18 julio, 2024

Muscle tissue: what it is, characteristics, types, functions, diseases

What is muscle tissue?

He muscle tissue It is in charge of orchestrating the movements and contractions of the body. It is made up of cells with the ability to contract, called myocytes. It is an abundant tissue, and in humans it makes up a little less than half of its entire mass.

There are three types of muscle tissue, which differ mainly in cell characteristics and location. These are skeletal, smooth, and cardiac muscle.

Skeletal muscle is striated, multinucleated, and voluntary. The cardiac also presents these structural characteristics, but it is involuntary. The smooth one lacks striations, presents a nucleus and its movements are involuntary.

The main function of muscle tissue is related to movement, both voluntary and involuntary. It directs the movements of the extremities and the trunk, and the movements of the internal organs (vasodilation, vasoconstriction, movements of the intestine, esophagus, stomach, etc). It also directs the movements of the heart cells in rhythmic beats.

Characteristics of muscle tissue

Muscles are tissues that have the ability to become excited and respond to a series of stimuli, such as changes in pressure, heat, light, among others.
It is responsible for the movement of organisms.
Muscles are characterized by their properties of contractility, extensibility, and elasticity.
It is composed of almost 80% water, which plays a vital role in contraction and provides a suitable medium for inorganic ions and organic compounds present in the tissue.
The proteins that make it up are of the contractile type: actin, myosin and tropomyosin.
It has high metabolic demands due to its contractile activity. You must maintain a balance between energy production (through cellular respiration) and waste product removal to maintain optimal muscle function.

functions of muscle tissue

In the most complex multicellular organisms, movement is orchestrated by muscle tissue. For this reason, the main function associated with muscle tissue is locomotion and movement, including functions associated with digestion, reproduction, and excretion, among others.

types of muscle tissue

In vertebrates there are three types of muscle cells that make up 60 to 75% of the total body weight.

skeletal muscle

It is also known as striated or voluntary muscle, because these structures can be consciously mobilized by the animal. Cells are multinucleated and arranged longitudinally.

Skeletal muscle is involved in body movements. Each muscle is directly attached to two or more bones by connective tissue. When the muscle contracts, the bones move around the joint that holds them together.

Of the total weight of the animal, the striated muscle corresponds to 40%, approximately. In humans, it has been found that in the female sex the proportion of striated muscle is lower.

The units that make up this system are made up of actin, myosin, and tropomyosin. Of the three, the most abundant is myosin and it is found in the primary filaments. Actin is in the secondary filaments and tropomyosin is in the I bands.

Smooth muscle

It lacks striae and is involuntary. This type of muscle is part of the walls of internal organs, such as the digestive tract, respiratory tract, urinary bladder, veins, arteries, among others. It is responsible for regulating the movements of these organs.

Smooth muscle contraction is much slower than skeletal muscle, but its contractions are longer.

cardiac muscle

It is found exclusively in the heart. It is composed of striated multinucleated fibers, reminiscent of skeletal muscle in many respects. The fibers have a syncytium pattern, but do not fuse with each other.

Unlike skeletal muscle, cardiac muscle generates ATP aerobically and uses fatty acids for its generation (and not glucose).

These muscles are specialized to respond to stimuli in a rhythmic manner, to make the heart beat. Like smooth muscle, it is innervated by the autonomic system, making it an involuntary muscle.

Phasic and tonic muscles

In vertebrates, muscles have two types of arrangements, phasic and tonic muscles. The former have insertions into structures and function in antagonistic pairs.

Tonic muscles are found in soft organs, such as the heart, urinary bladder, digestive tract, and in the walls of the body. These do not have origins or insertions comparable to the phasic musculature.

muscle cells

Each muscle is made up of a group of cells called muscle fibers, or myocytes, organized in parallel with respect to their neighbors. This structuring allows all the fibers to work at the same time.

To refer to muscle cells, the term «fiber» is used, since they are much longer than they are wide. However, they should not be confused with other types of fibers, such as collagen, for example.

Muscle tissue cells have their own nomenclature: the cytoplasm is known as the sarcoplasm, the cell membrane as the sarcolemma, the smooth endoplasmic reticulum is the smooth saccharcoplasmic reticulum, and the functional unit is the sarcomere.

Depending on the type of muscle, the cells vary in terms of their shape and number of nuclei. The most noticeable differences are:

striated muscle cells

These cells have a diameter between 5 and 10 um, and the length can reach several centimeters. This incredible size can be explained by the fact that each cell comes from many embryonic cells called myoblasts, which fuse to form a large, multinucleated structure. They are rich in mitochondria.

Structure and organization

These multinucleated units are called myotubes. The structure contains multiple tubes within a single plasma membrane and differentiate into a mature muscle fiber, or myofiber.

Each muscle fiber is made up of multiple parallel grouped subunits called myofibrils, which in turn are made up of a series of longitudinally repeated elements called sarcomeres.

Sarcomeres are the functional units of striated muscle and each one is delimited at its ends by the so-called Z line.

The “striated” appearance of muscle occurs because the myofibrils of a muscle fiber are made up of closely aligned sarcomeres, taking on a banded appearance under light microscopy.

The bands are formed by contractile proteins. The dark ones are formed mainly by myosin (mainly) and the light ones by actin.

smooth muscle cells

Anatomically, smooth muscle is composed of spindle cells (spindle-shaped), long, with sharp edges, and a centrally located nucleus. Although they are also made up of the proteins actin and myosin, they lack striations and no tubules or branches.

cardiac muscle cells

Like smooth muscle cells, heart muscle cells have several nuclei, although there are cells that only have one. They are shorter than those of skeletal muscle.

They are elongated and have multiple branches. The ends of the cell are blunt. They are rich in mitochondria, glycogen granules, and lipofuscin. When viewed under the microscope, a pattern of striae similar to those of skeletal muscle is observed.

Muscle tissue diseases

Muscular dystrophy

Duchenne muscular dystrophy is a recessive genetic disorder linked to the sex chromosome X. The cause is a mutation in the gene that codes for dystrophin, causing its absence in the muscle. Muscular dystrophy affects one child in 3,500.

Interestingly, in terms of size, the dystrophin gene is one of the largest known, at 2.4 Mb and a mRNA of 14 kB. Depending on which mutation occurs, the dystrophy can be more or less severe.

The main function of healthy dystrophin in muscle is structural, as it links actin filaments inside cells with proteins located in the cell membrane. Actin transmits the movement and force of the myofibrils to the membrane proteins, and then to the extracellular space.

The disease affects all muscles, causing weakness and also muscle atrophy. The first symptoms usually appear on the extremities of the body. As the disease progresses, patients will need wheelchairs.

rhabdomyolysis

Rhabdomyolysis is a pathology caused by necrosis (pathological cell death) of muscles. Specifically, it is not a disease, but a syndrome, which can be associated with multiple causes: excessive exercise, infections, drug and alcohol poisoning, among others.

When cells die, various substances are released into the bloodstream that, under normal conditions, would be inside muscle cells. The most common substances released are creatine phosphokinase and myoglobin.

Removal of these atypical compounds from the blood can be by dialysis or blood filtration.

Myasthenia gravis

The term myasthenia gravis has its origins in Latin and Greek, meaning «severe muscle weakness.» It is a chronic autoimmune pathology that affects the skeletal muscle of the body, causing loss of strength in them.

As the disease progresses, the weakness becomes more evident. It affects muscles that participate in basic daily activities such as eye movement, chewing, speech, swallowing food, etc.

myositis

Myositis is muscle inflammation. The causes of this inflammation vary widely, from injuries to autoimmune diseases. Two main categories are distinguished: polymyositis and dermatomyositis.

The first causes significant muscle weakness in the patient and affects the muscles located near the abdomen and trunk. The second pathology, in addition to causing muscle weakness, affects the skin.

Amyotrophic Lateral Sclerosis

Amyotrophic lateral sclerosis, Lou Gehrig’s disease or Charcot’s disease, is a neuromuscular condition that appears when the cells of the nervous system die progressively, causing muscle paralysis. In the long term this disease causes the death of the patient.

The disease is more common in older men. Stephen Hawking was an eminent physicist and is perhaps the most famous patient with amyotrophic lateral sclerosis.

injuries and tendinitis

The excessive use of the muscles can translate into medical conditions that affect the locomotor ability of the patient. Tendinitis usually affects the joints mostly and occurs due to excessive and forceful use of the same, such as the wrists or fingers.

References

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