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The heart is a muscular organ that pumps blood throughout the body. It is made up of cardiac muscle, also known as myocardium, which is one of the three types of vertebrate muscle tissues, the other two being skeletal muscle and smooth muscle. The heart has four muscular sections or chambers that briefly hold blood before moving it. The cardiac muscle cells are interconnected through intercalated discs, which allow action potentials to spread between cardiac cells by permitting the passage of ions between cells, producing depolarization of the heart muscle. The heart is an involuntary muscle, meaning it beats on its own without conscious input.
| Characteristics | Values |
|---|---|
| Type of muscle | Cardiac muscle, also called myocardium |
| Location | Middle layer of the heart |
| Involuntary/Voluntary | Involuntary |
| Function | Pumping blood through the body |
| Composition | Individual cardiac muscle cells (cardiomyocytes) joined by intercalated discs |
| Pacemaker cells | Located in the sinoatrial node (SA) and atrioventricular node (AV) |
| Sheets of muscle | Wrap around the left ventricle |
| Nutrients | Supplied by coronary arteries |
| Veins | Drain blood away through the coronary sinus into the right atrium |
| Regeneration | Cardiac muscle cells have minimal regeneration capabilities |
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What You'll Learn
Cardiac muscle cells
Cardiomyocytes are the individual cells that make up the cardiac muscle. They are tubular structures composed of chains of myofibrils, which are rod-like units within the cell. The myofibrils consist of repeating sections of sarcomeres, which are the fundamental contractile units of the muscle cells. Sarcomeres are composed of long proteins that organise into thick and thin filaments, called myofilaments. The thick myofilaments contain the protein myosin, and the thin myofilaments contain the protein actin. The myofilaments slide past each other as the muscle contracts and relaxes, producing the formation of "cross-bridges" that cause contraction of the heart and generation of force.
Specialised modified cardiomyocytes known as pacemaker cells set the rhythm of the heart contractions. They 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) positioned on the wall of the right atrium, near the entrance of the superior vena cava. Other pacemaker cells are found in the atrioventricular node (secondary pacemaker). Pacemaker cells carry the impulses that are responsible for the beating of the heart. They are distributed throughout the heart and are responsible for several functions.
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Pacemaker cells
The heart is made up of three layers—the pericardium, myocardium, and endocardium. The myocardium, or cardiac muscle, is the middle layer of the heart and is the only place in the body where cardiac muscle is found. The cardiac muscle is responsible for the contractility of the heart and, therefore, the pumping action.
The heart is comprised of many cell types, including cardiomyocytes and pacemaker cells. Pacemaker cells are highly specialized myocardial cells with an intrinsic ability to depolarize rhythmically and initiate an action potential. They are located primarily in the sinoatrial SA node and atrioventricular AV node, with some cells also in the bundle of His and Purkinje fibers. The SA node serves as the primary pacemaker, while the AVN and ventricular conduction system act as secondary (accessory) pacemakers. Pacemaker cells carry the impulses that are responsible for the beating of the heart. They are distributed throughout the heart and are responsible for several functions.
The pacemaker cells' action potential is divided into three phases instead of five, as phases one and two are absent. Phase four, a phase of gradual depolarization, is unique to the pacemaker cells. This gradual depolarization mainly occurs due to the slow influx of Na+ ions through the hyperpolarization-activated cyclic nucleotide-gated channel (HCN channel). This causes the membrane potential to change from -60mV to reach the threshold potential of -40mV. The slope of phase four determines the heart rate and is different for pacemaker cells in different regions. SA node pacemaker cells depolarize at a rate of 60 to 100 per minute, while the AV node does so at 40 to 60 per minute.
In the heart, action potentials are generated by specialized cell structures called pacemaker cells, which use them to control the rhythmic contraction of muscles. In cardiac pacemaker cells, action potentials occur when specialized channels in the cell membrane open and allow ions to flow into or out of the cell. This change in electric charge makes the cell more positive on the inside, which attracts more ions from neighboring cells and triggers a chain reaction that propagates the action potential along the heart muscle. This eventually leads to the contraction of the heart and pumps blood around our bodies.
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Intercalated discs
The heart is composed of cardiac muscle, also known as myocardium, which is one of three types of vertebrate muscle tissues, the others being skeletal muscle and smooth muscle. Cardiac muscle is an involuntary, striated muscle that constitutes the main tissue of the heart wall.
Fascia adherens junctions are anchoring sites for actin filaments. Desmosomes are intermediate filament anchoring structures. Gap junctions form intercellular channels that provide a low-resistance pathway for the direct cell-to-cell passage of electrical charges. The three types of junctions act together as a single area composita.
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Heart's pumping action
The heart is a muscular organ, with cardiac muscle (also known as myocardium) constituting the main tissue of its walls. This type of muscle is unique to the heart and is involuntary, meaning it contracts and relaxes without conscious control. The heart's primary function is to pump oxygenated blood to the rest of the body through the cardiovascular system, supplying vital oxygen and nutrients to cells and removing waste products.
The heart's pumping action is facilitated by the contraction of cardiac muscle cells, or cardiomyocytes, which are interconnected by intercalated discs. These discs enable the transmission of electrical impulses and facilitate the coordinated contraction of cardiomyocytes. The contraction of these cells occurs in a specific sequence, allowing the ventricles to squeeze in multiple directions simultaneously: longitudinally (shortening from apex to base), radially (narrowing from side to side), and with a twisting motion. This ensures that the maximum amount of blood is squeezed out of the heart with each heartbeat.
Pacemaker cells, located in the sinoatrial and atrioventricular nodes, play a crucial role in regulating the heart's pumping action. These cells spontaneously generate and transmit electrical impulses, setting the rhythm and rate of heart contractions. The impulses from pacemaker cells cause the release of calcium ions, which trigger the contraction of cardiomyocytes. This contraction mechanism requires a constant supply of oxygen and nutrients, delivered by the coronary arteries, to meet the high energy demands of the cardiac muscle.
The heart's pumping action is vital for maintaining blood circulation and supplying oxygen and nutrients to the body's organs and tissues. Without this pumping action, the blood would be unable to move through the circulatory system, leading to a breakdown in the proper functioning of other organ systems. Therefore, the coordinated contraction of cardiac muscle cells and the resulting pumping action of the heart are essential for the body's overall health and functionality.
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Heart conditions
The heart is made up of three layers: the pericardium, myocardium, and endocardium. The myocardium, or cardiac muscle, is the middle layer of the heart. It is one of three types of vertebrate muscle tissues, the others being skeletal muscle and smooth muscle.
Cardiac muscle is an involuntary, striated muscle. It is composed of individual cardiac muscle cells, or cardiomyocytes, joined by intercalated discs. These cells are surrounded by an extracellular matrix produced by supporting fibroblast cells. The intercalated discs allow action potentials to spread between cardiac cells by permitting the passage of ions between cells, producing depolarization of the heart muscle.
Specialized modified cardiomyocytes known as pacemaker cells set the rhythm of the heart contractions. They are located in the sinoatrial node (the primary pacemaker) and the atrioventricular node (secondary pacemaker). Pacemaker cells carry the impulses responsible for the beating of the heart and are distributed throughout the heart.
Cardiac muscle cells contract in a coordinated manner, allowing the ventricle to squeeze in several directions simultaneously. This contraction pumps blood through the heart and blood vessels. If this coordination breaks down, the heart may not pump at all, as may occur during abnormal heart rhythms such as ventricular fibrillation.
Some heart conditions that affect the muscles of the heart include:
- Congenital heart disease, which occurs when something goes wrong while the heart is forming before birth.
- Coronary artery disease, which is caused by blockages in the coronary arteries, the vessels that supply blood to the heart.
- Cardiomyopathy, a disease of the heart muscle that reduces its ability to pump blood.
- Heart failure, when the heart is not strong enough to pump blood effectively.
- Endocarditis, an infection of the heart's valves or lining.
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Frequently asked questions
The heart is made up mostly of cardiac muscle, also known as myocardium. This is one of three types of vertebrate muscle tissues, the others being skeletal muscle and smooth muscle.
The primary function of the cardiac muscle is to pump blood into circulation by generating sufficient force. The heart is the main organ of the cardiovascular system, a network of blood vessels that pumps blood throughout the body.
The cardiac muscle contracts in a similar manner to skeletal muscle. Electrical stimulation triggers the release of calcium from the cell's internal calcium store. The heart's pacemaker cells, located in the sinoatrial node, generate and send out electrical impulses that cause the heart to beat.
