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Cardiac muscle, also known as myocardium or heart muscle, is one of the three major categories of muscles in the human body, the other two being skeletal muscle and smooth muscle. Cardiac muscle is composed of cardiac muscle cells, also known as cardiomyocytes, which are striated, branched, and contain many mitochondria. These cells are joined by intercalated discs and encased by collagen fibres and other substances that form the extracellular matrix. The contraction of cardiac muscle cells is involuntary and regulated by the sinoatrial node of the heart, allowing the heart to pump blood efficiently throughout the body.
| Characteristics | Values |
|---|---|
| Other names | Heart muscle, myocardium |
| Type of muscle tissue | Involuntary, striated muscle |
| Location | Middle layer of the heart wall |
| Blood supply | Coronary circulation |
| Composition | Individual cardiac muscle cells joined by intercalated discs, encased by collagen fibers and other substances that form the extracellular matrix |
| Contraction mechanism | Release of calcium from the cell's internal calcium store, the sarcoplasmic reticulum |
| Function | Pump blood into circulation by generating sufficient force |
| Contractile unit | Sarcomeres |
| Rhythm regulation | Sinoatrial node of the heart, which contains pacemaker cells |
| Nutrient supply | Coronary arteries |
| Waste removal | Veins and a capillary network |
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What You'll Learn
Cardiac muscle cells
Specialized modified cardiomyocytes known as pacemaker cells set the rhythm of the heart contractions. These cells are located in the sinoatrial node, which serves as the heart's primary pacemaker. They generate and send out electrical impulses that control the heart rate and determine how fast the heart pumps blood.
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Heart muscle layers
The heart is a muscular organ with three layers of tissue: the endocardium, the myocardium, and the epicardium.
The endocardium is the innermost layer of the heart. It lines the inner surfaces of the heart chambers and covers the cardiac valves. The endocardium has two layers: the inner layer, which is made of endothelial cells, and the second layer, which is a subendocardial connective tissue that is continuous with the connective tissue of the myocardium. The endocardium is not cardiac muscle, but rather simple squamous epithelial cells.
The myocardium is the thickest layer of the heart and is made up of cardiac muscle cells, or cardiomyocytes. These cells are striated, branched, and contain many mitochondria. Each cell contains a single, centrally located nucleus surrounded by a cell membrane called the sarcolemma. The myocardium is responsible for the contractile function of the heart, generating force to pump blood throughout the body.
The epicardium is the outermost layer of the heart and is equivalent to the visceral layer of the serous pericardium. It is composed of loose connective tissue and fat, and is lined externally by simple squamous epithelial cells (mesothelium). The epicardium forms part of the pericardial sac that surrounds, protects, and lubricates the heart.
The heart muscle is one of three major categories of muscles found in the human body, the other two being smooth muscle and skeletal muscle. The primary function of cardiac muscle is to pump blood into circulation by generating sufficient force. The rhythmic contraction of cardiac muscle is regulated by the sinoatrial node of the heart, which serves as the heart's pacemaker.
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Contractions and pumping
The primary function of cardiac muscle is to pump oxygenated blood into circulation by generating sufficient force. The heart is made up of three layers—the pericardium, myocardium, and endocardium. The myocardium, or cardiac muscle, forms a thick middle layer between the outer layer of the heart wall (the pericardium) and the inner layer (the endocardium).
Cardiac muscle cells (cardiomyocytes) are the contractile myocytes of the cardiac muscle. They are striated, branched, and contain many mitochondria. Each myocyte contains a single, centrally located nucleus surrounded by a cell membrane known as the sarcolemma. The sarcolemma contains voltage-gated calcium channels, which are specialized ion channels that skeletal muscle does not possess.
The cardiac action potential 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 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. When a cardiac muscle cell contracts, the myosin filament pulls the actin filaments toward each other, causing the cell to shrink.
The sheets of muscle that wrap around the left ventricle closest to the endocardium are oriented perpendicularly to those closest to the epicardium. When these sheets contract in a coordinated manner, they allow the ventricle to squeeze in several directions simultaneously – longitudinally (becoming shorter from apex to base), radially (becoming narrower from side to side), and with a twisting motion (similar to wringing out a damp cloth) to squeeze the maximum possible amount of blood out of the heart with each heartbeat.
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Electrical impulses
Cardiac muscle, or myocardium, is a type of muscle tissue that forms the heart. It is one of three types of vertebrate muscle tissues, the others being skeletal muscle and smooth muscle. The heart is made up of three layers—the pericardium, myocardium, and endocardium. The myocardium forms a thick middle layer between the outer layer of the heart wall (the pericardium) and the inner layer (the endocardium).
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. These contractile functions of the heart require ATP, which can be obtained through various substrates, including fatty acids, carbohydrates, proteins, and ketones.
The coordination between the pacemaker cells and the cardiac muscle cells is essential for the heart's pumping action. The pacemaker cells generate electrical impulses, known as action potentials, which stimulate the cardiac muscle cells to contract and relax in a synchronized manner. This coordination ensures that the heart can efficiently pump blood and supply oxygen and nutrients to the body.
The electrical impulses also play a role in the excitation-contraction coupling process. When an electrical stimulation, or cardiac action potential, triggers a cardiac muscle cell, it causes a rise in calcium levels. This increase in calcium leads to the sliding of the cell's myofilaments past each other, resulting in the contraction of the cell. This process is vital for the heart's ability to contract and pump blood effectively.
In summary, electrical impulses are integral to the functioning of cardiac muscle. They regulate the rhythmic contractions, control the heart rate, and facilitate the excitation-contraction coupling process, ultimately enabling the heart to pump blood efficiently and meet the metabolic demands of the body.
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Diseases and conditions
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 heart wall. The myocardium forms a thick middle layer between the outer layer of the heart wall (the pericardium) and the inner layer (the endocardium).
Cardiac muscle cells, also called cardiomyocytes, are the contractile myocytes of the cardiac muscle. They are responsible for the contractility of the heart and, therefore, the pumping action. The primary function of cardiac muscle is to pump blood into circulation by generating sufficient force.
- Ischemic conditions caused by a restricted blood supply to the muscle, such as angina and myocardial infarction (heart attack).
- Viral infections, which are a major cause of cardiomyopathy.
- Cardiomyopathy caused by another disease or its treatment, including complex congenital heart disease, nutritional deficiencies, uncontrollable fast heart rhythms, or certain types of chemotherapy for cancer.
- Cardiomyopathy linked to a genetic defect.
- Heart muscle inflammation, or myocarditis, most commonly caused by a viral infection or sometimes caused by the body's own immune system.
- Damage to the heart muscle caused by drugs such as alcohol, long-standing high blood pressure or hypertension, or persistent abnormal heart racing.
Cardiomyopathy can lead to heart failure, which is when the heart is no longer able to pump enough blood to meet the body's needs due to structural and functional defects of the myocardium.
There are five main categories of non-ischemic cardiomyopathy:
- Dilated cardiomyopathy (DCM): Dilation and impaired contraction of one or both ventricles that cannot be explained by coronary artery disease, valvular disease, or hypertension.
- Hypertrophic cardiomyopathy (HCM): A thickened left ventricle and abnormal heart contraction caused by a mutation in the sarcomere protein genes.
- Restrictive cardiomyopathy (RCM): Impaired ventricular filling with nondilated ventricles.
- Arrhythmogenic right ventricular cardiomyopathy (ARVC): Replacement of the myocardium with fibrofatty tissue, leading to an increased predisposition to ventricular tachycardia and sudden cardiac death.
- Unclassified cardiomyopathies.
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Frequently asked questions
Cardiac muscle, also called myocardium, is one of the three major categories of muscles in the human body, the other two being smooth muscle and skeletal muscle.
Cardiac muscle is responsible for the contractility of the heart and, therefore, the pumping action. The heart pumps blood into circulation by generating sufficient force.
Unlike skeletal muscle, cardiac muscle is under involuntary control. Cardiac muscle also exhibits rhythmic contractions.
Cardiac muscle cells (cardiomyocytes) are striated, branched, and contain many mitochondria. They are connected end-to-end by intercalated discs to form long fibres.
Cardiomyopathy refers to a group of medical conditions that affect cardiac muscle tissue and impair the heart's typical ability to pump blood.
