Dominic Stone
Cardiac muscle is an involuntary striated muscle that constitutes the main tissue of the heart wall. It is one of three major types of muscle, the others being skeletal and smooth muscle. The cardiac muscle is self-exciting, meaning it has its own conduction system and is not reliant on the nervous system. This is in contrast to skeletal muscle, which requires conscious or reflex nervous stimuli. The self-exciting nature of cardiac muscle is due to specialized cardiac muscle cells called autorhythmic cells, which can generate an action potential without external stimulation by nerve cells. These autorhythmic cells initiate and distribute cardiac electrical impulses throughout the heart muscle, causing it to beat.
| Characteristics | Values |
|---|---|
| Type of muscle | Involuntary, striated muscle |
| Location | Walls of the myocardium |
| Composition | Individual cardiac muscle cells or cardiomyocytes joined by intercalated discs |
| Function | Contraction and relaxation |
| Regulation | Autonomic nervous system |
| Resistance to fatigue | High due to a large number of mitochondria enabling continuous aerobic respiration |
| Contractility | Regulated by changes in intracellular calcium concentration |
| Source of calcium | Sarcoplasmic reticulum and extracellular sources |
| Excitation-contraction coupling | Unique mechanism called calcium-induced calcium release |
| Autorhythmicity | Self-excitable and able to generate action potentials without external stimulation |
| Conduction system | Has its own conduction system, the cardiac conduction system (CCS) |
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What You'll Learn
- Cardiac muscle is self-exciting, with its own conduction system
- It is involuntary and intrinsically controlled
- Cardiac muscle is autorhythmic, able to generate an action potential without nerve cells
- Excitation-contraction coupling is unique to cardiac muscle
- Calcium-induced calcium release is important for contraction
Cardiac muscle is self-exciting, with its own conduction system
The cardiac muscle is an involuntary striated muscle that constitutes the main tissue of the heart wall. It is one of three major types of muscle, the others being skeletal and smooth muscle. The cardiac muscle is self-exciting, meaning it has its own conduction system. This is in contrast to skeletal muscle, which requires either conscious or reflex nervous stimuli.
The self-exciting nature of cardiac muscle is due to its autorhythmicity, which is a property exhibited by specialised cardiac muscle cells called autorhythmic cells. These cells are self-excitable, able to generate an action potential without external stimulation by nerve cells. They can initiate their own depolarisation signal and depolarise the whole heart without the use of a neurosignal. This is made possible by the presence of ion channels that are activated by hyperpolarisation, allowing the entry of Na+ ions into the cell and resulting in a spontaneous slow depolarisation.
The action potential generated by these autorhythmic cells is an electrical impulse that triggers the release of calcium from the cell's internal calcium store, the sarcoplasmic reticulum. This release of calcium ions is known as calcium-induced calcium release and is unique to cardiac muscle. The high concentration of calcium promotes actin-myosin bridging, leading to cardiac muscle contraction. The sarcoplasmic reticulum also plays a crucial role in regulating intracellular calcium concentration, which is essential for controlling cardiac contractility.
The cardiac conduction system (CCS) is composed of these specialised autorhythmic cells and is responsible for initiating and distributing cardiac electrical impulses throughout the heart muscle, causing it to beat. The CCS includes the Sinoatrial Node (SA Node) and the Atrioventricular Node (AV Node), which act as the primary pacemaker and a delay signal, respectively. The SA node generates action potentials at a rate of about 60-100 times per minute in humans, setting the rhythm of the heart contractions.
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It is involuntary and intrinsically controlled
The cardiac muscle is an involuntary muscle tissue that is found only within the heart. It is a type of striated muscle, constituting the main tissue of the heart wall. The cardiac muscle is self-exciting, meaning it has its own conduction system. This is in contrast to skeletal muscle, which requires conscious or reflex nervous stimuli to function.
The cardiac muscle is intrinsically controlled and is adapted to be highly resistant to fatigue. It has a large number of mitochondria, enabling continuous aerobic respiration. The heart is so tuned to aerobic metabolism that it is unable to pump sufficiently in ischaemic conditions. At basal metabolic rates, about 1% of energy is derived from anaerobic metabolism. This can increase to 10% under moderately hypoxic conditions.
The cardiac muscle is myogenic, meaning that the pacemaker serves only to modulate and coordinate contractions. The cardiac muscle cells would still fire in the absence of a functioning SA node pacemaker, although in a chaotic and ineffective manner. This condition is known as fibrillation. The heart can still beat properly even if its connections to the central nervous system are completely severed.
The cardiac muscle is autorhythmic, meaning it can initiate its own depolarization signal and depolarize the whole heart without the use of a neurosignal. Action potentials (electrical impulses) in the heart originate in specialized cardiac muscle cells, called autorhythmic cells. These cells are self-excitable, able to generate an action potential without external stimulation by nerve cells. The sequence of electrical events during one full contraction of the heart muscle involves an excitation signal (an action potential) created by the sinoatrial (SA) node. The wave of excitation spreads across the atria, causing them to contract. Upon reaching the atrioventricular (AV) node, the signal is delayed, allowing the ventricles to fill.
The SA node is located in the right uppermost atrial wall and is called the primary pacemaker. It is a self-exciting tissue that generates an action potential at a rate of about 60-100 times per minute in humans. The AV node is located in the interatrial septum and acts as a backup pacemaker, although it is slower.
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Cardiac muscle is autorhythmic, able to generate an action potential without nerve cells
Cardiac muscle is a type of involuntary striated muscle that forms the main tissue of the heart wall. It is one of three major types of muscle, the others being skeletal and smooth muscle. The cardiac muscle is self-exciting, with its own conduction system, unlike skeletal muscle, which requires conscious or reflex nervous stimuli. This self-exciting property is called autorhythmicity, and it is exhibited by only cardiac muscle cells, not skeletal or smooth muscles.
Autorhythmicity refers to the ability of cardiac muscle cells to initiate their own depolarization signal and depolarize the whole heart without a neurosignal. These specialized cardiac muscle cells, called autorhythmic cells, are self-excitable and can generate an action potential without external stimulation by nerve cells. This means that the heart can still beat properly even if its connections to the central nervous system are completely severed. The action potential is a result of the sequence of electrical events during one full contraction of the heart muscle.
The action potential is created by the sinoatrial (SA) node, which consists of specialized muscle cells known as pacemaker cells. These pacemaker cells are only weakly contractile and are located in the sinoatrial node (the primary pacemaker) on the wall of the right atrium. The SA node generates an action potential at a rate of about 60-100 times per minute in humans, which causes the heart to beat. The wave of excitation then spreads across the atria, causing them to contract. Upon reaching the atrioventricular (AV) node, the signal is delayed, allowing the ventricles to fill before contracting.
The action potential triggers the release of calcium from the cell's internal calcium store, the sarcoplasmic reticulum, through a process known as calcium-induced calcium release. This increase in intracellular calcium concentration leads to cardiac muscle contraction. The cardiac muscle cells are linked by gap junctions, which allow the action potential to travel rapidly from cell to cell, enabling the heart to work as a unit.
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Excitation-contraction coupling is unique to cardiac muscle
The cardiac muscle is self-exciting, which means it has its own conduction system. This is in contrast to skeletal muscle, which requires conscious or reflex nervous stimuli. Cardiac muscle cells are autorhythmic, meaning they can initiate their own depolarization signal and depolarize the heart without neurosignals. This is made possible by specialized cardiac muscle cells called autorhythmic cells, which can generate an action potential without external stimulation by nerve cells.
Excitation-contraction coupling is a process unique to cardiac muscle that describes the conversion of an electrical stimulus (action potential) into a mechanical response (muscle contraction). It involves the entry of calcium ions (Ca2+) into the cell cytoplasm, triggering calcium-induced calcium release from the sarcoplasmic reticulum. This increase in intracellular calcium concentration leads to actin-myosin bridging and subsequent cardiac muscle contraction.
The process of excitation-contraction coupling in cardiac muscle begins with an excitation signal (an action potential) created by the sinoatrial (SA) node, which is the heart's natural pacemaker. This excitation signal spreads across the atria, causing them to contract. When the signal reaches the atrioventricular (AV) node, it is delayed.
The depolarization wavefront then hits the cardiomyocytes, causing a few calcium ions to flow through gap junctions. If a threshold membrane potential is reached, sodium channels open, and ions move across the cell membrane. T-tubules play a crucial role in this process by helping to bring calcium deep into the cell. The calcium ions then bind to ryanodine receptors on the sarcoplasmic reticulum, releasing even more calcium into the cell through calcium-induced calcium release.
This release of calcium activates two contractile proteins, actin and myosin, which are responsible for cell contraction. The calcium ions bind to Troponin C, causing tropomyosin to move aside and expose binding sites on actin for myosin heads. With the help of ATP, the myosin heads attach to the actin filaments and slide them past each other, resulting in cardiac muscle contraction.
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Calcium-induced calcium release is important for contraction
Calcium-induced calcium release (CICR) is a biological process where calcium activates calcium release from intracellular Ca2+ stores, such as the sarcoplasmic reticulum. CICR was initially discovered in skeletal muscle, but it is now known to be a widely occurring cellular signaling process present in many non-muscle cells, such as insulin-secreting pancreatic beta cells, epithelium, and other cells. CICR is important for excitation-contraction coupling in cardiac muscle, which is a process that describes the conversion of an electrical stimulus (action potential) into a mechanical response (muscle contraction).
In cardiac muscle, CICR occurs when an action potential depolarizes the cell membrane, activating voltage-gated Ca2+ channels (e.g., L-type calcium channels). This results in a Ca2+ influx that activates ryanodine receptors on the sarcoplasmic reticulum membrane, leading to the release of more Ca2+ into the cytosol. The high concentration of calcium promotes actin-myosin bridging and subsequent cardiac muscle contraction. At the end of the contraction, the calcium concentration inside the sarcoplasm declines, with 80% of the calcium being reabsorbed into the sarcoplasmic reticulum via an ATP-dependent pump.
The cardiac muscle is an involuntary striated muscle found in the walls of the myocardium. It is self-exciting, meaning it has its own conduction system and can initiate its own depolarization signal without the use of a neurosignal. This self-exciting nature of cardiac muscle, along with its ability to generate action potentials independently, contributes to its autorhythmicity, where it can contract at set intervals to determine heart rate.
The inherent contractile activity of the heart is regulated by the autonomic nervous system, and cardiac contractility is influenced by changes in intracellular Ca2+ concentration ([Ca2+]i). The process of CICR in cardiac muscle ensures that the required levels of Ca2+ are released to facilitate contraction. Thus, CICR plays a crucial role in excitation-contraction coupling, which is unique to cardiac muscle, and is essential for proper heart function.
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Frequently asked questions
Yes, cardiac muscle is self-exciting, meaning it has its own conduction system. This is in contrast with skeletal muscle, which requires either conscious or reflex nervous stimuli.
Autorhythmicity is the ability of cardiac muscle cells to depolarize to threshold and fire action potentials on their own, at set intervals which determine heart rate.
Calcium ions play a key role in cardiac muscle contraction. An increase in intracellular calcium concentration increases cardiac muscle contractility. Calcium-induced calcium release is a process where calcium ions bind to ryanodine receptors on the sarcoplasmic reticulum, causing the release of larger stores of calcium, which promotes actin-myosin bridging and subsequent cardiac muscle contraction.
The Sinoatrial Node (SA Node) is a primary pacemaker located in the right uppermost atrial wall. It is composed of specialized muscle cells that are self-exciting and autorhythmic. These cells generate action potentials at a rate of about 60-100 times per minute in humans, which then spread across the atria, causing them to contract.
Cardiac muscle is one of three major types of muscle, the others being skeletal and smooth muscle. Unlike skeletal muscle, cardiac muscle is involuntary and does not require external stimulation to contract. Smooth muscle, on the other hand, is slower to contract and relax than skeletal muscle and can maintain a contraction longer with a given amount of ATP.
