Logan Steele
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 forms the heart. The primary function of cardiac muscle is to pump blood into circulation by generating sufficient force. The cardiac muscle receives a constant flow of blood from the coronary arteries, which originate from the aortic root and lie on the outer or epicardial surface of the heart. The coronary veins then drain the blood away into the right atrium.
| Characteristics | Values |
|---|---|
| Type of Muscle Tissue | Cardiac |
| Other Names | Heart muscle, myocardium |
| Found in | Heart |
| Consists of | Three layers: Pericardium, myocardium, and endocardium |
| Contraction | Rhythmic, wave-like contractions known as the heartbeat |
| Controlled by | Sinoatrial node of the heart |
| Cells | Striated, branched, contain many mitochondria, and are under involuntary control |
| Calcium Channels | Voltage-gated |
| T-Tubules | Bigger and wider than those in skeletal muscle |
| Cardiomyopathies | Ischemic conditions caused by a restricted blood supply to the muscle such as angina, and myocardial infarction |
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What You'll Learn
Cardiac muscle cells
Cardiac muscle, also called heart muscle or myocardium, is one of three types of vertebrate muscle tissues, the others being skeletal muscle and smooth muscle. The myocardium forms a thick middle layer between the outer layer of the heart wall (the pericardium) and the inner layer (the endocardium). The cardiac muscle is composed of individual cardiac muscle cells, also called cardiomyocytes, which are joined by intercalated discs.
The individual cardiac muscle cell (cardiomyocyte) is a tubular structure composed of chains of myofibrils, which are rod-like units within the cell. The myofibrils consist of repeating sections of sarcomeres, which are the fundamental contractile units of the muscle cells. Sarcomeres are composed of long proteins that organise into thick and thin filaments, called myofilaments. Thin myofilaments contain the protein actin, and thick myofilaments contain the protein myosin. The myofilaments slide past each other as the muscle contracts and relaxes. This process is activated by the release of calcium from the sarcoplasmic reticulum (SR) when delivering an action potential to the muscle, in a process called excitation-contraction coupling. The sliding of actin and myosin past each other produces the formation of “cross-bridges”, which causes contraction of the heart and generation of force.
Within the intercalated disc, there are three different types of cell junctions: fascia adherens, desmosomes, and gap junctions. The transverse side of the intercalated discs runs perpendicular to the muscle fibres at the Z lines and provides a structural component via fascia adherens and desmosome connections. The lateral side of the discs contains gap junctions that permit intercellular communication by allowing ions from one cardiomyocyte to move to a neighbouring cell without having to be excreted into the extracellular space first. The low resistance of the gap junctions allows depolarisation to spread quickly throughout the syncytium, facilitating the rapid transmission of action potentials to produce a synchronised contraction of the cardiomyocytes in unison.
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Cardiomyopathies
Cardiac muscle, also known as myocardium, is one of the three types of vertebrate muscle tissues, the other two being skeletal muscle and smooth muscle. The myocardium forms a thick middle layer of the heart wall, with the outer layer being the pericardium and the inner layer being the endocardium. The heart wall is a three-layered structure, with the myocardium sandwiched between the inner endocardium and the outer epicardium.
There are several types of cardiomyopathies, and they differ based on their cause and how they affect the heart's structure and function. All types of cardiomyopathies weaken the heart muscle, and certain types are temporary and improve over time. Some people have overlapping cardiomyopathies, for example, a person may have both dilated cardiomyopathy and arrhythmogenic cardiomyopathy. Dilated cardiomyopathy is a common type of cardiomyopathy that occurs when the ventricles (the lower chambers of the heart) weaken and become larger, causing the heart to work harder. The heart becomes less able to pump blood throughout the body and can lead to heart failure or irregular heartbeats called arrhythmias.
Hypertrophic cardiomyopathy occurs when the heart muscle becomes larger and thicker than normal. The thickened areas can block the ventricles, making it more difficult for the heart to pump blood. Arrhythmogenic cardiomyopathy is a rare condition that develops when fatty or scarred tissue replaces the normal muscle tissue in the right ventricle, leading to an irregular heartbeat. Restrictive cardiomyopathy is another rare form of cardiomyopathy that causes the ventricles to stiffen, resulting in a reduced ability to fill with blood and, consequently, a decreased amount of blood pumped to the rest of the body. Peripartum cardiomyopathy is a rare but serious form of cardiomyopathy that can occur during or after pregnancy, typically developing late in pregnancy or within a few months after giving birth.
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Electrical impulses
The cardiac conduction system is the network of nodes, cells, and signals that controls the heartbeat. Each heartbeat is the result of electrical signals travelling through the conduction pathway of the heart. The cardiac conduction system sends out thousands of signals per day to keep the heart beating.
The heart's pumping action is regulated by an electrical conduction system that coordinates the contraction of the various chambers of the heart. An electrical stimulus is generated by the sinus node (also called the sinoatrial node, or SA node). This is a small mass of specialised tissue located in the right upper chamber (atria) of the heart. The sinus node generates an electrical stimulus regularly, 60 to 100 times per minute under normal conditions. The atria are then activated. The electrical stimulus travels down through the conduction pathways and causes the heart's ventricles to contract and pump out blood. The 2 upper chambers of the heart (atria) are stimulated first and contract for a short period of time before the 2 lower chambers of the heart (ventricles). The electrical impulse travels from the sinus node to the atrioventricular node (also called the AV node). The AV node is the secondary pacemaker.
There are two major types of cardiac muscle cells: myocardial contractile cells and myocardial conducting cells. The myocardial contractile cells constitute 99% of the cells in the atria and ventricles. They conduct impulses and are responsible for the contractions that pump blood through the body. The myocardial conducting cells form the conduction system of the heart. They are generally much smaller than the contractile cells and have few of the myofibrils or filaments needed for contraction. Their function is similar in many respects to neurons, although they are specialised muscle cells. Myocardial conduction cells initiate and propagate the action potential (the electrical impulse) that travels throughout the heart and triggers the contractions that propel the blood.
T-tubules in cardiac muscle are bigger and wider than those in skeletal muscle, but fewer in number. They lie close to the cell's internal calcium store, the sarcoplasmic reticulum. The functions of T-tubules include rapidly transmitting electrical impulses known as action potentials from the cell surface to the cell's core.
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Cardiac output
The cardiac output is dependent on the heart rate and the stroke volume, or the volume of blood ejected by the heart with each beat. The stroke volume is not equal to the end-diastolic volume but is the end-diastolic volume minus the end-systolic volume. The heart rate is determined by signals from the sinoatrial node, which automatically depolarizes at an intrinsic rate of 60 to 100 times each minute. The cardiac output is also affected by the phase of respiration, with intra-thoracic pressure changes influencing diastolic filling and, consequently, the cardiac output.
The cardiac output is calculated using the formula:
\[ \text:{Cardiac Output} = {Heart Rate} \times {Stroke Volume} \]
The cardiac output can be measured using several methods, some of which are more invasive than others. The most accurate method involves measuring the oxygen breathed in and taking blood samples from an artery and a catheter in the pulmonary artery. Other methods include using electrodes on the chest to measure heart rate and stroke volume, and transthoracic echocardiography to estimate stroke volume.
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Contractile functions
The primary function of cardiac muscle is to pump oxygenated blood into circulation by generating sufficient force. The contractile functions of the heart require ATP, which can be obtained through various substrates, including fatty acids, carbohydrates, proteins, and ketones. The contractile functions of the heart involve the following:
Electrical Impulses
The cardiac cycle is the performance of the human heart from the beginning of one heartbeat to the beginning of the next. It consists of two periods: diastole, during which the heart muscle relaxes and refills with blood, and systole, during which the heart contracts and pumps blood. The cardiac action potential triggers muscle contraction by increasing the concentration of calcium within the cytosol. This rise in calcium causes the cell's myofilaments to slide past each other in a process called excitation-contraction coupling.
Calcium Ions
Calcium ions play two critical roles in the physiology of cardiac muscle. Their influx through slow calcium channels accounts for the prolonged plateau phase and absolute refractory period that enable cardiac muscle to function properly. Calcium ions also combine with the regulatory protein troponin in the troponin-tropomyosin complex, which removes the inhibition that prevents the heads of the myosin molecules from forming cross-bridges with the active sites on actin that provide the power stroke of contraction.
T-Tubules
T-tubules in cardiac muscle are bigger and wider than those in skeletal muscle, but fewer in number. They lie close to the cell's internal calcium store, the sarcoplasmic reticulum, with which they form a combination known as a diad. The functions of T-tubules include rapidly transmitting electrical impulses, helping to regulate the concentration of calcium within the cell, and mechano-electric feedback.
Contractile Cells
The contractile cells contract and propel the blood. The normal path of transmission for the conductive cells is the sinoatrial (SA) node, internodal pathways, atrioventricular (AV) node, atrioventricular (AV) bundle of His, bundle branches, and Purkinje fibers. The action potential for the conductive cells consists of a prepotential phase with a slow influx of Na+ followed by a rapid influx of Ca2+ and outflux of K+.
Sheets of Muscle
Within the myocardium, there are several sheets of cardiac muscle cells or cardiomyocytes. When these sheets contract in a coordinated manner, they allow the ventricle to squeeze in several directions simultaneously – longitudinally, radially, and with a twisting motion – to squeeze the maximum possible amount of blood out of the heart with each heartbeat.
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Frequently asked questions
Cardiac muscle, also called the myocardium, is one of three major categories of muscles found within the human body, the others being smooth muscle and skeletal muscle. It is an involuntary, striated muscle that constitutes the main tissue of the wall of the heart.
No, cardiac muscle is not avascular. Blood is brought to the myocardium by the coronary arteries and drained away by the coronary veins into the right atrium.
The primary function of cardiac muscle is to pump blood into circulation by generating sufficient force. The contractile functions of the heart require ATP, which can be obtained through various substrates, including fatty acids, carbohydrates, proteins, and ketones.
