (proprioceptors) that capture information from muscle fibers and pass on to sensory nerve fibers that transmit the spinal cord. The main function of the efferent fibers is to control body posture and coordinate muscles whose contraction is opposed.

What are the Functions of Muscular Tissues?

During muscle fiber rest, the energy molecule ATP binds to ATPase, present in the heavy portion of myosin (Head). However, myosin ATPase cannot break ATP (Dephosphorylate) by itself, requiring Actin to serve as an enzymatic cofactor for myosin ATPase.

When the motor nerve fiber releases Ach into the myoneural cleft, it causes a process of membrane depolarization that extends into the muscle fiber thanks to the T tubules and this potential reaches the sarcoplasmic reticulum due to the presence of the triads. The sarcoplasmic reticulum membrane is stimulated to depolarize and this opens Ca2 channels, passively releasing the ion into the sarcoplasm.

The free Ca² in the sarcoplasm binds to the TnC portion of Troponin, which pushes the tropomyosin filament into the thin filament (Actin plus Troponin plus Tropomyosin), by doing this Actin is exposed and interacts with the myosin filament, Myosin- ATP. Now that Actin is interacting with myosin, the myosin ATPase enzyme already has its co-factor to break the molecule down.

The ATPase enzyme breaks down the energy molecule of ATP (Adenosine Triphosphate) into ADP (Adenosine Diphosphate) plus Pi (Inorganic Phosphate). This breakdown, called dephosphorylation, releases a large amount of energy that was stored in the ATP molecule.

The high energy that the ATP molecule generates when dephosphorylating causes the myosin head to deform and as the actin filament is interacting with the myosin head, this actin filament is pushed which causes the Actin to slide over the myosin.

When the actin filament slides over the myosin, it decreases the size of the muscle fiber and increases its thickness, causing the fiber to contract.

When displacing Actin, the myosin head detaches from the actin filament and returns to its original shape, however, new actin-myosin ligaments are made when the actin filament slides over the myosin filament in such a way that with each contraction the ligaments that generated that contraction are undone and new ligaments with other myosin heads are made to guarantee the next contraction and so susceptibly.

When the cell finishes its depolarization, Ca² returns to the sarcoplasmic reticulum in an active way, which interrupts the contractile activity due to the fact that the TnC portion of Troponin no longer has Ca² to provoke its characteristic reaction that leads to muscle fiber contraction. This cycle of contraction and relaxation is known as excitation-contraction coupling.

Types of Fiber

Skeletal muscle tissue has two types of fibers: Red Fibers (Type I), White Fibers (Type II). The difference between the three is in the amount of myoglobin they have. Myoglobin is a hemoprotein, which can store oxygen, being very necessary in muscles that have high activity requiring high oxygen consumption. Myoglobin, when bound to oxygen, has a dark red color, which gives the characteristic color of most muscles.

Red Fibers or Type I, are rich in myoglobin in their sarcoplasm, this guarantees them a slow, oxidative contraction and continues using glucose and fatty acids as a source of energy and performing. They are smaller fibers with numerous mitochondria, which guarantees a good aerobic energy performance.

The White Fibers of Type II are low in myoglobin, tense their color more light red, this guarantees a fast, oxidative and discontinuous contraction. Due to the small number of myoglobin, little oxygen is stored and this gas is thrown directly into the mitochondria. Type II fibers are divided into two subclasses: IIa, which are fast and resistant to fatigue and IIb, which are fast but accumulate lactic acid very quickly which causes fatigue and muscle soreness. Some authors consider that IIb muscle fibers are a third type of fiber, called mixed or intermediate fiber.

Muscles may have a preference for the type of fibers with some muscles composed more of white fibers and others more of red fibers depending on the function and energy requirement.

Cardiac Striated Muscle Tissue (Cardiac Muscle)

The heart is a predominantly muscular organ as it is the contractile activity of the heart muscle that gives the heart its function as the body’s blood pump. Cardiac muscle fibers are similar to skeletal muscle fibers, however, some particularities of cardiac muscle make it a muscle tissue of its own.

Cardiac muscle has striations, similar to skeletal muscle, but not as well highlighted as in skeletal muscle. Cardiac muscle is involuntary, being controlled by the parasympathetic and sympathetic activity of the autonomic nervous system. Shorter (90 µm and length) and branched, in such a way that one fiber makes projections to the others and the fibers can project longitudinally, transversely and obliquely in the same lamina.

The core of cardiac muscle fibers is at the center of the fiber and, unlike skeletal muscle fibers which are multinucleated, these fibers usually have only one or two nuclei.

Cardiac muscle fibers also have a much greater amount of mitochondria in their cytoplasm than do skeletal muscle fibers.

The sarcoplasmic reticulum is connected with the tubules of the transverse cisternae (T tubules) in a less intense way than in the skeletal muscle fibers in such a way that in addition to the triads there is the presence of dyads.

A peculiar feature of cardiac muscle fibers is the marked presence of lipofuscin, a golden brown pigment made up of phospholipids and proteins located close to the cell nucleus. Lipofuscin is usually present in cells that do not multiply and have a long life, such as cardiac muscle fibers.

Cardiac myocytes do not have the ability to regenerate, if there is any damage to cardiac muscle tissue, fibroblasts will proliferate in the injured space and fill it with collagen fibers, forming a kind of scar.

Interleaved Disks

The intercalated or intercalated discs is a very characteristic structure of cardiac muscle fibers. Under the light microscope, it appears as strongly stained transverse lines that appear at irregular intervals along the edges.

The great opacity of intercalated discs is due to the presence of junctional complexes (adhesion zonules, gap junctions and desmosomes) between cardiac muscle fibers. The adhesion zonules, unlike the epithelium, are irregularly arranged. Desmosomes act both in the adhesion zonule and in the regions where there are gap junctions and their importance is to prevent cardiac cells from becoming disarranged during contractile activity. Gap-like gap junctions are essential for the proper contractile functioning of the heart because it is through these gap junctions that ions pass that cause one cell to excite the other in such a way that the action potential generated in only one point in the heart (Brand Cells) -step), can propagate to all heart cells, as in a syncytial form.

Secretory Function

Cardiac muscle fibers, mainly the cells of the left atrium, synthesize an atrial peptide called: Atrial Natriuretic Peptide (ANP) that are stored in the form of granules and released into the blood, acting as an important hormone regulating plasma volume.

PNA increases the secretion of Na by the renal tubules, which causes water to also be secreted following the osmotic gradient, which reduces the plasma volume of the blood, which controls blood volume and blood pressure.

Other Cardiac Structures

Some muscular structures of the heart were modified and formed cells with specific functions to the heart.

Pacemaker cells are an example of modified muscle cells and their function is to generate, by themselves, an action potential that is conducted throughout the heart by contracting muscle fibers.

Another modified muscle cells present in the heart are the Purkinje fibers which are specialized to conduct the action potential generated in the atria to the ventricles.

Contraction

Smooth muscle cells, upon receiving neurotransmitters from the autonomic nervous system, allow the entry of Ca² from the caveolae into the sarcoplasm. In the sarcoplasm, Ca² binds to calmodulin, forming the calmodulin-Ca² complex. phosphorylation of myosin II molecules. When myosin II molecules are phosphorylated, they stretch on the actin filament. Under the action of the myosin II ATPase enzyme, ATP is broken down and releases energy to move the myosin head over Actin and slide, a process similar to the contraction of skeletal muscle.

Like actin myofilaments, myosin II are linked to a network of structures called dense bodies, which are round, amorphous regions scattered throughout the cytoplasm of the muscle cell. When one cell contracts, the others are also stimulated to contract as they are pulled by these dense bodies. How to tighten the end of a shoelace and the entire shoelace twitches. During contraction, the cell nucleus passively deforms, taking on a rough, curled or helical appearance.

Hormones that can act on smooth muscle can have an effect on the sarcoplasmic concentration of cyclic AMP that leads to the activation of the kinase enzyme independent of the entry of Ca² into the cell. Hormones can increase or decrease the sarcoplasmic concentration of cyclic AMP, consequently leading to an increase or decrease in contraction.