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The human body has more than 600 muscles, and one of the most important is the cardiac muscle, also called the myocardium. It is one of three types of vertebrate muscle tissues, the others being skeletal muscle and smooth muscle. The cardiac muscle is an involuntary, striated muscle that constitutes the main tissue of the heart wall. The heart is made up of three layers—the outer layer or pericardium, the middle myocardium, and the inner endocardium. The contraction of cardiac muscle cells produces force and shortening in these bands of muscle, with a resultant decrease in the heart chamber size and the consequent ejection of blood into the pulmonary and systemic vessels.
| Characteristics | Values |
|---|---|
| Types | Cardiac muscle, smooth muscle, skeletal muscle |
| Location | Walls of the heart |
| Appearance | Striated |
| Control | Involuntary |
| Contraction | Rhythmic |
| Blood supply | Coronary circulation |
| Composition | Individual cardiac muscle cells, intercalated discs, collagen fibers, extracellular matrix |
| Calcium | Stored in sarcoplasmic reticulum |
| T-tubules | Bigger and wider than those in skeletal muscle |
| Action potential | Divided into five phases |
| Electrical coupling | Excitation-contraction coupling |
| Gap junctions | Allow propagation of coordinated action potentials |
| Desmosomes | Anchor cardiac muscle fibers together |
| Sarcomeres | Contractile units |
| Pacemaker cells | Set the rhythm of heart contractions |
| Cardiomyopathy | Disease that affects cardiac muscle |
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What You'll Learn
Cardiac muscle structure
Cardiac muscle, also known as 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 or epicardium) and the inner layer (the endocardium). The myocardium is surrounded by a thin outer layer called the epicardium (AKA visceral pericardium) and an inner endocardium. Coronary arteries supply blood to the cardiac muscle, and cardiac veins drain this blood.
Cardiac muscle cells (also called cardiomyocytes) are the contractile myocytes of the cardiac muscle. They are surrounded by an extracellular matrix produced by supporting fibroblast cells. These cardiomyocytes are joined together at their ends by intercalated discs to create a syncytium of cardiac cells. Intercalated discs enable the rapid transmission of electrical impulses through the network, enabling the syncytium to act in a coordinated contraction of the myocardium.
Cardiac muscle cells contain mitochondria, which convert oxygen and glucose into energy in the form of adenosine triphosphate (ATP). Under a microscope, cardiac muscle cells appear striated or striped due to alternating filaments that comprise myosin and actin proteins. The myofilaments of cardiac muscle are arranged in a similar pattern to skeletal muscle, resulting in cross-striations.
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How it contracts
Cardiac muscle, also called the myocardium, is one of three major categories of muscles in the human body, the others being smooth muscle and skeletal muscle. The myocardium forms a thick middle layer of the heart wall, between the outer layer (the pericardium) and the inner layer (the endocardium).
Cardiac muscle contracts in a similar way to skeletal muscle, but with some differences. The process of contraction begins with electrical stimulation in the form of a cardiac action potential. This triggers the release of calcium from the cell's internal calcium store, the sarcoplasmic reticulum. This rise in calcium causes the cell's myofilaments to slide past each other in a process called excitation-contraction coupling.
The calcium attaches to troponin C, causing tropomyosin to detach from the myosin-binding sites on actin. Actin and myosin then form a cross-bridge, and contraction occurs. Cross-bridges last as long as calcium is attached to troponin C. When calcium is no longer bound to troponin C, the actin-binding site is covered up, ending contraction and allowing the muscle to relax.
The rate at which the heart contracts and the synchronization of atrial and ventricular contraction are dependent on the electrical properties of the cardiac muscle cells. The cardiac muscle must contract with enough force and blood to supply the metabolic demands of the entire body. This is termed cardiac output and is defined as heart rate multiplied by stroke volume, which is determined by the contractile forces of the cardiac muscle and the frequency at which they are activated.
Cardiac muscle cells (cardiomyocytes) are connected by intercalated discs, which consist of three types of cell-cell junctions: the actin filament anchoring fascia adherens junctions, the intermediate filament anchoring desmosomes, and gap junctions. These interconnections allow the cardiomyocytes to contract together synchronously to enable the heart to work as a pump.
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Cardiomyopathies
Dilated cardiomyopathy is a common type of cardiomyopathy that occurs when the ventricles, the lower chambers of the heart, weaken and become larger. This causes the heart to work harder to pump blood. Peripartum cardiomyopathy is a rare but serious type of dilated cardiomyopathy that can occur during or after pregnancy.
Hypertrophic cardiomyopathy occurs when the heart muscle becomes larger and thicker than normal, which can block the ventricles and make it harder for the heart to pump blood. This type of cardiomyopathy is often inherited, and first-degree relatives should be screened for the disease. It is estimated that only 1 in 5 people with hypertrophic cardiomyopathy are diagnosed, as many do not exhibit symptoms.
Arrhythmogenic cardiomyopathy is a rare condition that develops when fatty or scarred tissue replaces the normal muscle tissue in the right ventricle, causing an irregular heartbeat. This type of cardiomyopathy usually affects teens or young adults and increases the risk of cardiac arrest.
Restrictive cardiomyopathy is another rare form of cardiomyopathy, causing the ventricles to stiffen and reducing the amount of blood pumped to the rest of the body. Cardiomyopathies can be acquired due to another disease, condition, or factor, or inherited through genes passed on from a parent. In many cases, the cause of cardiomyopathy is unknown, especially when it occurs in children. Treatment options for cardiomyopathies include lifestyle changes, medications, surgery, implanted devices to correct arrhythmias, and other nonsurgical procedures.
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Electrical impulses
Cardiac muscle, also known as heart muscle or myocardium, is one of the three types of vertebrate muscle tissues, the others being skeletal muscle and smooth muscle. The cardiac muscle constitutes the main tissue of the heart wall and is responsible for its pumping action. The rhythmic contraction of the cardiac muscle is involuntary and regulated by the sinoatrial node (SA node) of the heart, which serves as the heart's pacemaker.
The cardiac conduction system is a network of nodes, cells, and signals that controls the heartbeat. This system generates and propagates electrical impulses that trigger the contraction of the heart muscle, propelling blood throughout the body. The SA node, located in the upper part of the right atrium, acts as the heart's natural pacemaker by sending electrical impulses that initiate the heartbeat. These impulses then travel through the conduction pathways, causing the heart's ventricles to contract and pump blood.
The cardiac muscle cells, or cardiomyocytes, are shorter and have smaller diameters than skeletal muscle cells. They exhibit an alternating pattern of dark A bands and light I bands, attributed to the arrangement of myofilaments and fibrils organized in sarcomeres. T-tubules, which are larger and wider in cardiac muscle than in skeletal muscle, play a crucial role in transmitting electrical impulses from the cell surface to the cell's core. They also regulate calcium concentration within the cell, contributing to excitation-contraction coupling.
The action potential, or activation of the muscle, is divided into five phases, each caused by changes in the permeability of the plasma membrane to potassium, sodium, and calcium ions. The electrical stimulation triggers the release of calcium from the sarcoplasmic reticulum, the cell's internal calcium store. This rise in calcium causes the cell's myofilaments to slide past each other, resulting in muscle contraction.
Additionally, gap junctions between adjacent cardiomyocytes enable the propagation of coordinated action potentials, allowing for electrical coupling. Cardiac desmosomes, another type of intercellular structure, anchor cardiac muscle fibers together and maintain the structural integrity of the heart. These electrical impulses and the resulting contractions work together to ensure the efficient pumping of blood through the heart and circulation throughout the body.
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Calcium's role
Calcium plays a crucial role in the electrical activity and pumping function of the heart. Calcium particles enter the heart muscle cells during each heartbeat, contributing to the electrical signal that coordinates the heart's function. These calcium particles carry an electrical charge and play a key role in the link between electrical activation and mechanical contraction.
During each heartbeat, calcium particles enter the heart muscle cells and contribute to the electrical signal that moves from cell to cell. This electrical signal coordinates the uniform contraction of the heart muscle cells, allowing them to squeeze together and pump blood effectively. The calcium particles bind to specialized machinery within the cells, initiating contraction and making the cells contract in a process called excitation-contraction coupling.
The heart muscle cells, or cardiomyocytes, have voltage-gated calcium channels, which are specialized ion channels that skeletal muscle cells do not possess. These calcium channels help regulate the concentration of calcium within the cell. The calcium particles also contribute to the electrical signal that moves from one cardiomyocyte to the next through gap junctions, ensuring synchronized contraction.
Additionally, calcium plays a role in the relaxation phase of the cardiac cycle. When calcium particles are removed from the heart cells, it triggers relaxation, allowing the heart to refill with blood before the next heartbeat. This removal of calcium through ion channels helps control the amount of calcium inside the cell and prepares the heart for the next contraction.
The proper regulation of calcium is vital for the heart's function. Abnormalities in calcium handling can lead to heart rhythm disorders and directly impair the pumping function or relaxation of the heart.
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Frequently asked questions
A cardiovascular muscle, also known as cardiac muscle or myocardium, is one of the three types of vertebrate muscle tissues found in the human body. The other two types are skeletal muscle and smooth muscle.
Cardiovascular muscles are involuntary, striated muscles that constitute the main tissue of the heart wall. They are composed of individual cardiac muscle cells, called cardiomyocytes, joined by intercalated discs and encased by collagen fibres and other substances that form the extracellular matrix.
Cardiovascular muscles contract with enough force and blood to supply the metabolic demands of the entire body. This is termed cardiac output and is defined as heart rate multiplied by stroke volume. The contraction of individual cardiac muscle cells produces force and shortening in these bands of muscle, resulting in a decrease in heart chamber size and the ejection of blood into the pulmonary and systemic vessels.
