Victor Forge
The heart is a vital organ that pumps blood throughout the body. It is made up of muscle and tissue and has four muscular sections or chambers that briefly hold blood before moving it. The heart wall is a three-layered structure with a thick layer of myocardium or cardiac muscle forming the bulk of the heart. This middle layer is sandwiched between the outer layer of the heart wall (the pericardium or epicardium) and the inner layer (the endocardium).
| Characteristics | Values |
|---|---|
| Name | Cardiac muscle, myocardium, heart muscle |
| Number of layers | 3 |
| Outer layer | Pericardium, visceral pericardium, epicardium, outer layer of the heart wall |
| Middle layer | Myocardium, thick layer of myocardium |
| Inner layer | Endocardium, inner layer of the heart wall, simple squamous epithelial cells |
| Muscle cells | Cardiomyocytes, contractile myocytes, rectangular, branching cells |
| Muscle cell structure | Chains of myofibrils, repeating sections of sarcomeres, thick and thin filaments (myofilaments) |
| Thick filaments | Myosin |
| Thin filaments | Actin |
| Contraction mechanism | Calcium ions bind to troponin and tropomyosin, uncovering binding sites on actin; myosin binds to actin, pulling thick filaments along thin filaments |
| Relaxation mechanism | Calcium concentration falls, troponin and tropomyosin cover binding sites on actin |
| Pacemaker cells | Located in sinoatrial node, atrioventricular node, bundle of His, Purkinje fibers |
| Contraction regulation | Autonomic nervous system, sympathetic nervous system |
| Contraction frequency | Determined by metabolic demand |
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What You'll Learn
The heart's three layers
The heart is a muscular organ that pumps blood throughout the body. It is the main organ of the cardiovascular system, a network of blood vessels that supplies blood and oxygen to all parts of the body. The heart is composed of four muscular chambers that briefly hold blood before moving it.
The heart wall has three layers, each with its own function: the outer epicardium, the middle myocardium, and the inner endocardium. The heart is surrounded by a fluid-filled sac called the pericardium, which serves to protect this vital organ. The outer layer of the pericardium is called the visceral pericardium or epicardium. It forms the inner layer of the pericardium and functions to protect the inner heart layers and assist in the production of pericardial fluid, which helps reduce friction between the pericardial membranes.
The myocardium is the thickest layer of the heart wall, composed of cardiac muscle fibres, which enable the heart to contract. The thickness of the myocardium varies in different parts of the heart, with the left ventricle being the thickest layer as it is responsible for generating the power needed to pump oxygenated blood from the heart to the rest of the body. The myocardium houses the pacemaker cells and specialised cardiomyocytes that generate and conduct the cardiac electric impulses.
The endocardium is the innermost layer of the heart wall, lining the internal chambers of the heart and the heart valves. It provides a smooth surface for efficient blood flow inside the heart chambers. It also houses capillaries to supply blood to the heart muscles, nerve fibres, and heart conduction cells.
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Cardiac muscle cells
Cardiomyocytes are surrounded by an extracellular matrix produced by supporting fibroblast cells. They are joined together at their ends by intercalated discs to create a syncytium of cardiac cells. Within the intercalated disc, there are three different types of cell junctions: fascia adherens, desmosomes, and gap junctions. The lateral side of the discs contains gap junctions that permit intercellular communication by allowing ions from one cardiomyocyte to move to a neighbouring cell without having to be excreted into the extracellular space first.
Specialised modified cardiomyocytes known as pacemaker cells set the rhythm of the heart contractions. Pacemaker cells are only weakly contractile without sarcomeres and are connected to neighbouring contractile cells via gap junctions. They are located in the sinoatrial node (the primary pacemaker) and the atrioventricular node (secondary pacemaker).
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The role of pacemaker cells
The heart is a vital organ that pumps blood throughout the body. It is composed of muscle and tissue, with cardiac muscle tissue, or myocardium, forming the bulk of the heart. The heart wall has three layers: the outer epicardium, or visceral pericardium, the thick middle layer of myocardium, and the inner endocardium.
The heart's natural pacemaker is the sinus node, or sinoatrial node (SA node), a small mass of specialized cells in the top of the right atrium, or upper chamber of the heart. The SA node is made up of specialized cardiomyocytes known as pacemaker cells, which can spontaneously generate cardiac action potentials, or electrical impulses, that cause the heart to beat. These action potentials are rapid electrical changes that occur across the membrane of certain cells and are propagated through the heart's electrical conduction system.
Pacemaker cells are responsible for setting the rhythm and pace of the heartbeat. They are distributed throughout the heart and are connected to neighbouring contractile cells via gap junctions, which enable them to locally depolarize adjacent cells and contract in a coordinated manner. This allows all contractile cells of the heart to act in sync and contract as a unit.
In most cases, the SA node acts as the primary pacemaker and regulates the heart's sinus rhythm. However, if the SA node is damaged or non-functional, a secondary pacemaker may set the pace. This is typically represented by cells in the atrioventricular node (AV node), located between the atria and ventricles within the atrial septum. If the AV node also fails, Purkinje fibers may act as a default or "escape" pacemaker.
In cases where the heart's natural pacemaker is defective or blocked, an artificial pacemaker may be implanted to replace its functions. These devices detect abnormal heartbeats and deliver electrical impulses to restore a normal heart rhythm.
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Calcium's role in contraction
The heart is a vital organ that pumps blood throughout the body. It is composed of cardiac muscle tissue, or myocardium, which forms the bulk of the heart. This muscle tissue is made up of over three billion heart muscle cells, each with its own machinery to contract and relax.
Calcium plays a crucial role in the contraction of the heart. The heart beats more than two billion times during an average person's lifetime, and calcium particles are responsible for linking electrical activation to mechanical contraction. During each heartbeat, calcium particles with an electrical charge enter the heart muscle cells and contribute to the electrical signal that moves from cell to cell.
Calcium ions bind to the protein troponin, along with tropomyosin, which then uncovers key binding sites on actin. Myosin, in the thick filament, can then bind to actin, pulling the thick filaments along the thin filaments. This process is known as cross-bridge cycling. The influx of calcium ions into the cell increases the driving force for calcium release through open ryanodine receptors (RyRs) and also increases the number of open RyRs.
When the concentration of calcium within the cell falls, troponin and tropomyosin once again cover the binding sites on actin, causing the cell to relax. This relaxation allows the heart to refill with blood before the next heartbeat.
Abnormal calcium movement can impair the contraction or relaxation of the heart, hindering its normal pump function. This can lead to heart failure and potentially deadly heart rhythm disorders.
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How cardiac muscle differs from other muscles
The heart is a vital organ that pumps blood throughout the body. It is made up of muscle and tissue and contains four muscular sections or chambers that briefly hold blood before moving it. The heart wall is a three-layered structure, with a thick layer of myocardium or cardiac muscle tissue forming the bulk of the heart.
Cardiac muscle is one of three types of muscle tissues found in the human body, the others being smooth muscle and skeletal muscle. Unlike skeletal muscle, cardiac muscle is not under voluntary control. It is composed of individual cardiac muscle cells or cardiomyocytes joined by intercalated discs and encased by collagen fibres and other substances that form the extracellular matrix.
Cardiac muscle contracts in a similar manner to skeletal muscle, but with some key differences. Electrical stimulation triggers the release of calcium from the cell's internal store, the sarcoplasmic reticulum. As the thick and thin filaments slide past each other, the cell becomes shorter and fatter. Calcium ions bind to the protein troponin, which then uncovers binding sites on actin. Myosin in the thick filament can then bind to actin, pulling the thick filaments along the thin filaments. When the concentration of calcium within the cell falls, the cell relaxes.
Cardiac muscle cells are highly dependent on aerobic respiration and require a constant flow of blood to provide oxygen and nutrients. The coronary arteries supply blood to the myocardium, and the coronary veins drain the blood away.
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Frequently asked questions
The heart is covered by the myocardium, also known as the cardiac muscle. It is one of three types of muscle tissues in the body, the other two being skeletal and smooth muscle.
The myocardium forms the bulk of the heart and is responsible for the heart's contractility, allowing it to pump blood into circulation. It receives electrical impulses from the brain and pacemaker cells, which trigger the release of calcium and subsequent contraction of the heart muscle.
The myocardium is a thick layer of muscle tissue sandwiched between the outer epicardium (or visceral pericardium) and the inner endocardium. The endocardium lines the cardiac chambers and covers the cardiac valves. The myocardium itself is composed of individual cardiac muscle cells or cardiomyocytes, which are joined by intercalated discs and encased by collagen fibres.
