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The cardiac muscle, also called the myocardium, is one of three major categories of muscles found within the human body, along with smooth muscle and skeletal muscle. Unlike skeletal muscle, which is the only voluntary muscle tissue in the human body, cardiac muscle is not under conscious control. Instead, it is regulated by the autonomic nervous system and various hormones that modulate heart rate to control blood pressure. The primary function of the cardiac muscle is to pump blood into circulation by generating sufficient force through contraction. This process is fuelled by ATP, which can be obtained through various substrates, including fatty acids, carbohydrates, proteins, and ketones.
| Characteristics | Values |
|---|---|
| Location | Heart and the beginnings of the great vessels of the heart |
| Consistency | Involuntary |
| Composition | Cardiac muscle cells, collagen fibres, intercalated discs, and other substances that form the extracellular matrix |
| Cell shape | Short cylindrical, branching cells about 80 µm in length and 15 µm in diameter |
| Cell nucleus | Single, centrally placed nucleus (or occasionally two nuclei) |
| Cell contents | Mitochondria, myofibrils, myofilaments, actin, myosin, calcium, sodium, potassium, glycogen deposits, triglycerides, etc. |
| Cell junctions | Gap junctions, desmosomes, intercalated discs |
| Cell functions | Contraction, excitation-contraction coupling, autorhythmicity, electrical impulses, etc. |
| Cell requirements | ATP, oxygen, nutrients, etc. |
| Primary function | Pump blood into circulation by generating sufficient force |
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What You'll Learn
Cardiac muscle is involuntary
Cardiac muscle, also called myocardium, is one of three major categories of muscles in the human body, the others being skeletal muscle and smooth muscle. The myocardium forms a thick middle layer of the heart, between the outer layer of the heart wall (the pericardium) and the inner layer (the endocardium).
The individual cardiac muscle cells, or cardiomyocytes, are contractile cells that work together to pump blood from the heart. Each cardiomyocyte needs to contract in coordination with its neighbouring cells, forming a functional syncytium. This coordination is achieved through gap junctions between adjacent cardiomyocytes, which allow for the propagation of coordinated action potentials from one cell to the next in a phenomenon known as electrical coupling. The cardiac action potential lasts up to 20 times longer than the skeletal myocyte action potential, allowing for the complete ejection of blood from each cardiac chamber.
The contractile functions of the heart require ATP, which can be obtained through various substrates, including fatty acids, carbohydrates, proteins, and ketones. Aerobic production is the primary utilisation process, although the heart may use anaerobic processes to a limited extent. The cardiac action potential is divided into five phases: resting, upstroke, early repolarisation, plateau, and final repolarisation. This process involves the release of calcium from the sarcoplasmic reticulum, which causes the cell's myofilaments to slide past each other in a process called excitation-contraction coupling.
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It pumps blood into circulation
The primary function of the cardiac muscle is to pump blood into circulation. The cardiac muscle is also called the myocardium, and it is one of three major categories of muscles in the human body, the other two being smooth muscle and skeletal muscle. The myocardium forms the thick middle layer of the heart, sandwiched between the outer layer of the heart wall (the pericardium) and the inner layer (the endocardium).
The 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 meet the metabolic demands of the entire body. The contractile functions of the heart require ATP, which can be obtained through various substrates, including fatty acids, carbohydrates, proteins, and ketones. The cardiac muscle cells (cardiomyocytes) are the contractile cells of the cardiac muscle. The cells are surrounded by an extracellular matrix produced by supporting fibroblast cells.
The cardiac muscle does not relax and prepare for the next heartbeat by simply ceasing contraction; instead, this occurs through an active process called Lusitropy. During Lusitropy, the regulatory protein phospholamban controls the rate at which the SERCA pumps calcium into the sarcoplasmic reticulum. The sympathetic nervous system can increase Lusitropy through beta-1 adrenergic stimulation by phosphorylation of phospholamban with cAMP-dependent protein kinase (PKA). When phosphorylated, the phospholamban ceases inhibition of the SERCA, allowing it to increase the rate of calcium intake and relaxation of the cardiac muscle.
The cardiac muscle is not under voluntary control. Instead, it is intrinsically controlled and autorhythmic, with specialized pacemaker cells that set the rhythm of the heart contractions and directly control heart rate. These pacemaker cells are self-excitable and able to depolarize to threshold and fire action potentials on their own at set intervals that determine heart rate. They are distributed throughout the heart and carry the impulses responsible for the beating of the heart. The pacemaker cells respond to signals from the autonomic nervous system (ANS) to speed up or slow down the heart rate.
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The heart's pacemaker cells
The heart's pacemaker, also known as the sinus node, is a small mass of specialized cells in the top of the right atrium (upper chamber of the heart). It is the heart's natural rhythm generator, producing electrical impulses, known as cardiac action potentials, which control the rate of contraction of the cardiac muscle, that is, the heart rate.
The pacemaker cells are comprised of sinoatrial (SA) and atrioventricular (AV) nodes, which are known to fire spontaneously, sending electrical activity throughout the heart, and do not require stimulation to initiate their action. This autorhythmicity transpires because of funny current channels, which allow sodium ions to leak continuously into the cell, slowly raising the membrane potential until a certain threshold is reached, causing depolarization of the cell. This subsequently opens calcium channels, causing calcium ions to enter the cell, further raising the membrane potential. After a positive membrane potential is sensed, potassium channels open, causing an outward flow of ions, returning the membrane potential to its resting potential.
The SA node is the primary pacemaker of the heart, and its cells have the quickest rate of spontaneous depolarization, thus they initiate action potentials the quickest. The action potential generated by the SA node passes down the electrical conduction system of the heart, and depolarizes the other potential pacemaker cells at the AV node to initiate action potentials before these other cells have had a chance to generate their own spontaneous action potential, thus they contract and propagate electrical impulses.
The pacemaker cells are connected to neighboring contractile cells via gap junctions, which enable them to locally depolarize adjacent cells. Gap junctions allow the passage of positive cations from the depolarization of the pacemaker cell to adjacent contractile cells. This starts the depolarization and eventual action potential in contractile cells. Having cardiomyocytes connected via gap junctions allows all contractile cells of the heart to act in a coordinated fashion and contract as a unit, all while being in sync with the pacemaker cells.
In the case of SA node dysfunction, a group of cells further down the heart will become its pacemaker. This center is typically represented by cells inside the atrioventricular node (AV node), which is an area between the atria and ventricles, within the atrial septum.
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Intercalated discs and desmosomes
Intercalated discs (ICDs) are highly specialised intercellular contact areas that connect adjacent cardiomyocytes. They are composed of three major complexes: desmosomes, fascia adherens, and gap junctions. Intercalated discs are fundamental to cellular function and regulation, facilitating the arrangement of cells into organised layers, thereby maintaining tissue integrity and efficient electromechanical coupling.
Desmosomes are intercellular structures that anchor cardiac muscle fibres together and are vital in maintaining the structural integrity of the heart. They consist of plaques of cadherin molecules, a transmembrane glycoprotein that spans the gap between cells and anchors the desmosome to desmin filaments in the cytoplasm. The desmosome joins intermediate filaments in one cell to those in its neighbour. In the adherens junction, the extracellular part of cadherin molecules binds to those from the adjacent cell. Desmosomes and adherens junctions are necessary for mechanically coupling and reinforcing cardiomyocytes.
The plicate region of the intercalated disc consists of the desmosome, as an individual mechanical component, and the area composita or fascia adherens, where both the desmosomes and adherens junction proteins are intermingled. Desmosomes are formed by the homo or heterophilic interactions of desmoglein 2 (DSG2) and desmocollin 2 (DSC2), where AJs are formed by the N-cadherin (N-cad). Desmosomes are anchored to the intermediate filament protein, desmin, via the armadillo proteins plakoglobin (PG) and plakophilin (PKP) 2, which further interacts with the plaque protein desmoplakin, which then interacts with desmin.
The role of intercalated discs and their components, including desmosomes, has been the subject of numerous studies, particularly in relation to cardiac disease. Human genetic studies have identified mutations in almost every known component of the desmosomal complex in patients with arrhythmogenic right ventricular cardiomyopathy (ARVC). The truncated plakoglobin of Naxos disease, for example, disrupts plakoglobin’s ability to interact with other desmosomal proteins within the ICD, altering normal desmosome structure and leading to abnormal electrical coupling of myocytes and an increase in cardiac arrhythmias.
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Electrical coupling and impulses
The cardiac conduction system is a network of nodes, cells and signals that controls the heartbeat. Electrical signals move through the heart, causing it to beat and pump blood around the body.
The sinoatrial (SA) node, located in the upper right atrium, acts as the heart's natural pacemaker. It sends electrical impulses that initiate the heartbeat. The SA node is regulated by the autonomic nervous system, which controls the speed of the electrical signals. The electrical impulse then travels from the SA node to the atrioventricular (AV) node. The AV node is comprised of pacemaker cells that fire spontaneously, sending electrical activity throughout the heart without requiring stimulation. This autorhythmicity is due to the continuous leakage of sodium ions into the cell, which slowly raises the membrane potential until a threshold is reached, causing depolarization and the subsequent opening of calcium channels. This results in an influx of calcium ions, further raising the membrane potential. Finally, potassium channels open, allowing an outward flow of ions and returning the membrane potential to its resting state.
The myocardial conducting cells form the conduction system of the heart and are responsible for initiating and propagating the electrical impulse that triggers contractions. These cells are similar in function to neurons but are specialised muscle cells.
The cardiac muscle cells, or cardiomyocytes, are connected by intercalated discs that contain gap junctions. These gap junctions allow for the propagation of coordinated action potentials from one cell to the next, a phenomenon known as electrical coupling. This enables the cardiomyocytes to contract together synchronously, ensuring the heart functions effectively as a pump.
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Frequently asked questions
Cardiac muscle, also called myocardium or heart muscle, is one of three types of vertebrate muscle tissues, the others being skeletal muscle and smooth muscle. It is an involuntary, striated muscle that constitutes the main tissue of the wall of the heart.
The primary function of 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 cardiac muscle cells contract in a wave-like pattern so that the heart can work as a pump.
Unlike skeletal muscle, cardiac muscle is under involuntary control. It is not under voluntary control. The natural pacemaker of the heart is made of cardiac muscle tissue that stimulates other cardiac muscle cells to contract.
