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The myocardium is the muscular layer of the heart. It consists of cardiac muscle cells (cardiac myocytes [also known as cardiac rhabdomyocytes] or cardiomyocytes) arranged in overlapping spiral patterns.
What is Myocardium?
It is the heart’s muscular middle layer, wedged between the epicardium and the endocardium. It is sometimes referred to as the cardiac mass. Cardiac muscle cells (also known as heart muscle cells, cardiomyocytes, cardiac myocytes, or cardiac rahbdomyocytes) and fibroblasts make up the myocardium.
The heart is the human body’s primary circulatory organ, which pumps blood throughout the body. The heart is housed in the “pericardium,” a thin fibroelastic, double-layered, fluid-filled sac. The outer fibrous or parietal pericardium and the interior serous or visceral pericardium are the two layers of the pericardium.
The middle layer of the heart’s wall; the muscular material of the heart lying in the middle, i.e. between the epicardium and the endocardium (biology term). Mûs + karda are two Ancient Greek words that mean “muscle” and “heart,” respectively.
The heart’s walls are divided into three layers:
1. Epicardium: The epicardium is the heart’s outermost layer. Underneath the mesothelial cells lie connective and adipose tissues. The visceral layer of the serous pericardium is likewise made up of this layer. There are coronary arteries and veins, lymphatic vessels, and nerves beneath the epicardium.
2. Myocardium: This is the heart’s muscle layer, which is responsible for the heart’s pumping activity. It accounts for 95 percent of the mass of cardiomyocytes and is the thickest layer in the heart wall. The pressure present in each chamber determines the thickness of the myocardial layer. As a result, the issue arises as to which of the four chambers of the heart has the thickest myocardium. The atrium has a thin myocardial layer, whereas the ventricles, particularly the left ventricle, have the thickest myocardium.
In fact, the left ventricular myocardium in mature animals is three times thicker than the right ventricular myocardium. This is fascinating, and it raises the question of why the left ventricle is thicker than the right. Because the left ventricle pumps blood across the body and also against the higher pressure in contrast to the right ventricle, the left ventricle’s walls are thicker than the right ventricle’s.
3. Endocardium: This is the deepest layer of the heart that borders the inner wall as well as the heart valves. The endocardium is further split into two layers:
(a) an inner endothelial cell layer that borders the heart chamber.
(b) a subendocardial layer that continues the connective tissue of the myocardial connective tissue layer. This subendocardial layer houses the heart’s impulse-conduction mechanism.
The heart is a muscular organ in vertebrates that pumps blood to various areas of the body. It circulates blood by causing rhythmic contractions. The heart’s wall is made up of three layers: the epicardium (outermost), the myocardium (middle), and the endocardium (innermost) (innermost).
Cardiac Muscle Histology
It is critical to comprehend the meaning of cardiac muscles. In a nutshell, cardiac muscles are striated, involuntary muscles made up of cardiac muscle cells, or cardiomyocytes.
One of the three types of muscles identified in vertebrates are cardiac muscles (the other two being skeletal muscles and smooth muscles). Cardiac muscles have certain characteristics of both smooth and skeletal muscles. Cardiac muscles, like skeletal muscles, are striated and may cause powerful contractions as well as begin continual contractions. Cardiac muscles, on the other hand, have certain distinct features.
The cardiomyocytes are attached to the fibrous skeleton of the heart in a helical or overlapping spiral arrangement. This helical pattern produces a three-dimensional structural network that is complicated.
Cardiac muscle fibres are elongated cylindrical in form, with a length of 50–100 m and a width of 10–25 m. The muscle fibres are made up of discrete quadrangular cells with a clear oval nucleus in the centre. Muscle cells that are rectangular in shape are frequently branched and connected end to end to create a syncytium.
An intercalated disc is formed when two cardiac muscle cells come together to produce a unique junctional complex. Three key components are included in these intercalated discs:
1. Desmosomes connect the cytoskeleton’s intermediate filaments.
2. Gap junctions with low electrical resistance that allow excitation to spread.
3. Adhering junctions, also known as fascia adherens, are linkages between actin filaments that allow contraction to be transmitted.
An intercalated disc is a structural term that refers to the double membranes created by closely linked cells and desmosomes joined by gap junctions. This is necessary for the transmission of electrical impulses from one cell to the next.
Gap junctions facilitate the electrical connection of muscle cells, allowing the heart muscles to pulse in unison. Cardiac muscle cells can create powerful, continuous, and rhythmic contractions on their own.
The autonomic nervous system and hormones, on the other hand, can affect the contractility of heart muscle cells. Intercalated discs appear as faint lines perpendicular to the long axis of the heart muscle fibres under a microscope.
A brown colour pigment situated in a perinuclear location may generally be detected in an adult human. The buildup of lipids, phospholipids, and proteins following lipid peroxidation is the brown colour pigment “lipofuscin,” which is called a “wear and tear” pigment.
The cardiac myocyte’s functional unit for contraction is the sarcomere. The contractile fibres of the sarcomeres are encircled by transverse discs known as Z-bands within each myocyte. A number of sarcomeres are stacked end to end and circumferentially in each myocyte, producing a “cable effect” within the cell.
Myosin and actin are two essential proteins present in heart muscle cells. The heavy filaments are made up of myosin, whereas the thin filaments are made up of actin. These two proteins work together to produce the myofibrillar filament, which is responsible for contractile activity in cardiac muscle tissues.
Due to the high energy demand for regular contractions, cardiac muscle cells’ sarcoplasm (i.e., the cytoplasm of cardiac myocytes) is unusually abundant in mitochondria. The cardiac myocytes use oxidative phosphorylation to satisfy their energy needs. As a result, these muscles demand a constant supply of oxygen. Coronary arteries transport blood to the myocardium. Glycogen granules are also found in the myofibrils to support the increased energy demand.
The myocardial has the highest oxygen demand and is the most severely impacted by reduced blood flow. Ischemia, or a reduction in blood flow, can be dangerous for these muscles. Cardiovascular muscles are also resistant to exhaustion. Myogenic in nature, cardiac muscles generate their own action potential. However, some modified muscle cells are specialised to generate the stimulus for the heartbeat and convey the impulse to various parts of the myocardial system.
The Sinoatrial node, atrioventricular node, bundle of His, and Purkinje fibres are all specialised cells that make up the cardiac conduction system. Contractile cells make up 99 percent of the myocardium, whereas the myocardial conducting system makes up only 1% of the cells.
Conducting myocardial cells are specialised muscle cells that work similarly to neurons. The action potential is initiated and distributed throughout the heart by these conducting cells, and the contracting myocardial cells pick it up to pump the heart rhythmically and regularly.
It is critical to comprehend the function of the myocardial in order to comprehend the circulatory system. The cardiac muscles’ principal role is to promote heart contractions and relaxation.
The contraction of the cardiac muscles causes blood to be pumped from the ventricles to the rest of the body, while the relaxation of the cardiac muscles enables blood to enter the atrium. The heart’s pounding circulates blood throughout the body, ensuring that every cell and tissue receives oxygen and nutrients.
Cardiac muscles are primarily controlled by an independent neurological system that sends out timed nerve impulses to the heart cells, causing them to contract and relax in a regular manner.
The sarcoplasmic reticulum is stimulated by an action potential or nerve impulse, resulting in the release of calcium ions (Ca2+) into the cytoplasm. Troponin is induced to release tropomyosin by cytoplasmic Ca2+ ions. Tropomyosin that has been released changes its location, allowing myosin to bind to actin.
The stored ATP molecules are subsequently used by myosin to shorten each sarcomere. In the absence of the impulse, however, fast Ca2+ reabsorption into the sarcoplasmic reticulum occurs. Troponin re-anchors itself to tropomyosin in the absence of Ca2+, resulting in cardiac muscle cell relaxation. Twitch contractions occur in the heart muscle, with a protracted refractory time followed by short relaxation intervals. The heart has to relax in order to fill the atrium with blood for the next cycle to begin.
The force on the walls of the heart chamber is generated when all of the cardiac muscles act in unison. The cardiac muscle sheets are arranged in a planar pattern, with each muscle perpendicular to the others. When the heart contracts, it does so in numerous directions as a result of this. As a result of the contraction of the numerous layers of cardiac muscle fibre, the ventricle and atrium decrease from top to bottom and side to side. This causes the ventricles to pump and twist vigorously, pushing blood throughout the body.
It’s crucial to remember that cardiac muscle passes via aerobic metabolism, which mostly uses lipids and carbs.
Primary (genetic) and secondary (non-genetic) cardiac disorders are the two kinds of myocardial dysfunction (mostly acquired but may be precipitated for genetic reasons).
i. Cardiac Myopathy
Cardiomyopathies are heart illnesses caused by a faulty myocardium. Cardiomyopathy is one of the leading causes of illness and death worldwide. Cardiomyopathies are classified into five groups based on their clinical manifestations:
1. Dilated Cardiomyopathy: Ventricular dilation and congestive heart failure symptoms. Some of the primary contributing causes of this disease are arterial hypertension, myocarditis, alcohol misuse, or tachyarrhythmias.
2. Hypertrophic Cardiomyopathy: Hypertrophic cardiomyopathy is caused by ventricular hypertrophy, particularly in the left ventricle. This cardiomyopathy is thought to be passed on through the generations via the sarcomere.
3. Restrictive Cardiomyopathy: Scarring and stiffness of the ventricle walls, as well as impaired diastolic filling of the heart, are seen in this condition.
4. Arrhythmic Cardiomyopathy: Arrhythmic cardiomyopathy is caused by genetically faulty desmosomes. Non-ischemic cardiomyopathy is characterised by arrhythmias. The bulk of the instances recorded had an issue with the right ventricle, but there have been a few occurrences of left ventricles as well.
5. Unclassified Cardiomyopathy: Unclassified cardiomyopathy is a kind of cardiomyopathy that does not fit into any of the other categories.
ii. Myocardial Infarction (Heart Attack)
The heart tissues demand a large amount of oxygen and energy that is delivered continuously. The coronary arteries carry oxygen from the lungs to the heart. These arteries, on the other hand, are predisposed to the development of atheromas, which are aberrant deposits of fatty acids, cholesterol, and other cell detritus. If these atheromas are big, they impede or decrease blood and oxygen flow to the cardiac cells, resulting in a disease known as myocardial infarction or heart attack.
Myocardial ischemia is a condition in which the oxygen flow to the heart cells is reduced. The absence of oxygen causes heart tissue to die. However, the damaged region is healed as part of the human body’s natural physiological reaction. Though the mending causes fibrous tissue to grow at the location, this disrupts the usual propagation and conduction of excitatory impulses, resulting in aberrant cardiac contractions. These asynchronous contractions cause cardiac arrhythmias, or irregular heartbeats, such as ventricular fibrillation.
Cardiomyopathy is a heart muscle illness that lasts a long time.
1. Ischemic Cardiomyopathy: Diffuse coronary artery disease can cause prolonged heart ischemia, resulting in dilated cardiomyopathy. This can happen after one or more silent myocardial infarction events.
2. Metabolic Cardiomyopathy: Diabetes mellitus causes high blood glucose levels, which leads to cardiac dysfunction and metabolic cardiomyopathy.
3. Peripartum Cardiomyopathy: Peripartum cardiomyopathy is defined as left ventricular systolic dysfunction occurring within one month after childbirth or five months after delivery.
4. Tachycardia-induced Cardiomyopathy: Tachycardia-induced cardiomyo-pathy is caused by a chronic and consistently high heart rate (> 110 beats per minute), as observed in prolonged ventricular tachycardia or atrial fibrillation. It can eventually lead to cardiac failure if it is not addressed.
If cardiomyopathy is not addressed, it might result in the following complications:
1. Heart Failure: Ineffective heart pumping leads to inadequate blood to satisfy your body’s demands, which can be fatal.
2. Blood Clots: Blood clots can form, and these clots can limit blood supply to organs such as the heart and brain, finally leading to stroke.
3. Valve Problems: Cardiomyopathy can cause the heart to expand, causing the heart valves to close incorrectly.
4. Cardiac Arrest and Rapid Death: Cardiomyopathy can cause cardiac arrhythmias, which can cause fainting or death.
Myocarditis is caused by inflammation of the myocardium. This might be due to a number of factors, including viral infection (infectious myocarditis), toxic chemicals, medication allergies, bacterial, fungal, or parasite infection, and an autoimmune disease. In myocarditis, both cardiomyocytes and cardiac vascular endothelial cells can be damaged and lost. The white blood cells infiltrate the heart muscle wall as a result of this.
Interstitial cardiac fibrosis, wall motion anomalies, arrhythmias, heart failure, myocardial infarctions, decreased ejection fraction, and sudden cardiac death can all occur as a result of this. Chest discomfort, dyspnea, and flu-like symptoms are common symptoms of myocarditis, although it can also be asymptomatic.
v. Heart Failure
Heart failure or congestive heart failure, is a frequent end-stage route and symptom of cardiac dysfunction that can occur for a number of pathophysiological reasons. Heart failure occurs when the heart is unable to pump enough blood throughout the body, causing congestion, decreased organ perfusion, and functional impairment. Heart failure can be classified as acute or chronic, right heart vs. left heart, or systolic vs. diastolic, depending on the cause, location, and duration.
Each of these disorders has its own clinical manifestations and features. Myocardial injury or infarction, persistent hypertension, valve dysfunction, arrhythmias, cardiomyopathies, and other causes and risk factors can all lead to heart failure. Shortness of breath, extreme fatigue, and leg edema are all frequent signs of coronary heart failure. The use of medicines such as ACE inhibitors and beta-blockers, among other things, is crucial in the treatment of heart failure.
vi. Perioperative Myocardial Injury
Perioperative myocardial damage is a myocardial complication that can develop after non-cardiac surgery and is often underappreciated. It’s important to emphasise that this condition is not the same as a myocardial infarction. Perioperative myocardial damage is more likely in patients under the age of 65 who have a history of atherosclerotic disease.
There is a significant rise in cardiac troponin T plasma concentration in this situation (hs-cTn). This disease is frequently diagnosed and exhibited without the presence of chest discomfort, dyspnea, or other classic heart damage signs. Because most of the typical symptoms are absent, this disease is frequently misdiagnosed.
It is critical to do hs-cTn screening perioperatively to rule out the occurrence of this syndrome. The risk of 30-day death after noncardiac surgery has been demonstrated to be substantially increased by perioperative myocardial damage.
Biological Importance of Myocardium
The heart is the body’s primary pumping organ. The pumping of blood throughout the body is caused by the contraction and relaxation of the heart. The cells and tissues of the body require a certain quantity of oxygen, which is provided by the circulating blood, for optimal functioning.
The myocardium, or cardiac muscles, are in charge of the heart’s rhythmic and continual contraction and relaxation, or pumping activity. Cardiac muscles are one of the first embryonic organs that form and operate throughout life. These muscles are part of a complicated system that includes coronary arteries, cardiac lymphatics, and autonomic nerves.
Cardiomyocytes make up the heart’s thickest layer. The myocardial is such a vital organ in the body that its malfunction can be fatal. Cardiovascular disease is the biggest cause of mortality in the globe. The myocardium is involved in a variety of cardiac disorders, resulting in contractile dysfunction, cell death, and ventricular pump failure.
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