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Cardiac muscle tissue, or myocardium, is a type of muscle tissue that forms the heart. It is one of three types of muscle tissue in the body, the other two being skeletal and smooth muscle tissue. The myocardium is responsible for keeping the heart pumping and relaxing normally through involuntary movements. It is made up of cardiomyocytes, which are contractile cells that allow the heart to pump. These cells contract and relax in a coordinated fashion, generating the pressure needed to pump blood through the circulatory system.
| Characteristics | Values |
|---|---|
| Definition | Cardiac muscle tissue, or myocardium, is a type of muscle tissue that forms the heart. |
| Location | Cardiac muscle tissue exists only in the heart. |
| Function | Keeps the heart pumping and relaxing normally. |
| Composition | Cardiac muscle cells (also called cardiomyocytes) are the contractile myocytes of the cardiac muscle. |
| Shape | A healthy adult cardiomyocyte has a cylindrical shape that is approximately 100μm long and 10–25μm in diameter. |
| Contraction | Cardiac muscle contracts involuntarily and keeps the heart pumping blood around the body. |
| Pacemaker Cells | Pacemaker cells generate electrical impulses, or action potentials, that tell cardiac muscle cells to contract and relax. |
| Conditions | Cardiomyopathy, myocarditis, ischemic heart disease, hypertrophic cardiomyopathy, dilated cardiomyopathy, restrictive cardiomyopathy |
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What You'll Learn
The cardiac muscle's role in the heart's pumping function
The human body contains three kinds of muscle tissue: skeletal, smooth, and cardiac. Cardiac muscle tissue, or myocardium, is a type of muscle tissue that forms the heart. It is the only organ that is also a muscle. It is made of a special type of muscle tissue and is the only place in the body with cardiac tissue.
Cardiac muscle tissue works to keep the heart pumping through involuntary movements. This is one feature that differentiates it from skeletal muscle tissue, which is voluntary. It does this through specialised cells called pacemaker cells. These control the contractions of the heart. The nervous system sends signals to pacemaker cells that prompt them to either speed up or slow down the heart rate. Pacemaker cells are connected to other cardiac muscle cells, allowing them to pass along signals. This results in a wave of contractions of the cardiac muscle, which creates the heartbeat.
Cardiac muscle cells are the contracting cells that allow the heart to pump. Each cardiomyocyte needs to contract in coordination with its neighbouring cells. If this coordination breaks down, the heart may not pump at all, as may occur during abnormal heart rhythms such as ventricular fibrillation. Cardiomyocytes are the individual cells that make up the cardiac muscle. The primary function of cardiomyocytes is to contract, which generates the pressure needed to pump blood through the circulatory system.
Cardiac muscle fibres have their own auto-rhythmicity. Unlike smooth or skeletal muscle, which require neural input for contraction, cardiac fibres have their own pacemaker cells like the sinoatrial (SA) node that spontaneously depolarizes. These depolarizations occur at a consistent pace, but the pacemaker cells can also receive input from the autonomic nervous system to decrease or increase the heart rate depending on the body's requirements.
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How cardiac muscle cells contract and relax
Cardiac muscle tissue, or myocardium, is a type of muscle tissue that forms the heart. It is responsible for keeping the heart pumping and relaxing normally. The heart contains pacemaker cells that produce the depolarization and action potentials to drive cardiac cell contraction.
Cardiac muscle cells contract and relax in a similar manner to skeletal muscle cells, although there are some important differences. A contraction begins with a characteristic flow of ions across the cell membrane known as an action potential. The action potential subsequently triggers muscle contraction by increasing the concentration of calcium within the cytosol. This increase in calcium causes the cell's myofilaments to slide past each other in a process called excitation-contraction coupling or the sliding filament theory. There are two kinds of myofilaments: thick filaments composed of the protein myosin, and thin filaments composed of the proteins actin, troponin, and tropomyosin. As the thick and thin filaments slide past each other, the cell becomes shorter and fatter. In a mechanism known as cross-bridge cycling, calcium ions bind to the protein troponin, which, along with tropomyosin, then uncover key binding sites on actin. Myosin, in the thick filament, can then bind to actin, pulling the thick filaments along the thin filaments. This process uses ATP to power the contraction.
When the concentration of calcium within the cell falls, troponin and tropomyosin once again cover the binding sites on actin, causing the cell to relax. Intracellular calcium is removed by the sarcoplasmic reticulum, dropping the concentration of intracellular calcium. This decrease in intracellular calcium concentration returns the troponin complex to its inhibiting position on the active site of actin, ending contraction as the actin filaments return to their initial position, relaxing the muscle.
The myocardial contractile cells constitute the bulk (99%) of the cells in the atria and ventricles. These cells conduct impulses and are responsible for contractions that pump blood through the body. The myocardial conducting cells (1% of the cells) form the conduction system of the heart. Their function is similar in many respects to neurons, although they are specialized 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.
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The impact of diseases on cardiac muscle
Cardiac muscle tissue, or myocardium, is a type of muscle tissue that forms the heart. It is one of three types of muscle in the body, the other two being skeletal and smooth muscle. The myocardium is surrounded by a thin outer layer called the epicardium (or visceral pericardium) and an inner endocardium. Coronary arteries supply blood to the cardiac muscle, and cardiac veins drain it. The primary function of cardiac muscle is to pump blood into circulation by generating sufficient force through rapid, involuntary contraction and relaxation.
Diseases affecting the cardiac muscle have a significant impact on health worldwide. Ischemic heart disease, for instance, is the leading cause of morbidity and mortality globally. It is characterised by an imbalance between the supply and demand of oxygenated blood for cardiac tissue, often due to a diminished blood supply to the heart. This can lead to irreversible loss of cardiac function. Myocarditis, an inflammation of the heart muscle, can also cause damage to the myocardium.
Cardiomyopathy is another condition that affects the heart muscle, causing the heart to work inefficiently. It can lead to arrhythmias, heart failure, heart valve disease, and cardiac arrest. Cardiomyopathy can cause the heart to stiffen, enlarge, thicken, and form scar tissue. There are several types of cardiomyopathy, including dilated cardiomyopathy, which causes the cardiac muscle tissue of the left ventricle to stretch and the heart's chambers to dilate; hypertrophic cardiomyopathy (HCM), a genetic condition that interrupts blood flow out of the ventricles; and restrictive cardiomyopathy (RCM), which causes the ventricle walls to stiffen and reduces the heart's ability to pump blood.
Cardiac muscle diseases can have a major impact on exercise performance. Abnormal cardiac function can affect the muscles, vasculature, and lungs, impairing exercise capacity. Atherosclerotic heart disease, for example, is often diagnosed through exercise testing, which is used to measure functional capacity and evaluate treatment efficacy.
Treatments for cardiac muscle diseases can include lifestyle changes, medications, and procedures. Maintaining a healthy weight, getting regular exercise, reducing stress, and avoiding tobacco and alcohol products can help strengthen the heart and improve cardiac muscle performance.
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The structure of cardiac muscle tissue
Cardiac muscle tissue, also known as myocardium, is a specialised type of muscle found only in the heart. It is one of the three major categories of muscles in the human body, the other two being smooth muscle and skeletal muscle. The cardiac muscle is responsible for the contractility of the heart and, in turn, the pumping action. The heart must contract with enough force to pump blood and supply the metabolic demands of the entire body.
Cardiac muscle cells, also known as cardiomyocytes, are small and cylindrical in shape. They are usually branched and possess an endomysium and perimysium but do not have an epimysium. Most of these cells contain one nucleus, but some have two. The nucleus houses all of the cell's genetic material. These cells also contain mitochondria, which convert oxygen and glucose into energy in the form of adenosine triphosphate (ATP). Cardiac muscle cells appear striated or striped under a microscope. These stripes occur due to alternating filaments that comprise myosin and actin proteins.
The cardiac muscle cells are connected by interlocking regions of thickened sarcolemma called intercalated discs. These discs provide attachment points, giving the tissue a branched pattern. They also allow cardiac muscle tissue to function as a functional syncytium, enabling the entire tissue section to contract synchronously. The intercalated discs contain gap junctions, which control the passage of ions and help propagate electrical signals during muscle contraction. The intercalated discs also have an abundance of desmosomes that anchor the cells to each other and prevent cell separation during contraction.
The thick (myosin) and thin (actin, troponin, and tropomyosin) protein filaments are arranged into contractile units, with the sarcomere extending from Z line to Z line. Sarcomeres are specific portions of myofibrils located between two Z lines and are responsible for the striated appearance of cardiac tissue. They are composed of thick and thin filaments. The thick filaments are composed of polymerised myosin type II protein and are attached to the band called the M line in the middle of the sarcomere. The thin filaments consist of polymers of the protein alpha-actin and are attached to the Z lines.
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Clinical tests to evaluate cardiac muscle function
The primary function of the cardiac muscle is to pump blood into circulation by generating sufficient force. To accomplish this, the cardiac muscle has distinct features that allow it to contract and relax in a coordinated fashion and resist fatigue.
Cardiac muscle diseases have a significant impact on health worldwide, and clinicians must understand the physiology and management of these diseases. Various clinical tests are employed to evaluate cardiac muscle function and identify any abnormalities. Here are some detailed descriptions of clinical tests used to assess cardiac muscle function:
Echocardiogram
An echocardiogram is a standard test that uses sound waves or ultrasound to create images of the heart. This test allows physicians to visualise the heart's movement and blood flow through it. Echocardiograms can be performed as a transthoracic echocardiogram (TTE) or a transesophageal echocardiogram (TEE). TEE provides more detailed images by inserting a thin tube with an echo transducer down the patient's throat, close to the upper chambers of the heart. Echocardiograms help identify valvular abnormalities, masses, pericardial disease, congenital abnormalities, and pulmonary hypertension. They are also useful in diagnosing congestive heart failure and cardiomyopathies.
Electrocardiogram (ECG or EKG)
An ECG or EKG is a non-invasive test that uses electrodes placed on the body's surface to record the heart's electrical activity and rhythms. These electrical rhythms lead to the depolarisation of the heart, resulting in the contraction of the myocardium. By measuring the electrical activity, cardiologists can determine if parts of the heart are enlarged or overworked.
Cardiac Biomarkers
Blood tests can be performed to identify enzymes and proteins that indicate heart disease or cardiac damage. For example, troponin measures the levels of cardiac proteins troponin T and troponin I, which are released upon damage to the cardiac muscle. Creatine kinase (CK) is another enzyme released from cardiac and skeletal muscle following damage.
Cardiac Magnetic Resonance Imaging (MRI)
Cardiac MRI is a valuable tool for diagnosing heart diseases or problems with blood vessels. It provides detailed images of the heart muscle, chamber sizes, function, and connecting blood vessels. Cardiac MRI can help detect scarring, inflammation, tumours, or clots in the heart. It is often used when other imaging tests, such as echocardiograms or chest X-rays, are not clear.
Cardiac Computed Tomography (CT) Scan
A cardiac CT scan, also known as a CAT scan, is a non-invasive imaging test that uses X-rays to create detailed two-dimensional and three-dimensional images of the heart and its blood vessels. This test helps identify heart diseases or problems with the heart or blood vessels supplying blood to the body. However, it is important to note that cardiac CT scans have some risks, including rare cases of kidney damage from contrast dye and a slight risk of cancer due to radiation exposure.
Exercise Cardiac Stress Test
Also known as an exercise tolerance test (ETT), this test evaluates the sufficiency of the heart's blood supply and the normalcy of the heart rhythm. It is often performed in conjunction with imaging tests, such as echocardiograms, to elevate the heart rate and study the heart's function under stress.
These clinical tests provide valuable information about cardiac muscle function and play a crucial role in the diagnosis and management of cardiac diseases.
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Frequently asked questions
Cardiac muscle is a type of muscle tissue that forms the heart. It is one of three types of muscle in the body, the other two being skeletal and smooth muscle.
Cardiac muscle tissue works to keep the heart pumping through involuntary movements. It contracts and relaxes to pump blood through the cardiovascular system.
Conditions that affect cardiac tissue can impact the heart's ability to pump blood around the body. For example, myocarditis is a condition where the heart muscle becomes inflamed, often due to a viral infection. This can reduce the heart's pumping function, leading to heart failure.
