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The heart is a muscular organ that pumps blood throughout the body. It is made up of a type of muscle tissue called cardiac muscle, or myocardium, which is one of three types of vertebrate muscle tissues, the others being skeletal muscle and smooth muscle. The myocardium forms the thick middle layer of the heart, between the outer layer of the heart wall (the pericardium) and the inner layer (the endocardium). It is composed of individual cardiac muscle cells, or cardiomyocytes, which are rectangular, branching cells that contract involuntarily to pump blood through the body.
| Characteristics | Values |
|---|---|
| Name | Cardiac muscle, myocardium, heart muscle |
| Location | Middle layer of the heart |
| Composition | Individual cardiac muscle cells (cardiomyocytes) joined by intercalated discs, encased by collagen fibres and other substances that form the extracellular matrix |
| Cell Composition | Chains of myofibrils, which are rod-like units within the cell |
| Myofibril Composition | Repeating sections of sarcomeres, which are the fundamental contractile units of the muscle cells |
| Sarcomere Composition | Long proteins that organize into thick and thin filaments (myofilaments) |
| Thick Filament Composition | Myosin protein |
| Thin Filament Composition | Actin protein |
| Contraction Mechanism | Release of calcium from the sarcoplasmic reticulum triggers the myofilaments to slide past each other in a process called excitation-contraction coupling |
| Relaxation Mechanism | Outward flow of potassium ions returns the membrane potential to its resting potential |
| Contraction Strength Determiner | Concentration of calcium in the myocyte |
| Contraction Speed Determiner | Beta-1 adrenergic receptors on the surface of the cell |
| Pacemaker Cells | Sinoatrial (SA) and atrioventricular (AV) nodes, which generate electrical impulses and control heart rate |
| Blood Supply | Coronary circulation |
| Functions | Pumping blood into circulation, generating sufficient force |
| Conditions | Cardiomyopathies, including hypertrophic, dilated, restrictive, and arrhythmogenic right ventricular dysplasia |
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What You'll Learn
Cardiac muscle cells
Specialised modified cardiomyocytes known as pacemaker cells set the rhythm of the heart contractions. These cells are only weakly contractile without sarcomeres and are connected to neighbouring contractile cells via gap junctions. They are distributed throughout the heart and are responsible for generating and sending out electrical impulses.
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Pacemaker cells
The heart is a muscular organ that pumps blood throughout the body. It is made up of three layers—the pericardium, myocardium, and endocardium. The myocardium is the muscular middle layer of the heart. The heart has four chambers—two on the top (atria) and two on the bottom (ventricles).
The heart's natural rhythm generator is the cardiac pacemaker. It employs pacemaker cells that produce electrical impulses, known as cardiac action potentials, which control the rate of contraction of the heart. These pacemaker cells are highly specialized myocardial cells with an intrinsic ability to depolarize rhythmically and initiate an action potential.
The SA node controls the rate of contraction for the entire heart muscle because its cells have the quickest rate of spontaneous depolarization, thus they initiate action potentials the fastest. The action potential generated by the SA node passes down the electrical conduction system of the heart, and depolarizes the other potential pacemaker cells at the atrioventricular (AV) node to initiate action potentials before these other cells have had a chance to generate their own spontaneous action potential. The AV node is an area between the atria and ventricles, within the atrial septum.
If the SA node does not function, or if the impulse generated is blocked before it travels down the electrical conduction system, a group of cells further down the heart will become its pacemaker. This is typically represented by cells inside the AV node. If the AV node also fails, Purkinje fibers are occasionally capable of acting as the default or "escape" pacemaker.
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Contractions
The heart is a muscle, but it is not like the biceps in your upper arms or the quads in your thighs. It is a hard-working, involuntary muscle that beats thousands of times a day to keep you alive.
Cardiac muscle, also called heart muscle or myocardium, is one of three types of vertebrate muscle tissues found in the human body. The other two types are skeletal muscle and smooth muscle. Cardiac muscle constitutes the main tissue of the wall of the heart and is composed of individual cardiac muscle cells, or cardiomyocytes, joined by intercalated discs.
Cardiac muscle contracts in a similar manner to skeletal muscle, although with some important differences. Electrical stimulation in the form of a cardiac action potential triggers the release of calcium from the cell's internal calcium store, the sarcoplasmic reticulum. This release of calcium causes the cell's myofilaments to slide past each other in a process called excitation-contraction coupling. The sliding of actin and myosin past each other produces the formation of "cross-bridges," which causes contraction of the heart and generation of force. The concentration of calcium in the myocyte is the critical factor that determines how much force is generated with each contraction.
The primary function of cardiac muscle is to pump blood into circulation by generating sufficient force. The mechanism behind each coordinated contraction involves the cardiac muscle and electrical impulses. These contractile functions of the heart require ATP, which can be obtained through various substrates, including fatty acids, carbohydrates, proteins, and ketones.
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Cardiomyopathies
The heart is a muscular organ that pumps blood throughout the body. It is made up of three layers: the pericardium, myocardium, and endocardium. The myocardium is the muscular middle layer of the heart. The heart also has four chambers, two on top (atria) and two on the bottom (ventricles).
Cardiomyopathy is a disease of the heart muscle that makes it harder for the heart to pump blood to the rest of the body. Cardiomyopathy causes the heart muscle to become enlarged, thick, or rigid. As cardiomyopathy progresses, the heart becomes weaker and less able to pump blood throughout the body. This can lead to heart failure or irregular heartbeats called arrhythmias. A weakened heart can also cause other complications, such as heart valve problems.
There are several types of cardiomyopathies, including dilated cardiomyopathy, hypertrophic cardiomyopathy, arrhythmogenic cardiomyopathy, and restrictive cardiomyopathy. Dilated cardiomyopathy occurs when the ventricles, the lower chambers of the heart, weaken and become larger. This makes the heart work harder to pump blood. Hypertrophic cardiomyopathy occurs when the heart muscle becomes larger and thicker than normal, which can block the ventricles and make it harder for the heart to pump blood. Arrhythmogenic cardiomyopathy is a rare condition that develops when fatty or scarred tissue replaces normal muscle tissue in the right ventricle, causing an irregular heartbeat. Restrictive cardiomyopathy is also rare and causes the ventricles to stiffen, preventing them from relaxing and filling with enough blood to pump to the rest of the body.
Cardiomyopathy can be acquired due to another disease, condition, or factor, or it can be inherited when the gene for the disease is passed on from a parent. In some cases, cardiomyopathy may have no signs or symptoms and require no treatment. However, in other cases, it can develop quickly with severe symptoms and serious complications, requiring treatment such as lifestyle changes, medications, surgery, or implanted devices to correct arrhythmias.
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Cardiac muscle tissue
The human body contains three kinds of muscle tissue: skeletal, smooth, and cardiac. Cardiac muscle tissue, also called the myocardium, is a type of muscle tissue that forms the heart. It contracts and releases involuntarily, keeping the heart pumping blood around the body.
Cardiac muscle cells (cardiomyocytes) are striated, branched, and contain many mitochondria. Each cardiomyocyte contains a single, centrally located nucleus surrounded by a cell membrane known as the sarcolemma. The sarcolemma of cardiac muscle cells contains voltage-gated calcium channels, which are specialized ion channels that skeletal muscles do not possess.
Cardiac muscle cells contain branched fibres connected via intercalated discs that contain gap junctions and desmosomes. These interconnections allow the cardiomyocytes to contract together synchronously, enabling the heart to work as a pump. When a cardiac muscle cell contracts, the myosin filament pulls the actin filaments towards each other, causing the cell to shrink. The cell uses ATP to power this contraction.
The heart has four muscular sections or chambers that briefly hold blood before moving it. The muscular ventricular septum originates from the bottom of the ventricle, with a membranous septum forming shortly after, joining with the aortic-pulmonary septum as it twists down and fuses. The heart's contractile functions require ATP, which can be obtained through various substrates, including fatty acids, carbohydrates, proteins, and ketones.
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Frequently asked questions
The heart is made of cardiac muscle, also called myocardium.
Cardiac muscle is one of three types of muscle tissues in the body, the others being skeletal and smooth muscle.
Cardiac muscle contracts and relaxes involuntarily, keeping the heart pumping blood around the body.
Cardiac muscle appears striated or striped under a microscope. These stripes occur due to alternating filaments that comprise myosin and actin proteins.
