TYPES OF MUSCLES: CHARACTERISTICS, FUNCTIONS AND CLASSIFICATION - ANATOMY AND PHYSIOLOGY - 2024

Human muscle types can be grouped into smooth muscle and striated muscle. Striated muscle, in turn, is classified into two groups known as skeletal striated muscle and cardiac striated muscle.

A muscle is a tissue made up of cells called "muscle fibers" that have the ability to contract in the face of electrical stimuli, that is, to reduce their length, generating mechanical forces.

Some types of muscles (Source: BruceBlaus via Wikimedia Commons)

Muscle tissue allows joint displacement, body movement, and ambulation. It also participates in the fulfillment of specific functions in specialized tissues such as digestive tissue, blood vessels, the bronchial tree, and the heart, among others.

The muscles also make up the sphincters, which are muscular structures that surround a tube, allowing it to be opened or closed, promoting the emptying of the content inside.

Skeletal muscle is structurally related, as its name implies, with bones and joints, while smooth muscle is associated with visceral functions and cardiac skeletal muscle is proper to the function of the heart as a pump.

A fundamental difference between the different types of muscles is that one group is under voluntary control of the nervous system (the skeletal muscles), others are involuntary muscles (the visceral muscles, which are smooth muscles), and still others have automatic functions (such as muscle cardiac).

Just as neurons do, muscle fibers can be excited by mechanical, chemical, or electrical stimuli, generating an action potential that is transmitted along their plasma membrane. However, these cells have a contractile mechanism that is activated by this action potential.

Contraction of muscle fibers is possible due to the presence of contractile proteins called actin and myosin, the union of which represents one of the molecular "motors" that converts the chemical energy from ATP hydrolysis into movement.

Characteristics of the types of muscles

To facilitate understanding and analysis, we will separate the characteristics of the three main types of muscles: skeletal, cardiac, and smooth muscles.

Characteristics of skeletal skeletal muscle

This type of muscle is characterized by the fact that each of its cells (muscle fibers) is surrounded by connective tissue, which electrically isolates them from others. For this reason, each muscle fiber must be innervated by a nerve fiber that is under the voluntary control of the nervous system.

The set of muscle fibers innervated by a single nerve fiber is called a "motor unit" and this unit responds in unison to the stimulation of its nerve fiber.

Larger motor units are generally used for “gross” movements, but small motor units are used for fine and delicate movements that require a high degree of control.

The functional unit of a skeletal muscle is known as a "sarcomere." Each sarcomere is bounded by two “Z lines” and is made up of actin and myosin filaments (contractile proteins) interdigitated with each other.

The areas within successive sarcomeres that contain only fine active filaments make up the so-called "clear areas" or "clear striations" that are observed in the light microscope. The areas of the sarcomeres that contain thick myosin filaments give rise to the "dark striations" of skeletal muscles.

Skeletal muscle contraction involves the sliding of actin and myosin fibers (one over the other) and not the shortening of these protein fibers.

Characteristics of the cardiac skeletal muscle

The heart is made up of a special kind of striated muscle that, unlike skeletal muscle, has tight connections between its fibers that allow it to function as a syncytium.

Histological section of the heart muscle (Source: Alexander G. Cheroske via Wikimedia Commons)

It is an automatic muscle, that is, it is a muscle capable of producing its own stimulation (contraction), without the need for the function of the nervous system. The cardiac innervation of the nervous system only provides a mechanism of control of the contractile function, but does not originate it.

The contractile apparatus of the heart that allows it to function as a pump is also made up of sarcomeres delimited by two Z lines. Its fibers or muscle cells (cardiac myocytes) are branched and linked together through structures called "intercalary discs. "And" gap joints ".

Intercalary discs are low resistance structures through which electrical excitation can be conducted from one cell to another.

Cardiac muscle (Source: 1020 Cardiac Muscle.jpg: OpenStax Collegederivative work: Miguelferig via Wikimedia Commons)

Cardiac “automatism” is in charge of specialized muscle cells that generate spontaneous and rhythmic electrical activity that is transmitted to the atria so that they contract in unison and, with a certain delay, pass to the ventricular system, which sequentially contracts after these.

Smooth muscle characteristics

Smooth muscle differs from skeletal muscle in that it does not have transverse striations visible under the microscope. It also has actin and myosin as a sliding contractile apparatus, but these proteins are not arranged in a regular and orderly manner as is the case in skeletal muscle.

Instead of Z lines, smooth muscle muscle fibers have dense bodies in their cytosol that are attached to the plasma membrane and which in turn are attached to actin filaments. In general, these muscles have few mitochondria and their mechanical activity depends on glucose metabolism.

Smooth muscle vs, skeletal muscle (Source: OpenStax via Wikimedia Commons)

They are involuntary muscles, that is to say, they are innervated by nerve fibers that are not under the control of the will (as much as you want, you cannot induce the movement of your intestines voluntarily).

There are several types of smooth muscles, some with automatic activity (such as the fibers of the heart muscle) and some not.

Types of muscles: classification

The muscles of the human body can be classified in several ways. The fundamental classification is histological, which separates the muscles according to the presence or absence of striations when histological sections are observed under the light microscope.

The most extensive classifications are used for striated muscles, which can be separated according to their shape or the type of movement they perform.

- Histological classification

According to the observation of histological muscle sections in an optical microscope, it can be seen that there are two types of muscles, some that present transverse striations (as light and dark areas along the entire surface of the muscle cells) and others not.

In this way, muscles can be classified as striated muscles, those with the aforementioned transverse striations, and smooth muscles, those that do not.

Striated muscle

Striated muscles are of two types: skeletal muscle and cardiac muscle. The fundamental difference between these two lies in their function. Each cell in skeletal muscle functions in isolation from the others, while cells in the heart muscle function as a syncytium.

Smooth muscle

Histological section of the smooth muscle (Source: Juan Carlos Fonseca Mata via Wikimedia Commons)

Functionally speaking, smooth muscle can be subclassified as visceral or unitary smooth muscle and as multi-unit smooth muscle.

The first works as a syncytium, that is, all tissue cells behave as one (stimulation of one produces the contraction of all); meanwhile, the second is made up of individual units that produce delicate, graduated contractions.

Visceral smooth muscle is found on all walls of the hollow viscera, such as the musculature of the intestine, ureters, and uterus. Multiunit smooth muscle is unique to the iris (in the eye).

Although these are involuntary muscles, each cell of the multi-unit muscle is connected to a nerve fiber, in the same way that it happens with the muscle fibers of skeletal skeletal muscle.

In visceral smooth muscle, the junctions of nerve fibers are rare, since excitation spreads rapidly through the tight junctions between its cells. In addition, these cells respond to hormonal stimuli and other substances in the circulation.

Blood vessels are characterized by the presence of both types of smooth muscle (visceral and multi-unit) in their walls.

- Classification of skeletal muscle according to the type of movement

According to the type of movement they can perform, skeletal striated muscles are classified into:

- Extenders: those that increase the angle of the joints. An example of these muscles is the crural quadriceps of the lower limb anterior thigh.

- Flexors: those that decrease the joint angle. An example of a flexor muscle is the biceps brachii, located in the arm.

- Abductors: they are the muscles that move away the limb to which they are associated from the midline of the body. The main abductor muscles are the gluteus medius, gluteus minimus, and triquetrum.

- Adductors: bring the member to which they are associated closer to the midline of the body. Five examples found on the inner thighs are: the pectineus muscle, the long adductor, the medial rectus, the short adductor and the adductor magnus.

- Elevators: move one of their insertion sites "up". In the mandible are the temporal masseter, the medial pterygoid and the lateral pterygoid; there are also the external intercostal muscles.

- Depressors: these are the muscles that move one of their insertion sites “down”. Examples of this group of muscles are the internal intercostal muscles and the triangular muscle of the lips, which depresses the angle of the mouth.

- Rotators: perform the rotation of a bone around its axis. This group also includes the supinator muscles and the pronator muscles, which participate in the external or internal rotation movement of a limb. Examples of these are the latissimus dorsi muscle and the infraspinatus muscle.

- Sphincters: are those muscles capable of closing an opening or a tube. These include the internal sphincter muscle of the anus and the urethra.

- Classification of skeletal muscle according to its group action

According to the type of group action that the muscles of the same member perform, these are classified as:

- Agonists: are the muscles that produce a movement.

- Antagonists: are the muscles that oppose a movement.

- Synergists: the muscles that work together to generate movement that no muscle could produce on its own. Synergistic action can be seen in adduction of the hand at the wrist, where the anterior ulnar muscle flexes and adducts the hand; to produce adduction only, the posterior ulnar must counteract flexion.

- Fixative muscles: these are the muscles that prevent the movement of a bone, keeping it firm and allowing other muscles to act.

- Classification of skeletal muscle according to shape

According to their shape, skeletal muscles can be:

- Fusiform or elongated: they have narrow ends and wider centers.

- Unipenniform: they resemble the middle of a feather, that is, the fibers are perpendicular to one of the sides of the tendon from which they originate.

- Bipenniforms: they are similar in shape to a feather, since their fibers "come out" perpendicularly on both sides of their tendon of origin.

- Multipenniform: the fibers of these muscles arise from several tendons; These muscles have a fairly complex organization, such as the deltoid muscle, which is found in the shoulder.

- Widths: they have all their diameters more or less the same size.

- Flat: these are those muscles that tend to be fan-shaped. These are very thin and wide muscles, like the pectoralis major muscle.

- Short: they are short muscles and have little elongation capacity. A good example is the muscles of the face.

- Biceps: are those muscles that at one end are joined with a tendon to the bone and at the other they are divided into two muscle portions, each with a different tendon that joins it to the bone; Likewise, there are triceps and quadriceps, which instead of having two segments have three or four, each one joined by a tendon at its ends.

- Digastrics: they are made up of two muscle bundles joined at one end to a single tendon.

- Polygastric: they have more than two muscle bundles joined by the same tendon to the bone at one end. An example of these muscles is the rectus abdominis muscle.

Features

Muscles are essential tissues for the functioning of most of the organic systems that make up us. Not only do they allow us joint movement and displacement that differentiates us from sessile organisms such as plants, but they also allow us to relate to the environment and to all the entities that surround us.

From a visceral point of view, muscles fulfill essential functions for life. For example, the heart pumps blood throughout the body, without which we could not live.

Smooth muscle, which is found in hollow viscera, is essential for the function of the gastrointestinal, genitourinary, and respiratory tracts, among others.

This type of muscle also makes up the walls of blood vessels, working to control blood pressure. In the eye there are muscles that control the opening and closing of the pupil, regulating the entry of light and facilitating vision.

They are also part of the sphincters in general, so they participate in functions such as defecation, the emission of urine, etc.

References

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