CARDIAC MUSCLE: STRUCTURE, PHYSIOLOGY, FUNCTIONS, DISEASES - ANATOMY AND PHYSIOLOGY - 2025

The cardiac muscle or myocardium (myo, muscle and cardio, heart) is the muscle tissue that forms the walls of the heart of vertebrates. It is in charge of mediating the propulsion of blood through the entire vascular system through rhythmic and constant contractions.

Within the classification of muscle tissue, the myocardium is considered striated muscle, because its myofibrils are organized in sarcomeres, visible under the microscope. The cells of this tissue are generally branched or with extensions and have a single nucleus.

Source: Modified from: Blausen.com staff. «Blausen gallery 2014». Wikiversity Journal of Medicine. DOI: 10.15347 / wjm / 2014.010. ISSN 20018762.

It is innervated by nerves of the autonomic nervous system, so it works involuntarily. This means that we cannot consciously modulate the heartbeat, unlike the movement of our legs and arms, which we can control, for example.

Regarding its cellular structure, one of its main characteristics is the presence of intercalated discs located between adjacent cells. They serve to provide mechanical force and ensure that the contraction force generated by a single cell is expanded to neighboring cells.

The cells that make up the heart muscle are capable of generating their endogenous action potentials at periodic intervals. There are specialized cells called "pacemaker cells" that impose a heart rate on the entire heart, generating the action potential and dispersing it throughout the organ.

The most common pathologies that affect the heart are myocardial infarction, cardiomyopathies and myocarditis. These have different causes, both genetic and induced by drugs, infections or unhealthy lifestyle habits. To avoid them, constant physical exercise and the consumption of a balanced diet are recommended.

Basic anatomical and physiological aspects of the heart muscle

Structure and types of muscle tissues

One of the most conspicuous characteristics of the animal kingdom is movement, which is directed in its vast majority by the muscular system. Muscle cells function as molecular motors capable of transforming the ATP molecule, which is chemical energy, into mechanical energy.

The proteins involved in the contraction process are myosin and actin. For this reason, they are known as "contractile proteins."

In all animals, muscle is classified into two large groups: striated and smooth. In vertebrates, the first category includes skeletal (associated with muscles) and cardiac muscle.

On the contrary, the smooth is mainly found lining the interior of the hollow organs. Later we will describe the most important differences between these structures.

General structure of the heart

From the inside out, the heart is made up of three layers: endocardium, myocardium, and pericardium.

The role of the endocardium is to prevent the blood from modifying the properties of clotting. The second layer is the myocardium and its function is contractile. Finally, the pericardium is made up of two layers of fibrous tissue and is responsible for protecting the pumping organ. In this article we will focus on describing the second layer.

Structure and histology of the myocardium

Cellular characteristics

Histologically, the cardiac muscle exists only in the myocardium and in the proximal portions of the aorta and the vena cava. The muscle type is striated and similar in structure to voluntary contraction skeletal muscles. That is, the muscles that allow our daily movements, such as walking, exercising, among others.

The cells that make up the heart muscle are characterized by having a single central nucleus and being joined together by intercalated discs. These cells may or may not have branches.

These cellular characteristics allow distinguishing cardiac muscle from the rest of the muscle types, namely skeletal and smooth.

They are similar to skeletal muscle in their striated structure, since both this arrangement of contractile fibers can be observed. In contrast, myocardial cells have a single nucleus, whereas skeletal muscle cells are multinucleated.

Intercalary discs

Intercalar discs are complex interdigitations that exist between adjacent cells, and have three types of specializations: fascia adherens, macula adherens, and cleft junctions.

- The fascia adherens, composed of numerous filaments and related to the union of sarcomeres.

- The macula adherens, which is found in the intercalated discs and prevents the separation of cells during contraction.

- Slit junctions or gap junctions that allow direct ionic contact for electrical communication.

Therefore, although the cells are mononucleated, they actually function as a syncytium (a cell with multiple nuclei). In this way, myocardial cells behave as a whole (as a single unit muscle).

In addition to contractile cells, the myocardium also has a certain percentage of connective tissue made up of parallel collagen fibers. The function of this structure is to maintain the bond between cells and promote energy transmission.

Ultrastructure of the myocardium

Electron microscopy has helped elucidate the ultrastructure of these cardiac cells, and it has been found that compared to skeletal muscle:

- Cardiac cells have longer T tubules, - Each T tubule is associated with a terminal cistern forming dyads and not forming triads

- The sarcoplasmic reticulum is less defined.

The cells that make up cardiac muscle tissue are called cardiac myocytes, and the orientation just described is related to their function: it enables pressure to be exerted in the right direction.

The triads formed by invaginations of the sarcoplasmic reticulum occur because their expansions are located with two contacts to the T tubules, which continue to the outside of the cell membrane.

In addition, they have the typical organelles of a eukaryotic cell with high energy requirements, since they are cells that must contract more than 75 times per minute in a constant and rhythmic way.

As for the mitochondria, organelles responsible for the production of energy aerobically, they are particularly abundant in this cell type and they are grouped parallel to the axis in which the myofibrils run. They work hard to maintain a steady heartbeat.

Myocardial cell types

Not all cells of the heart are contractile, there are also excitable cells with a pacemaker function.

Cells with pacemaker activity are responsible for the rhythmic generation of action potentials and for conducting them throughout the organ. They are responsible for the periodic excitation of the heart. These are not very abundant, around 5% and they do not have the capacity to contract.

The second type are the most abundant (95% of the total cells of the heart mass) and perform the ordinary contraction work that allows efficient pumping of blood. The action potential occurs in five stages, with the resting membrane potential corresponding to -90mV.

Innervation

The cardiac muscle is innervated by branches from both the sympathetic and parasympathetic systems.

There is a set of modified heart fibers called Purkinje fibers (named after their discoverer, Jan Evangelista Purkinje), located in the walls of the ventricle under the endocardium. These form the intracardiac conduction system and coordinate the contraction of the ventricles.

Together with the aforementioned fibers, the system that orchestrates the electrical conduction of the heart is made up of a couple of additional elements: sinoatrial node, internodal fibers, atrioventricular node, and bundle of His. The potential starts at the sinoatrial node (the heart's natural pacemaker) and spreads throughout the rest of the system.

The His-Purkinje system is a conduction system that specializes in optimizing the speed of transmission of action potentials generated in the heart. They are easily recognizable as they are the largest cells in the heart, and are made up of only a few muscle fibers.

Regeneration

Cardiac muscle tissue lacks the ability to regenerate cells. In the event of a myocardial infarction, the tissue dies and is gradually replaced by tissue dominated by fibroblasts. New studies seem to challenge this fact.

Features

The heart muscle is responsible for the rhythmic and continuous contraction of the heart, which functions as a pump that orchestrates the passage of blood throughout the circulatory system.

The continuous movement of blood throughout the body is necessary to maintain a constant supply of oxygen. In addition to this vital gas, a flow of nutrients and the removal of waste products occur.

Diseases

Cardiomyopathies, myocarditis and other diseases are a fairly heterogeneous set of pathologies that affect the myocardium.

Most of these disorders translate into heart failure. They can have genetic or environmental causes, which means that they can be caused by infections or negative lifestyle habits of the patient.

Below we will describe the most frequent and those of greatest medical importance.

Cardiomyopathy or cardiomyopathy

Cardiomyopathy is a pathology that affects the heart muscle and consists of a harmful alteration in its shape. Generally, this change in shape hinders normal movements of systoles and diastoles.

It is caused by a wide range of diseases (hypertension, valvular diseases, infectious diseases) or it can be induced by the excessive consumption of drugs, alcohol, as well as by side effects of the consumption of some medications to treat depression. There are three types of cardiomyopathies:

- Hypertrophic. It consists of the increase in the thickness of the tissue of the ventricles, particularly the interventricular septum.

- Dilated. It is the reduction in thickness of the walls of the heart, increasing the area of ​​the cavities and decreasing the contraction pressure.

- Restrictive. It consists of the stiffness of the ventricles, which affects the normal filling of the pump.

Myocarditis

Myocarditis includes inflammation of the heart muscle, a phenomenon that affects the normal functioning of the heart in general and its electrical system.

One of the consequences of this inflammatory event is the reduction of blood pumping. By affecting the electrical system, the heart loses its rhythm and can cause arrhythmias.

The causes of myocarditis are generally infectious of viral origin, but it can also occur as a side effect of taking a medication or a general inflammatory pathology that is also affecting the heart.

In Latin America, one of the most important causes of myocarditis is the presence of the Trypanosoma cruzi parasite, the causal agent of Chagas disease.

The general symptoms of myocarditis are: chest pain, feelings of fatigue and tiredness, shortness of breath and breathing difficulties or unstable heart rates, among others.

If the condition is severe it can weaken the heart significantly, resulting in decreased blood supply to the body. If clots form, they can reach the brain and cause a stroke.

Myocardial infarction

This pathology consists of the localized death of muscle cells. At the time of the obstruction of the blood flow there is a suppression of the blood distribution. If the heart experiences prolonged oxygen suppression, the muscle dies.

The main cause of myocardial infarction is the obstruction of the coronary arteries, preventing the normal circulation of blood. For the normal functioning of this vital organ it is necessary for the blood to circulate freely.

The artery can become obstructed by the presence of a clot in the blood, by atherosclerosis, diabetes or hypertension, among others. Some habits of the patient can increase the risk of heart attack, since it accelerates the deterioration of the arteries such as the consumption of diets high in cholesterol, smoking or the consumption of drugs.

The characteristic symptom of a myocardial infarction is pain and pressure in the chest that spreads to the upper extremities, neck and back. Breathing becomes difficult and the patient tends to increase sweating.

Heart attack can be prevented through the implementation of healthy lifestyle habits that involve quitting smoking and alcoholic beverages, a nutrient-balanced diet, and practicing aerobic exercise.

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