MUSCLE TISSUE: CHARACTERISTICS, FUNCTIONS, TYPES, CELLS - ANATOMY AND PHYSIOLOGY - 2024

The muscle tissue is responsible for orchestrating the movements and contractions of the body. It is made up of cells that have 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, differing mainly in cell characteristics and location. These are skeletal, smooth, and cardiac muscle.

Source: Berkshire Community College Bioscience Image Library Skeletal muscle is striated, multinucleated, and voluntary. The cardiac one also presents these structural characteristics, but it is involuntary. Finally, the smooth has no stretch marks, has a nucleus and its movements are involuntary.

The main function of muscle tissue is related to movement, both voluntary and involuntary.

It directs both the movements of the extremities and the trunk, as well as the movements of the internal organs (vasodilaction, vasoconstraction, bowel movements, stomach esophagus, etc.). It also directs the movements of the heart cells in rhythmic beats.

characteristics

Muscles are tissues that have the ability to become excited and respond to a series of stimuli, such as pressure changes, heat, light, among others. This tissue is responsible for the movement of organisms. Muscles are characterized by their properties of contractility, extensibility and elasticity.

Muscles are made up of almost 80% water, which plays a vital role in contraction and provides a suitable medium for inorganic ions and organic compounds present in tissue. The proteins that comprise it are of the contractile type: actin, myosin and tropomyosin.

Features

Movement is considered a property of living things and can occur in various ways.

All living cells exhibit movement of their intracellular components, amoebas (like various unicellular organisms) can explore their environment by movement, and some organisms have cilia and flagella that allow their movement.

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

Types

In vertebrates there are three types of muscle cells that constitute 60 to 75% of the total body weight. There is skeletal muscle, smooth muscle, and cardiac muscle. Below we will describe the details of each one:

Skeletal muscle

It is also called striated or voluntary muscle, because these structures can be consciously mobilized by the animal. Cells are multinucleated and arranged longitudinally. In the next section we will describe this arrangement in detail.

Skeletal muscle participates in body movements. Each muscle is directly attached to two or more bones through 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, the proportion of skeletal muscle has been found to be lower in the female sex.

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

Smooth muscle

The second type of muscle tissue is smooth muscle, characterized by the lack of stretch marks and by being involuntary. This type of muscle is found forming part of the walls of internal organs such as the digestive tract, the respiratory tract, the urinary bladder, the veins, the arteries, among other organs.

As we can guess, we are not able to move our intestines or contract our veins voluntarily as we do with our extremities. You can move an arm, but you cannot modulate your bowel movements, so this muscle is involuntary.

A similar type of smooth muscle exists in the invertebrate lineage and is called paramyosin filaments. We find them in mollusks and other groups.

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

Cardiac muscle

The heart muscle is found exclusively in the heart. It is composed of striated multinucleated fibers, reminiscent in several respects of skeletal muscle. The fibers are in a syncytium modality, but they 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 in responding to stimuli in a rhythmic way, to make the heart beat. Like smooth muscle, it is innervated by the autonomic system, making it an involuntary muscle.

Comparatively, cardiac muscle resembles smooth muscle in structure and is involuntary like striated muscles.

Phasic and tonic muscles

In the body of vertebrates, muscles have two types of arrangement: phasic and tonic muscles. The former have insertions in structures and function in antagonistic pairs.

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

Muscle cells

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

To refer to muscle cells, we use the term "fiber" as they are much longer than they are wide. However, we must avoid being confused with other types of fibers themselves, such as collagen fibers, 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 the sarcomere.

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

Striated muscle cells

The cells that are part of striated muscle have a diameter between 5 and 10 um, while the length can reach several centimeters.

This incredible size can be explained as each cell comes from many embryonic cells called myoblasts that fuse together to form a large, multinucleated structure. Also, these cells are rich in mitochondria.

Structure and organization

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

Each muscle fiber is made up of multiple subunits grouped in parallel called myofibrils, which in turn are made up of a series of longitudinally repeating 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 the muscle appears because the myofibrils of a muscle fiber are made up of sarcomeres that are very precisely aligned, acquiring a banded appearance under the light of the light microscope.

The bands are made up of 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-shaped cells with long, sharp edges and a centrally located nucleus.

Although they are also made up of the proteins actin and myosin, they lack striae and tubules or branches.

Heart 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.

Regarding their morphology, they are elongated and have multiple branches. The ends of the cell are blunt. They are rich in mitochondria, glycogen granules, and lipofucsin. When viewed under a microscope, we will observe a pattern of striations similar to those of skeletal muscle.

Muscle tissue diseases

There are several conditions that affect the muscles in humans. All of these conditions have consequences for locomotion - as the main function of muscles is to serve movement.

The term myopathy is used to describe the set of symptoms that result from a primary alteration in the striated muscle tissue. It is also called myopathic syndrome. That is, the term applies to any primary condition and in a broader sense it can also be applied to any muscle injury.

The most important diseases and medical conditions that affect muscle tissue are:

Muscular dystrophy

Duchenne muscular dystrophy is a condition caused by a recessive genetic disorder that is linked to the X sex chromosome. 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, with 2.4 Mb and a 14 kB messenger RNA. Depending on which mutation occurs, the dystrophy can be more or less severe.

The main function of healthy dystrophin in muscle is structural, since it binds actin filaments inside cells with proteins located in the cell membrane. The movement and force of the myofibrils are transmitted by this protein to the membrane proteins, and then to the extracellular space.

The disease is characterized by affecting all muscles, causing weakness in them and also muscle atrophy. The first symptoms usually appear in the extremities of the body. As the disease progresses, patients need to be transported by wheelchair.

Rhabdomyolysis

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

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

The removal of these atypical compounds from the blood can be through 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 disease that affects the skeletal muscle of the body, causing a loss of strength in them.

As the disease progresses, the weakness becomes more apparent. It affects muscles that participate in basic daily activities such as eye movement, chewing, speech, and swallowing of food, among others.

Myositis

Muscle inflation is designated as myositis. The causes of this inflammation vary widely, from injury to autoimmune diseases. Two main categories of this inflammatory condition are distinguished: polymyositis and dermatomyositis.

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

Amyotrophic Lateral Sclerosis

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

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

Injuries and tendonitis

Overuse of muscles can lead to medical conditions that affect the patient's locomotor ability. Tendonitis is a condition that usually affects the joints mainly and occurs due to excessive and forced use of the joints, such as the wrists.

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