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Cardiac muscle, or myocardium, is one of three types of vertebrate muscle tissues, the others being skeletal and smooth muscle. It is an involuntary, striated muscle that forms the main tissue of the heart wall. The cardiac muscle contracts in a similar manner to skeletal muscle, but with some important differences. Electrical stimulation triggers the release of calcium from the cell's internal store, the sarcoplasmic reticulum, which causes the cell's myofilaments to slide past each other in a process called excitation-contraction coupling. The Purkinje fibres are additional myocardial conductive fibres that spread the impulse to the myocardial contractile cells in the ventricles. The electrical impulse reaches all of the ventricular muscle cells in about 75 ms. The cardiac muscle has autorhythmicity, which means it can initiate an electrical potential at a fixed rate that spreads rapidly from cell to cell to trigger the contractile mechanism.
| Characteristics | Values |
|---|---|
| Type of muscle tissue | Cardiac muscle tissue, also known as heart muscle or myocardium, is one of three types of vertebrate muscle tissues, the others being skeletal muscle and smooth muscle. |
| Function | Cardiac muscle tissue produces involuntary movements, meaning they are automatic and a person cannot control them. |
| Composition | Cardiac muscle is composed of individual cardiac muscle cells (cardiomyocytes) joined by intercalated discs and encased by collagen fibres and other substances that form the extracellular matrix. |
| Shape | Cardiac muscle cells are roughly rectangular when viewed under a microscope. |
| Size | Cardiac muscle cells measure approximately 100-150 μm by 30-40 μm. |
| Contraction | Cardiac muscle contracts in a similar manner to skeletal muscle but with some important differences. Electrical stimulation triggers the release of calcium from the cell's internal calcium store, the sarcoplasmic reticulum, leading to cell contraction. |
| Rate of Contraction | The rate of contraction is controlled by specialised "pacemaker" cells that generate electrical impulses. |
| Striations | Cardiac muscle appears striated or striped under a microscope due to the alternating filaments of myosin and actin proteins. |
| Conduction System | The cardiac conduction system includes the sinoatrial node, the atrioventricular node, the atrioventricular bundle, the atrioventricular bundle branches, and the Purkinje cells. |
| Conduction Rate | The Purkinje fibres have a fast inherent conduction rate, with the electrical impulse reaching all ventricular muscle cells in about 75 ms. |
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What You'll Learn
Electrical stimulation triggers cardiac muscle contraction
Cardiac muscle, or myocardium, is one of three types of vertebrate muscle tissues, the others being skeletal muscle and smooth muscle. It is an involuntary, striated muscle that forms the thick middle layer of the heart wall.
Cardiac muscle cells, or cardiomyocytes, are the contractile myocytes of the cardiac muscle. They are surrounded by an extracellular matrix produced by supporting fibroblast cells. The cardiac muscle cells are joined at their ends by intercalated discs to form long fibres. Each cell contains myofibrils, specialised protein contractile fibres of actin and myosin that slide past each other.
The cardiac conduction system includes the sinoatrial node (SA node), the atrioventricular node (AV node), the atrioventricular bundle, the atrioventricular bundle branches, and the Purkinje cells. The SA node is the primary pacemaker, positioned on the wall of the right atrium, while the AV node is the secondary pacemaker. These 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, including generating and sending out electrical impulses.
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. The rise in calcium causes the cell's myofilaments to slide past each other in a process called excitation-contraction coupling. This process is similar to that of skeletal muscle, although there are some important differences. For example, cardiac muscle has autorhythmicity, or the ability to initiate an electrical potential at a fixed rate that spreads rapidly from cell to cell to trigger the contractile mechanism. This property is not found in smooth or skeletal muscle.
The Purkinje fibres are additional myocardial conductive fibres that spread the impulse to the myocardial contractile cells in the ventricles. They have a fast inherent conduction rate, and the electrical impulse reaches all of the ventricular muscle cells in about 75 ms. Since the electrical stimulus begins at the apex, the contraction also starts there and travels toward the base of the heart. This allows the blood to be pumped out of the ventricles and into the aorta and pulmonary trunk.
In cases of chronic arrhythmias, a cardiologist can implant an artificial pacemaker to deliver electrical impulses to the heart muscle, ensuring that the heart continues to contract and pump blood effectively.
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Calcium release from the sarcoplasmic reticulum
Calcium ions (Ca2+) play a crucial role in muscle contraction. The release of calcium from the sarcoplasmic reticulum, an intracellular calcium store, is essential for initiating and regulating muscle contractions in cardiac muscle cells. This process is triggered by electrical stimulation in the form of a cardiac action potential, which causes a rapid increase in calcium ions within the cell.
The sarcoplasmic reticulum is a specialized endoplasmic reticulum found in muscle cells, including cardiac muscle or myocardium. It is responsible for storing and releasing calcium ions, which are essential for muscle contraction. The release of calcium from the sarcoplasmic reticulum is a highly regulated process that involves the interaction of various proteins and ions.
One of the key proteins involved in calcium release from the sarcoplasmic reticulum is ryanodine receptor (RyR), which acts as a calcium release channel. Ryanodine receptors are activated by calcium ions, leading to the release of additional calcium from the sarcoplasmic reticulum. This process is known as calcium-induced calcium release and is crucial for maintaining the contraction of cardiac muscle cells.
Another important protein involved in calcium release from the sarcoplasmic reticulum is calsequestrin. It binds to calcium ions, helping to store them within the sarcoplasmic reticulum. Additionally, Ca2+-ATPase, also known as the calcium pump, plays a critical role in calcium uptake by the sarcoplasmic reticulum. It reabsorbs calcium ions after muscle contraction, reducing intramuscular calcium concentrations and enabling muscle relaxation.
Studies have shown that alterations in the function of these regulatory proteins can have significant implications for cardiac muscle function. For example, decreased levels of calsequestrin and reduced ryanodine receptor binding affinity have been associated with abnormal calcium transients in failing cardiac muscle. Furthermore, changes in Ca2+-ATPase activity have been linked to ischemic heart disease, heart failure, and hypertrophy of cardiac muscle.
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Excitation-contraction coupling
Cardiac excitation-contraction coupling (ECC) is the process by which an electrical signal generated by the sinoatrial node (the heart's natural pacemaker) is converted into a mechanical force that makes the heart contract. It is the relationship between electrical signals in the form of action potentials and mechanical changes in the heart muscle cells, called cardiomyocytes, that cause them to contract.
The process of excitation-contraction coupling can be broken down into several steps. Firstly, excitation occurs when an electrical signal causes calcium ions to enter the cell. This is followed by myofilament activation, where the released calcium ions bind to troponin C, causing tropomyosin to move out of the way and expose binding sites on actin for myosin heads. With the help of ATP, the myosin heads then attach to the actin filaments and slide them past each other, shortening the sarcomere and leading to cardiac muscle contraction.
The process of excitation-contraction coupling is influenced by cytosolic calcium concentrations, which are primarily regulated by beta-adrenoceptor-coupled mechanisms. Beta-adrenergic stimulation, for example, increases calcium entry into the cell through L-type calcium channels. This stimulation activates protein kinase, which in turn increases calcium entry into the cell. Other mechanisms, such as the activation of the IP3 signal transduction pathway, can also stimulate the release of calcium by the sarcoplasmic reticulum (SR).
ECC can be impaired in systolic heart failure, where there is a decreased influx of calcium into the cell through L-type calcium channels, resulting in reduced calcium release by the SR. Additionally, a decrease in TN-C affinity for calcium can lead to an increase in free calcium near the troponin complex, impacting the activating effect on cardiac contraction. Understanding ECC is crucial for comprehending the coordination of individual myocytes in the heart's pumping mechanism.
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Heart rate modulation by endocrine and nervous systems
Cardiac muscle, or myocardium, is one of three types of vertebrate muscle tissues, the others being skeletal and smooth muscle. It is an involuntary, striated muscle that constitutes the main tissue of the heart wall. The heart is a pump that moves blood through blood vessels, providing the body with oxygen and nutrients.
Heart rate modulation is influenced by the nervous and endocrine systems. The nervous system, including the brain and spinal cord, influences the cardiovascular system through the autonomic nervous system (ANS). The ANS is divided into the sympathetic nervous system (SNS) and the parasympathetic nervous system (PNS). The SNS and PNS cooperatively modulate heart rate through their respective branches. The endocrine system also plays a role in heart rate modulation through hormonal factors.
The heart has its own intrinsic conduction system, which includes the sinoatrial node (SA node), the atrioventricular node (AV node), the atrioventricular bundle, and the Purkinje fibres. The SA node is the primary pacemaker of the heart and sets the heart rate at approximately 100 beats per minute in the absence of external influences. The ANS and endocrine system influence the heart rate, allowing it to respond to the body's varying needs for oxygen and nutrients.
The SNS and PNS have different effects on heart rate. A high level of activity in the SNS is associated with a high LF (low-frequency band), while a high level of activity in the PNS is associated with a high HF (high-frequency band). The ratio of LF to HF power is used to estimate the relative activity of the SNS and PNS.
In summary, the heart rate is modulated by the nervous and endocrine systems, which work together to ensure the body receives adequate oxygen and nutrients. The ANS, through the SNS and PNS, plays a significant role in heart rate modulation, and its activity can be assessed through HRV analysis.
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Cardiac muscle cell structure
Cardiac muscle cells, also called cardiomyocytes, are the contractile myocytes of the cardiac muscle. They are rectangular in shape and are joined at their ends by intercalated discs to form long fibres. The intercalated discs contain gap junctions that relay electrical impulses from one cardiac muscle cell to another. The cardiac muscle cells are surrounded by an extracellular matrix produced by supporting fibroblast cells.
Cardiac muscle cells contain myofibrils, specialised protein contractile fibres of actin and myosin that slide past each other. These are organised into sarcomeres, the fundamental contractile units of muscle cells. The regular organisation of myofibrils into sarcomeres gives cardiac muscle cells a striped or striated appearance when viewed under a microscope. These striations are caused by lighter I bands composed mainly of actin and darker A bands composed mainly of myosin.
Cardiac muscle cells also contain mitochondria, which convert oxygen and glucose into energy in the form of adenosine triphosphate (ATP). The cell uses ATP to power its contractions. Calcium ions play a crucial role in the contraction process by combining with the regulatory protein troponin, which removes the inhibition that prevents the heads of the myosin molecules from forming cross-bridges with the active sites on actin. This mechanism provides the power stroke of contraction.
Cardiac muscle cells form a highly branched cellular network in the heart. They work together to produce the rhythmic, wave-like contractions known as the heartbeat. The contraction of individual cardiac muscle cells produces force and shortening in the bands of muscle, resulting in a decrease in heart chamber size and the consequent ejection of blood into the pulmonary and systemic vessels. The rate at which the heart contracts and the synchronisation of atrial and ventricular contraction are dependent on the electrical properties of the cardiac muscle cells and the conduction of electrical information between different regions of the heart.
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Frequently asked questions
Cardiac muscles, also called heart muscles or myocardium, are one of three types of vertebrate muscle tissues, the others being skeletal and smooth muscle.
Cardiac muscles react to electrical stimulation in the form of a cardiac action potential, which triggers the release of calcium from the cell's internal calcium store, the sarcoplasmic reticulum. The rise in calcium causes the cell's myofilaments to slide past each other in a process called excitation-contraction coupling.
Unlike skeletal and smooth muscles, cardiac muscles have the unique property of autorhythmicity, which is the ability to initiate an electrical potential at a fixed rate that spreads rapidly from cell to cell to trigger the contractile mechanism.
Pacemaker cells are specialized modified cardiomyocytes that set the rhythm of heart contractions. They generate electrical impulses, or action potentials, that tell cardiac muscle cells to contract and relax, controlling the heart rate and determining how fast the heart pumps blood.
Cardiac muscles contract in a similar manner to skeletal muscles, but with some differences. When a cardiac muscle cell contracts, the myosin filament pulls the actin filaments toward each other, causing the cell to shrink. This process is powered by adenosine triphosphate (ATP), which is produced by the mitochondria in the cell.
