Umair Gaines
Skeletal muscle and cardiac muscle share some characteristics but differ in their electrical properties. Cardiac muscle, unlike skeletal muscle, is under involuntary control and has the unique property of autorhythmicity, or the ability to initiate an electrical potential at a fixed rate that spreads from cell to cell, triggering contractions. This property allows the heart to pump blood into circulation. The contractile elements of skeletal and cardiac muscle are virtually identical, but the T-tubules that transmit electrical impulses are only found at the Z-discs in cardiac muscle, while in skeletal muscle they are found at the junction of the A and I bands. Gap junctions within the intercalated discs of cardiac muscle cells allow for the flow of sodium, potassium, and calcium ions between adjacent cells, enabling coordinated contractions.
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What You'll Learn
- Cardiac muscle cells, or cardiomyocytes, are shorter and smaller in diameter than skeletal muscle cells
- T (transverse) tubules allow electrical impulses to reach the interior of cardiac muscle cells
- The heart's junctional rhythm originates in the AV node
- Artificial pacemakers deliver electrical impulses to the heart muscle to speed up the heart rate
- Cardiac muscle cells contain branched fibres connected via intercalated discs
Cardiac muscle cells, or cardiomyocytes, are shorter and smaller in diameter than skeletal muscle cells
Cardiac muscle cells, also known as cardiomyocytes, are considerably shorter and smaller in diameter than skeletal muscle cells. They are rectangular, branching cells that typically contain a single, centrally located nucleus, surrounded by a cell membrane called the sarcolemma. This sarcolemma is a unique feature of cardiac muscle cells, as it contains voltage-gated calcium channels, which are specialised ion channels that skeletal muscle does not possess.
Cardiac muscle cells are made up of sarcomeres, which allow for contractility. These sarcomeres are composed of thick (myosin) and thin (actin) filaments, which interact to form the basis of the sliding filament theory. The arrangement of these filaments within the sarcomere produces a striated appearance, with alternating dark A-bands and light I-bands, similar to skeletal muscle. However, the myofibrils of cardiac muscle cells tend to be smaller in diameter than those of skeletal muscle cells.
The cardiac muscle cells are connected by intercalated discs, which contain gap junctions, desmosomes, and fascia adherens. These gap junctions allow for the rapid transmission of electrical signals, ensuring that the cardiac muscle cells contract in a coordinated manner. This electrical coupling, known as autorhythmicity, is a unique property of cardiac muscle, not found in skeletal or smooth muscle.
The primary function of cardiac muscle cells is to contract and generate the pressure required to pump blood through the circulatory system. This requires a large amount of energy, which is met by the presence of numerous mitochondria in the cardiomyocytes, producing large amounts of adenosine triphosphate (ATP) and storing oxygen in myoglobin.
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T (transverse) tubules allow electrical impulses to reach the interior of cardiac muscle cells
Cardiac muscle, also known as the myocardium, is one of the three major muscle categories found in the human body, the other two being smooth muscle and skeletal muscle. Cardiac muscle cells, or cardiomyocytes, are shorter and have smaller diameters than skeletal muscle cells.
Cardiac muscle cells contain T (transverse) tubules, which are membrane invaginations that penetrate from the surface plasma membrane, the sarcolemma, to the interior of the cell. These T-tubules are critical in allowing electrical impulses to reach the interior of cardiac muscle cells, triggering the contractile mechanism. This contractile mechanism is responsible for the pumping action of the heart, ensuring the circulation of blood to meet the body's metabolic demands.
T-tubules form a highly branched and interconnected network within cardiac muscle cells. They are unique to striated muscle cells and are rich in ion channels, particularly calcium channels, which are vital for excitation-contraction (EC) coupling. The presence of T-tubules enables the rapid conduction of electrical excitation and facilitates communication with the sarcoplasmic reticulum, resulting in coordinated contractions.
The dyad membrane structure formed by T-tubules and the sarcoplasmic reticulum is crucial for calcium signaling and EC coupling, which are necessary for the beat-to-beat contraction of the heart. T-tubules play a central role in maintaining normal cardiac function, and disruptions in their structure or function can lead to arrhythmias and heart failure.
In summary, T (transverse) tubules are essential structures in cardiac muscle cells that facilitate the transmission of electrical impulses to the interior of the cells, triggering contractions and enabling the heart to pump blood effectively throughout the body.
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The heart's junctional rhythm originates in the AV node
The heart is made up of three layers: the pericardium, myocardium, and endocardium. The myocardium, or cardiac muscle, is responsible for the contractility of the heart and, therefore, the pumping action. The cardiac muscle must contract with enough force to supply blood to the metabolic demands of the entire body.
Cardiac muscle, like skeletal muscle, is made up of sarcomeres that allow for contractility. However, unlike skeletal muscle, cardiac muscle is under involuntary control. Cardiac muscle also demonstrates striations, an alternating pattern of dark A bands and light I bands attributed to the precise arrangement of the myofilaments and fibrils that are organized in sarcomeres along the length of the cell.
The heart's sinoatrial node (SA node) is the default pacemaker and is located in the upper right atrium. It sends electrical signals that control the heartbeat. Under normal conditions, the SA node determines the rate at which the organ beats. The electrical activity of sinus rhythm originates in the SA node and depolarizes the atria.
However, when the SA node stops working or sends signals that are too slow or weak, the heart's junctional rhythm takes over, originating in the AV node. The AV node, or atrioventricular node, is a separate cardiac pacemaker located in the inferior-posterior right atrium. It sits within an anatomical region commonly referred to as the triangle of Koch. The AV node can act as a backup pacemaker when the SA node is not functioning correctly. Junctional rhythm is an abnormal heart rhythm resulting from impulses coming from a locus of tissue in the area of the AV node, the "junction" between the atria and ventricles. Depending on where the rhythm originates in the AV node, the atria can contract before, during, or after ventricular contraction.
Junctional rhythm is seen equally in men and women and can be intermittent in young children and athletes, especially during sleep. It is often found in patients with sinus node dysfunction and can be asymptomatic or present with a variety of symptoms, including chest pain, fatigue, and shortness of breath.
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Artificial pacemakers deliver electrical impulses to the heart muscle to speed up the heart rate
The human body has three major categories of muscles: skeletal muscle, smooth muscle, and cardiac muscle. Cardiac muscle, or myocardium, shares some characteristics with skeletal muscle but also has some unique properties. One such property is autorhythmicity, which is the ability to initiate an electrical potential at a fixed rate that spreads rapidly from cell to cell, triggering the contractile mechanism. This electrical potential causes the myocardium to contract, which leads to the heart's contraction.
Cardiac muscle cells, or cardiomyocytes, are shorter and have smaller diameters than skeletal muscle cells. They also demonstrate striations, with an alternating pattern of dark A bands and light I bands attributed to the arrangement of myofilaments and fibrils that are organized in sarcomeres. T (transverse) tubules allow electrical impulses to reach the interior of the cell. However, cardiac muscle has half as many T tubules as skeletal muscle, and it stores fewer calcium ions, requiring most of them to come from outside the cells.
When the heart experiences arrhythmias, or abnormal heart rhythms, a cardiologist may implant an artificial pacemaker to speed up the heart rate and restore a full sinus rhythm. Pacemakers are electronic devices that deliver electrical impulses to the heart muscle, ensuring that the heart continues to contract and pump blood effectively. They are connected to the heart by wires (leads) running inside a vein, and the tips of the wires are implanted in the wall of a heart chamber. The pacemaker can sense the person's heart rate and rhythm and send an electrical signal or impulse to make the heart beat at a faster rate when needed.
Pacemakers are typically programmable by cardiologists, either temporarily upon demand or on a continuous basis. Modern pacemakers are designed to work on demand, providing electrical impulses only when the heart's natural rate falls below a certain limit. Some pacemakers are also equipped with built-in defibrillators, which deliver an electric shock to reset the heart rhythm and prevent sudden cardiac arrest in the case of fast arrhythmias.
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Cardiac muscle cells contain branched fibres connected via intercalated discs
Cardiac muscle cells, or cardiomyocytes, are considerably shorter and smaller in diameter than skeletal muscle cells. They are also extensively branched and are connected to one another at their ends by intercalated discs. Intercalated discs are complex structures that connect adjacent cardiac muscle cells. They are composed of three types of cell junctions: desmosomes, fascia adherens junctions, and gap junctions.
Desmosomes are intercellular structures that anchor cardiac muscle fibres together, maintaining the structural integrity of the heart. They are vital in preventing individual muscle fibres from pulling apart during the stress of contraction. The remainder of the intercalated disc is composed of desmosomes.
Gap junctions connect the cytoplasms of neighbouring cells electrically, allowing cardiac action potentials to spread between cardiac cells by permitting the passage of ions between cells, producing depolarization of the heart muscle. This is known as electrical coupling. This, in turn, causes the contraction of the myocardium. Gap junctions within the intercalated discs allow impulses to spread from one cardiac muscle cell to another, allowing sodium, potassium, and calcium ions to flow between adjacent cells, propagating the action potential, and ensuring coordinated contractions. This electrical coupling allows the quick transmission of action potentials and the coordinated contraction of the entire heart.
Fascia adherens are anchoring sites for actin, and they connect to the closest sarcomere.
Intercalated discs support synchronized contraction of cardiac tissue in a wave-like pattern so that the heart can work like a pump. This is known as a functional unit of contraction or a syncytium.
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Frequently asked questions
Skeletal muscles are giant cylinders that support movement in the body. Unlike cardiac muscles, they do not have autorhythmicity, meaning they cannot initiate an electrical potential at a fixed rate that spreads from cell to cell to trigger the contractile mechanism.
Cardiac muscles, also called the myocardium, are considerably shorter with smaller diameters than skeletal muscles. They are responsible for the contractility of the heart and the pumping action. The primary function of cardiac muscle is to pump blood into circulation by generating sufficient force.
Autorhythmicity is the ability of cardiac muscles to initiate an electrical potential at a fixed rate that spreads rapidly from cell to cell to trigger the contractile mechanism. This property allows the heart to contract in a coordinated manner.
When arrhythmias become a chronic problem, the heart maintains a junctional rhythm, which originates in the AV node. In such cases, a cardiologist can implant an artificial pacemaker, which delivers electrical impulses to the heart muscle to ensure that the heart continues to contract and pump blood effectively.
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