Dominic Stone
The cardiac muscle, also known as the myocardium, is one of three types of muscles found in the human body, the other two being skeletal and smooth muscles. The myocardium forms the thick middle layer of the heart, with the outer layer being the pericardium and the inner layer, the endocardium. The cardiac muscle is involuntary, meaning it is not under conscious control. Instead, it is regulated by pacemaker cells that respond to signals from the autonomic nervous system and various hormones that modulate heart rate and blood pressure. These pacemaker cells generate electrical impulses, or action potentials, that trigger the contraction and relaxation of cardiac muscle cells, allowing the heart to pump blood.
| Characteristics | Values |
|---|---|
| Location | Only found in the heart |
| Function | To pump blood into circulation by generating sufficient force |
| Contraction | Controlled by specialised cardiac muscle cells called pacemaker cells |
| Control | Involuntary, not under conscious control |
| Structure | Striated, composed of individual cardiac muscle cells joined by intercalated discs |
| Shape | Tubular, rectangular, cylindrical |
| Size | 100–150μm by 30–40μm, 80 µm in length and 15 µm in diameter |
| Composition | Chains of myofibrils, sarcomeres, mitochondria, intercalated discs, gap junctions, desmosomes |
| Nucleus | Single, centrally placed, occasionally two |
| Nutrients | Coronary arteries, veins, capillary network |
| Action Potential | Divided into 5 phases: resting, upstroke, early repolarization, plateau, final repolarization |
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Involuntary control
Unlike skeletal muscle, cardiac muscle is under involuntary control. This means that a person cannot control it. 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 supply 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. Aerobic production is the core utilization process; however, the heart may use anaerobic processes in a limited capacity. The cardiac action potential lasts approximately 200 ms and is divided into five phases: resting, upstroke, early repolarization, plateau, and final repolarization.
The cardiac muscle does not relax and prepare for the next heartbeat simply by ceasing contraction; it occurs in an active process called Lusitropy. During Lusitropy, Sarco/endoplasmic reticulum Ca-ATPase (SERCA) pumps on the membrane of the sarcoplasmic reticulum use ATP hydrolysis to transfer calcium back into the sarcoplasmic reticulum (SR) from the cytosol. The regulatory protein phospholamban can control the rate at which the SERCA pumps calcium into the SR. Phospholamban reduces the transfer of calcium by the SERCA (sarcoplasmic reticulum Ca2+ ATPase) when bound together.
The contractile functions of the heart are coordinated by specialized cardiac muscle cells called pacemaker cells. These cells respond to signals from the autonomic nervous system (ANS) to speed up or slow down the heart rate. Pacemaker cells can also respond to various hormones that modulate heart rate to control blood pressure. The wave of contraction that allows the heart to work as a unit, called a functional syncytium, begins with the pacemaker cells. This group of cells is self-excitable and able to depolarize to threshold and fire action potentials on their own, a feature called autorhythmicity; they do this at set intervals that determine heart rate.
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Electrical impulses
The cardiac muscle, also called the 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 constitutes the main tissue of the wall of the heart. The cardiac muscle forms a thick middle layer between the outer layer of the heart wall (the pericardium) and the inner layer (the endocardium).
The pacemaker cells are connected to neighbouring contractile cells via gap junctions, allowing them to transmit electrical impulses throughout the heart. This electrical coupling enables the rapid propagation of action potentials from one cardiac muscle cell to the next, resulting in coordinated contractions. The gap junctions create a functional unit of contraction called a syncytium, where cardiac muscle cells work together to ensure efficient blood pumping.
The cardiac action potential involves several phases, including resting, upstroke, early repolarization, plateau, and final repolarization. During the resting phase, the Na/K ATPase pump maintains the negative intracellular potential by exchanging three sodium ions for two potassium ions. The action potential triggers the release of calcium from the sarcoplasmic reticulum, the cell's internal calcium store. This release of calcium initiates the excitation-contraction coupling process, causing the cell's myofilaments to slide past each other and resulting in muscle contraction.
The calcium dynamics within cardiac muscle cells are regulated by the Sarco/endoplasmic reticulum Ca-ATPase (SERCA) pumps, which use ATP hydrolysis to transfer calcium between the cytosol and the sarcoplasmic reticulum. The regulatory protein phospholamban can modulate the rate of calcium intake and relaxation of the cardiac muscle by controlling the SERCA pumps. Additionally, the sympathetic nervous system can influence lusitropy, or the active relaxation process, through beta-1 adrenergic stimulation, enhancing the rate of calcium intake and muscle relaxation.
In summary, electrical impulses generated by pacemaker cells are essential for coordinating the contractions of the cardiac muscle. These impulses propagate through gap junctions, resulting in the transmission of action potentials and subsequent calcium release, contraction, and relaxation of cardiac muscle cells. The regulation of calcium levels by SERCA pumps and the influence of the sympathetic nervous system on lusitropy further contribute to the control and fine-tuning of cardiac muscle function.
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Intercalated discs
The intercalated disc is composed of three electron-dense structures: adherens junctions, desmosomes, and gap junctions. Adherens junctions are specialized structures essential for the mechanical coupling between neighbouring cells. The three morphologically different forms of adherens junctions are puncta adherentia, zonula adherens, and fascia adherens, the latter being the morphology found in the intercalated disc. Cell-cell mechanical anchoring occurs at two crucial points: the extracellular space, where cadherins tightly bind to each other, and the intracellular space, where the cytoplasmic end of the cadherin is indirectly attached to the actin cytoskeleton.
Desmosomes are intercellular structures that anchor cardiac muscle fibres together and are vital in maintaining the structural integrity of the heart. Gap junctions, as their name suggests, are gaps between cells that still maintain a junction between them. They form intercellular channels that provide a low-resistance pathway for the direct cell-to-cell passage of electrical charges. Each gap junction channel is composed of two hexameric structures called connexons that dock across the extracellular space and form a permeable pore.
The size of the space separating two cardiac cells at the intercalated disc changes depending on the proximity to the various structures, as well as the vesicular activity between the two cells. Mathematical modelling and experimental evidence support the idea that the intercellular space may be critical to the propagation of electrical impulses via an electric field mechanism.
Mutations in the intercalated disc gene are responsible for various cardiomyopathies that can lead to heart failure. Ruptured intercalated discs, when seen on histopathology, have two main causes: microtome sectioning (visual artifact) and forceful myocardial contraction (caused by ventricular fibrillation or electrical injury).
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Pacemaker cells
The cardiac muscle, also called the myocardium, is one of the three major categories of muscles in the human body. Unlike skeletal muscle, cardiac muscle is not under voluntary control. The heart is made up of three layers—the pericardium, myocardium, and 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 and blood to supply the metabolic demands of the entire body.
The cardiac pacemaker is the heart's natural rhythm generator. It employs pacemaker cells that produce electrical impulses, known as cardiac action potentials, which control the rate of contraction of the cardiac muscle, that is, the heart rate. In most humans, these cells are concentrated in the sinoatrial (SA) node, the primary pacemaker, which regulates the heart’s sinus rhythm. Sometimes a secondary pacemaker sets the pace, if the SA node is damaged or if the electrical conduction system of the heart has problems.
The cells that make up the SA node are specialized cardiomyocytes known as pacemaker cells that can spontaneously generate cardiac action potentials. These signals are propagated through the heart's electrical conduction system. Only one percent of the heart muscle cells are conductive, the rest of the cardiomyocytes are contractile. The SA node controls the rate of contraction for the entire heart muscle because 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 neighbouring 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 allow all contractile cells of the heart to act in a coordinated fashion and contract as a unit. All the while being in sync with the pacemaker cells; this is the property that allows the pacemaker cells to control contraction in all other cardiomyocytes.
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Calcium
Cardiac muscle cells contain a plasma membrane called the sarcolemma, which houses voltage-gated calcium channels and other essential proteins. The sarcolemma invaginates into the cytoplasm of the cardiomyocyte, forming highly branched structures known as T-tubules. These T-tubules play a vital role in excitation-contraction coupling (ECC), which is the process by which an electrical stimulus triggers a mechanical response in the cardiac muscle cell.
During ECC, electrical stimulation in the form of a cardiac action potential prompts the release of calcium from the sarcoplasmic reticulum (SR), which is the cell's internal calcium store. This release of calcium causes a rise in intracellular calcium levels, activating the contractile machinery of the cardiomyocyte. The calcium ions (Ca++) enter the cardiomyocyte through the voltage-gated calcium channels in the sarcolemma, initiating a sustained depolarization phase known as the "plateau."
This sustained depolarization phase is unique to cardiac muscle and provides a longer contraction compared to skeletal muscle. The influx of calcium ions during this phase is crucial for the contraction process. The calcium ions bind to specific sites on the cardiomyocyte's contractile proteins, myosin and actin, causing them to slide past each other and resulting in the shortening of the cardiomyocyte. This sliding filament mechanism generates the force required for cardiac muscle contraction.
Additionally, the regulation of calcium levels within the cardiomyocyte is essential for maintaining proper cardiac function. The relaxation phase of the cardiac cycle, known as Lusitropy, involves the active transport of calcium back into the SR by Sarco/endoplasmic reticulum Ca-ATPase (SERCA) pumps. The rate at which these pumps operate is controlled by the regulatory protein phospholamban, which can be influenced by the sympathetic nervous system through beta-1 adrenergic stimulation. This process ensures the timely removal of calcium from the cytosol, allowing the cardiac muscle to relax and prepare for the next heartbeat.
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Frequently asked questions
Cardiac muscle, also called the 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 constitutes the main tissue of the wall of the heart.
The cardiac muscle is controlled by specialised cardiac muscle cells called pacemaker cells that directly control heart rate. The pacemaker cells respond to signals from the autonomic nervous system (ANS) to speed up or slow down the heart rate.
Pacemaker cells are specialised modified cardiomyocytes that set the rhythm of the heart contractions. They carry the impulses that are responsible for the beating of the heart. They generate electrical impulses, or action potentials, that tell cardiac muscle cells to contract and relax.
The primary function of cardiac muscle is to pump blood into circulation by generating sufficient force. The cardiac muscle must contract with enough force and blood to supply the metabolic demands of the entire body.
