Myocardium n., plural: myocardia Muscular portion of the heart located in between the epicardium and the endocardium Source: modified by Maria Victoria Gonzaga, ubraintv-jp.com.org, from the work of Blausen.com staff (2014). “Medical gallery of Blausen Medical 2014”. WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436. Source, CC BY 3.0.

Myocardium Definition

What is the myocardium of the heart? It is the muscular middle layer of the heart that is sandwiched between the epicardium and endocardium. Sometimes it is referred to as the mass of the heart. The myocardium is made up of two types of cells: cardiac muscle cells (also called heart muscle cells,cardiomyocytes, cardiac myocytes, or cardiac rahbdomyocytes) and fibroblast.

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The heart is the central circulatory organ in the human body that circulates the blood throughout the body. The heart is located in a thin fibroelastic, double-layered, fluid-filled sac known as “pericardium”. The two layers of the pericardium are the outer fibrous or parietal pericardium and the inner serous or visceral pericardium.

Myocardium (biology definition): The middle layer of the wall of the heart; the muscular substance of the heart located in the middle, i.e. in between the epicardium and the endocardium. Etymology: from Ancient Greek mûs (“muscle”) + kardía (“heart”).
Figure 1: The three layers of the heart wall. Source: Modified by Maria Victoria Gonzaga, ubraintv-jp.com, from OpenStax College, CC BY 3.0.

There are three layers in the walls of the heart namely:

Epicardium: It is the outermost layer of the heart wall. This contains mesothelial cells under which are connective and adipose tissues. This layer also constitutes the visceral layer of the serous pericardium. Below the epicardium coronary arteries and veins, lymphatic vessels, and nerves are present. Myocardium: This is the muscle layer of the heart responsible for the pumping action of the heart and occupies 95% of the cardiomyocytes mass and is the thickest layer in the heart wall. The thickness of the myocardial layer is dependant on the pressure which is present in each chamber. So, the question arises out of the four chambers of the heart, which chamber has the thickest myocardium. The myocardial layer is thin in the atrium while the myocardium is thickest in the ventricles specifically in the left ventricle. As a matter of the fact, in adult animals, the left ventricular myocardium is three times thicker than the right ventricular myocardium. This is quite intriguing and instigates the query as to why is the left ventricle thicker than the right ventricle. The walls of the left ventricle of the heart are thicker than the right ventricle since the left ventricle pumps the blood throughout the body and also against the higher pressure in compassion to the right ventricle. Endocardium: This is the innermost layer that lines the inner wall of the heart and also lines the heart valves. The endocardium is further subdivided into two layers: (1) an inner endothelial cell layer that lines the heart chamber and (2) the second layer is the subendocardial layer which is a connective tissue layer in continuation with the connective tissue of the myocardial connective tissue layer. The impulse-conduction system of the heart is placed in this subendocardial layer.
The heart of the vertebrates is a muscular organ that pumps blood to various parts of the body. It moves the blood through rhythmic contractions. The wall of the heart is made up of three layers: (1) epicardium (outermost), (2) myocardium (middle), and (3) endocardium (innermost).
Figure 2: Myocardium and endocardium. Source: Alessandro Scalese and Mikael Häggström, M.D., CC-BY-SA 4.0.

Cardiac Muscle Histology

What is cardiac muscle? It is important to understand the definition of cardiac muscles. Basically, cardiac muscles are the involuntary, striated muscles that are made up of cardiac muscle cells or cardiomyocytes.

Cardiac muscles are one of the three types of vertebrate muscles (the other two being, skeletal muscles and smooth muscles).Cardiac muscles share some of the features of both smooth muscles as well as skeletal muscles. Like skeletal muscles, cardiac muscles are striated and can induce strong contractions as well can initiate continuous contractions like smooth muscles. However, cardiac muscles possess certain unique characteristics.

The cardiomyocytes are arranged in a helical or overlapping spiral pattern and are fastened to the fibrous skeleton of the heart. This helical pattern results in a complex 3-dimensional structure network.

Cardiac muscle fibers are elongated cylindrical in shape, approximately 50–100 μm long and 10–25 μm wide. These muscle fibers are made up of distinct quadrangular cells having a centrally located clear oval nucleus. The rectangular-shaped muscle cells are often branched and joined end to end forming a syncytium.

The two cardiac muscle cells meet to form a special junctional complex known as an intercalated disc. These intercalated disks contain three essential components:

Figure 3: Cardiac muscle structure. (a) Cardiac muscle cells. (b) Cardiac muscle cells showing the nuclei and intercalated discs. (c) Desmosomes and gap junctions. Source: OpenStax.org

Structurally, intercalated disk represents the double membranes formed due to tightly bound cells by desmosomes that are connected by gap junctions. This is required for transferring cell-to-cell electrical impulse conduction. Gap junctions aid in the electrical coupling of the muscle cells, thereby promoting the synchronized beating of the heart muscles.

Cardiac muscle cells are capable of strong, continuous, and rhythmic contractions generated automatically. Though, the autonomic nervous system and hormones can alter the contractility of the heart muscle cells.

Under a microscope, intercalated disks can be seen as faint lines running perpendicular to the long axis of the cardiac muscle fiber. In the adult human, a brown color pigment located at a perinuclear position can be usually seen. This brown color pigment is “lipofuscin“, which is considered as “wear and tear” pigment, is the accumulation of the lipids, phospholipids, and proteins after lipid peroxidation.

Sarcomeres are the functional unit of the cardiac myocyte and contraction. Within each myocyte, the contractile fibers of the sarcomeres are surrounded by transverse discs, known as Z-bands. Each myocyte possesses a number of sarcomeres that are stacked end to end and circumferentially forming a “cable effect” within the cell.

There are two important proteins found in cardiac muscle cells are myosin and actin. The thick filaments are made up of myosin while thin filaments are of actin. These two proteins collectively form the myofibrillar filament that functions to carry out the contractile function in the cardiac muscle tissues.

The sarcoplasm (i.e., the cytoplasm of the cardiac myocytes) of the cardiac muscle cells is extremely rich in mitochondria due to the high energy requirement for regular contractions. Oxidative phosphorylation occurs in the cardiac myocytes to meet the energy requirements. Thus, these muscles require an uninterrupted supply of oxygen. The blood is supplied to the myocardium by coronary arteries. Additionally, glycogen granules are present in the myofibrils to support the additional energy requirement.

The myocardium has the largest oxygen requirement and is most critically affected by the reduction in blood flow. Reduction in blood flow i.e. ischemiacan be potentially critical for these muscles. Also, cardiac muscles are resistant to fatigue. Essentially, cardiac muscles create their own action potential, i.e. myogenic in nature; however, certain modified muscle cells are specialized to generate the stimulus for the heartbeat and conduct the impulse to different segments of the myocardium system.

These specialized cells form the myocardial conduction system that consists of the sinoatrial node, atrioventricular node, the bundle of His, and Purkinje fibers. 99% of the myocardium is made up of contractile cells while 1% of the cells are part of the myocardial conducting system.

The conducting myocardial cells are specialized muscle cells that function in the same way as neurons. These conducting cells initiate and spread the action potential throughout the heart while the contracting myocardial cells take up this action potential to pump the heart rhythmically and regularly.

Functions of the Myocardium

To understand the circulatory system, it is important to understand what is the function of the myocardium. The primary function of the cardiac muscles is to stimulate contractions and relaxation in the heart. The contraction action results in the pumping of the blood from the ventricles to the whole body while the relaxation action of the cardiac muscles allows the atrium to receive blood. This beating of the heart pumps the blood throughout the body ensuring that each cell and tissue of the body receives the blood supply.

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Cardiac muscles are essentially under the control of an autonomous nervous system that releases the timed nervous impulses signaling the heart cells to contract and relax in a rhythmic pattern.