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The human heart is a complex organ, and its primary function is to pump blood into circulation. This is achieved through the contraction of cardiac muscles, which is involuntary and occurs without external stimulation. This inherent ability of the cardiac muscle to contract is known as myogenic activity. The rhythmic contractions of the cardiac muscle are facilitated by specialised pacemaker cells, which generate electrical impulses that initiate the heart's contraction. These contractions are essential for regulating blood flow and ensuring continuous circulation. The cardiac muscle is also unique in its structure, with long fibres formed by linking numerous cardiac muscle cells end-to-end.
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What You'll Learn
- Cardiac muscle cells are striated, branched, and contain many mitochondria
- The heart is myogenic, generating a heartbeat without external stimulation
- Electrical activity in the heart regulates heart rate
- Cardiac muscle cells are shorter and smaller than skeletal muscle cells
- Cardiac muscle cells are linked into long chains by specialised cell junctions
Cardiac muscle cells are striated, branched, and contain many mitochondria
Cardiac muscle, also called the myocardium, is one of three major categories of muscles in the human body, the other two being smooth muscle and skeletal muscle. Cardiac muscle is made up of sarcomeres that allow for contractility. The cardiac muscle is responsible for the contractility of the heart and, therefore, the pumping action. The cardiac muscle must contract with enough force and blood to meet the metabolic demands of the entire body. This is termed cardiac output.
Cardiac muscle cells (cardiomyocytes) are striated, branched, and contain many mitochondria. Each myocyte 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 skeletal muscle does not possess. The cardiac muscle cells are long, branched cells, shaped like cylinders joined end-to-end, with one or two nuclei located centrally. The myofilaments of cardiac muscle are arranged in a similar pattern to skeletal muscle, resulting in cross-striations. The fibres are crossed by linear bands called intercalated discs, which provide attachment points that give the tissue its characteristic branched pattern.
The cytoplasmic regions between the sarcomere branches are filled with mitochondria and smooth endoplasmic reticulum (sER) called sarcoplasmic reticulum, which envelopes each myofibril. The membranous network of sarcoplasmic reticulum is traversed by structures called T tubules, which are extensions of the sarcolemma. The cardiac conducting cells form the conducting system of the heart.
The contractile functions of the heart require ATP, which can be obtained through various substrates, including fatty acids, carbohydrates, proteins, and ketones. Mitochondria are the major source of muscle ATP production. As the amount of ATP stored within the cardiac muscle is typically insufficient to meet even the short-term dynamic energy demands of these muscles, the mitochondria are largely responsible for the continued resynthesis of this ATP.
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The heart is myogenic, generating a heartbeat without external stimulation
The heart is indeed myogenic, meaning it has the inherent ability to contract and generate a heartbeat without any external stimulation. This is due to the myogenic activity of cardiac muscles, which are responsible for pumping blood throughout the body.
Cardiac muscle, also called the myocardium, is one of three major categories of muscles in the human body, the other two being smooth muscle and skeletal muscle. Unlike skeletal muscle, which is attached to bones and controlled by the nervous system, cardiac muscle is under involuntary control. It is composed of striated, branched muscle cells called cardiomyocytes, which contain many mitochondria and a single, centrally located nucleus surrounded by a cell membrane called the sarcolemma.
The sarcolemma of cardiac muscle cells contains voltage-gated calcium channels, which are specialized ion channels that skeletal muscles do not possess. These calcium channels play a crucial role in the electrical activity that regulates the heart rate. Myocardial conducting cells, which make up about 1% of the cells in the heart, initiate and propagate the electrical impulse that triggers contractions. This electrical impulse travels throughout the heart through intercalated discs, which are found at the junction of different cardiac muscle cells.
The ability of cardiac muscle to contract without external stimulation is facilitated by specialized pacemaker cells. These cells generate electrical impulses that initiate the rhythmic contraction of the heart, ensuring the continuous and efficient circulation of blood. The pacemaker cells' action potential is divided into three phases, with the continuous leakage of sodium ions into the cell causing a slow rise in membrane potential until a threshold is reached, resulting in the depolarization of the cell and the subsequent entry of calcium ions, further raising the membrane potential.
In summary, the heart's myogenic nature is a result of the inherent contractile properties of cardiac muscle and the electrical impulses generated by pacemaker cells, allowing it to generate a heartbeat without external stimulation and maintain the body's metabolic demands.
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Electrical activity in the heart regulates heart rate
The cardiac muscle, also known as the myocardium, is one of three major categories of muscles in the human body, the other two being smooth muscle and skeletal muscle. The primary function of the 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.
The electrical activity in the heart is responsible for regulating the heart rate. The heart's electrical conduction system is a network of nodes, cells, and signals that controls the heartbeat. Each heartbeat is the result of electrical signals travelling through the heart's conduction pathway. The cardiac conduction system sends thousands of signals per day to keep the heart beating.
The process begins with the sinoatrial (SA) node, a small mass of specialised tissue located in the right upper chamber (atria) of the heart. The SA node generates an electrical stimulus regularly, 60 to 100 times per minute under normal conditions. The electrical impulse then travels from the SA node to the atrioventricular (AV) node, where it is slowed down for a fraction of a second before continuing down the conduction pathway into the ventricles. The atria are stimulated first and contract briefly before the ventricles, ensuring that blood empties into the ventricles before they contract and pump out blood.
The electrical activity in the heart is generated by membrane permeability mechanisms that are similar to those in other excitable cells, such as nerve and skeletal muscle cells. The myocardial conducting cells, which make up 1% of the cells in the heart, initiate and propagate the action potential (electrical impulse) that triggers the contractions. The contractile force of the cardiac muscle and the frequency of its activation determine the cardiac output, which is defined as heart rate multiplied by stroke volume.
In summary, the electrical activity in the heart regulates heart rate by controlling the contraction and relaxation of the heart muscle, which in turn determines the cardiac output and blood flow throughout the body.
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Cardiac muscle cells are shorter and smaller than skeletal muscle cells
Cardiac muscle, also known as myocardium, is one of three major categories of muscles in the human body, the other two being smooth muscle and skeletal muscle. Cardiac muscle cells are shorter and smaller than skeletal muscle cells. The cardiac muscle cells, or cardiomyocytes, are about 15 µm in diameter and about 100 µm long, with a centrally positioned nucleus. In contrast, skeletal muscle cells are giant cylinders with a multinucleated condition, resulting from multiple myoblasts fusing to produce each muscle fiber.
The structural differences between cardiac and skeletal muscle cells are notable. Cardiac muscle cells are striated, with an alternating pattern of dark A bands and light I bands. This pattern is attributed to the precise arrangement of the myofilaments and fibrils that are organized in sarcomeres along the length of the cell. Skeletal muscle fibres also exhibit a striated pattern due to the arrangement of actin and myosin filaments in the cytoplasm of the cells. However, the myofibrils in cardiac muscle are not as ordered as in skeletal muscle. Additionally, the nuclei in cardiac muscle cells reside in the center, while in skeletal muscle cells, the nuclei are located at the periphery.
The contractile elements of cardiac and skeletal muscle cells are similar, with both containing myofibrils, myofilaments, and sarcomeres. However, there are some differences in their calcium ion channels and the presence of T-tubules. The sarcolemma of cardiac muscle cells contains voltage-gated calcium channels, which skeletal muscle does not possess. T-tubules are transverse structures that penetrate from the surface plasma membrane to the interior of the cell, allowing the electrical impulse to reach the interior. Cardiac muscle has fewer T-tubules than skeletal muscle because they are only found at the Z-discs, whereas in skeletal muscle, they are found at the junction of the A and I bands.
Despite these differences, cardiac and skeletal muscle cells share some important functions and characteristics. Both types of muscle cells are excitable tissues, with electrical impulses propagating in a regenerative manner. The contractile functions of both cardiac and skeletal muscle require ATP, which can be obtained from various substrates. Additionally, both types of muscle cells demonstrate autorhythmicity, the ability to initiate an electrical potential that spreads rapidly from cell to cell, triggering the contractile mechanism.
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Cardiac muscle cells are linked into long chains by specialised cell junctions
Cardiac muscle, or myocardium, is one of three major categories of muscles in the human body, alongside smooth muscle and skeletal muscle. It is responsible for the contractility of the heart and, therefore, the pumping action. Cardiac muscle cells, also known as cardiomyocytes, are striated, branched, and contain many mitochondria. They are under involuntary control.
The specialised cell junctions that link cardiac muscle cells include intercalated discs, which themselves contain three types of cell junction: desmosomal junctions, adherent junctions, and gap junctions. Desmosomal junctions tightly link adjacent cells via anchors involving intermediate filaments. Adherent junctions anchor the actin fibres of the sarcomeres to each end of the cell. Gap junctions facilitate the passage of membrane excitation, enabling the synchronisation of muscle contraction.
The function of gap junctions in the conduction system of the heart is particularly important. The mammalian cardiac conduction system (CCS) coordinates the pumping function of the heart by producing and distributing the initial depolarisation signal in a specific spatiotemporal pattern. This is accomplished by a network of specialised cardiomyocytes that have the ability to generate and propagate action potentials, leading to atrial contraction and subsequent ventricular contraction. Gap junctions between adjacent cardiac muscle cells enable electrical activation to spread directly from muscle cell to muscle cell.
Studies have shown that defects in cell junction structures and their components are associated with conduction impairments in the CCS. Damaged cardiac muscle cells have extremely limited abilities to repair themselves or replace dead cells via mitosis.
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Frequently asked questions
Cardiac muscle, also called the myocardium, is one of three major categories of muscles in the human body, the other two being smooth muscle and skeletal muscle.
Cardiac muscle is responsible for the contractility of the heart, which is what pumps blood into circulation. The contractile functions of the heart require ATP, which can be obtained through various substrates, including fatty acids, carbohydrates, proteins, and ketones.
Cardiac muscle is unique because it has the inherent ability to contract without external stimuli. This type of muscle is only found in the heart and is responsible for pumping blood throughout the body.
Myogenic activity is the ability of muscles to generate a heartbeat by themselves and without any other stimulation. The heart beats on its own because of the myogenic activity of cardiac muscles.
Yes, smooth muscles, which are found in the walls of many internal organs such as the intestines, blood vessels, and the bladder, also exhibit myogenic activity. Unlike skeletal muscles, smooth muscles can contract without external stimulation.
