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Cardiac muscle, also known as myocardium, is an involuntary, striated muscle that forms the heart's thick middle layer. It is one of the three types of muscle tissues in the body, the other two being skeletal and smooth muscle. Unlike skeletal muscle, cardiac muscle is not under conscious control and contracts involuntarily to pump blood around the body. The cardiac muscle cells are joined by intercalated discs and encased by collagen fibres and other substances that form the extracellular matrix. While the cardiac muscle is highly resilient, it has minimal regeneration capabilities, and diseases affecting it, known as cardiomyopathies, are the leading cause of death in developed countries.
| Characteristics | Values |
|---|---|
| Voluntary control | No |
| Skeletal muscle similarities | Striated, sarcomeres |
| Skeletal muscle differences | No multiple nuclei, voltage-gated calcium channels |
| Other differences | No neural input for contraction, has intercalated discs |
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What You'll Learn
Cardiac muscle don't have the ability to regenerate
Cardiac muscle, also known as myocardium, is one of the three major categories of muscle found in the human body, the other two being smooth muscle and skeletal muscle. It is an involuntary, striated muscle that forms the bulk of the heart. The cardiac muscle is responsible for the contractility of the heart, which is the basis for its pumping action.
Cardiac muscle cells 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 muscles do not possess. The cardiac muscle cells are connected by intercalated discs, which contain gap junctions and desmosomes. These interconnections allow the cardiomyocytes to contract together synchronously, enabling the heart to work as a pump.
Cardiac muscle cells have baroreceptors that detect high blood volume through increased stretch in the atrium wall. In response, the cells in the atrium wall generate a hormone called atrial natriuretic peptide, which functions to decrease blood volume by increasing sodium excretion in the kidneys.
Cardiac muscle cells do not have the ability to regenerate. Researchers have found that the heart has a very limited regenerative power, and the rate of cell division does not increase in response to injury. However, some studies have shown that cardiac muscle cells can divide and regenerate in very limited amounts. For example, researchers from UCLA's Eli and Edythe Broad Center of Regenerative Medicine and Stem Cell Research have directly measured the division of heart muscle cells, proving that while such division is rare, it does occur. Additionally, studies on zebrafish heart regeneration have shown that fish myocardium replaces lost cardiomyocytes through the proliferation of existing cardiomyocytes.
Despite the limited regenerative abilities of cardiac muscle cells, there is ongoing research and development in the field of cardiac muscle regeneration. Scientists are exploring the use of human stem cells to repair damaged hearts and create functioning strips of ventricular muscle. These advancements raise the possibility of using induced pluripotent stem cell (iPS) technology to generate muscle tissue to repair diseased hearts.
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They don't have multiple nuclei
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 composed of individual cardiac muscle cells, or cardiomyocytes, joined by intercalated discs. Each cell contains a single, centrally located nucleus surrounded by a cell membrane called the sarcolemma.
The sarcolemma of cardiac muscle cells contains voltage-gated calcium channels, which are specialized ion channels that skeletal muscle does not possess. The presence of these calcium channels in the sarcolemma is essential for the release of calcium from the sarcoplasmic reticulum during electrical stimulation, triggering cardiac muscle contraction.
In contrast to skeletal muscle cells, which often contain multiple nuclei, cardiac muscle cells typically contain only one nucleus. This structural difference is significant because it allows cardiac muscle cells to contract in a coordinated and synchronous manner, enabling the heart to function effectively as a pump.
The absence of multiple nuclei in cardiac muscle cells also relates to their unique function. Cardiac muscle cells are responsible for the contractility of the heart, generating the force required to pump blood through the circulatory system. The coordinated contraction of these cells ensures the efficient pumping of blood from the heart, demonstrating the importance of their distinct structure, including the presence of a single nucleus.
The primary function of cardiomyocytes is to contract, and their rectangular shape and single nucleus contribute to their ability to work together to pump blood effectively. This contractile function is essential for maintaining blood circulation and meeting the metabolic demands of the entire body.
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They don't require neural input for contraction
Cardiac muscle, also known as myocardium, is an involuntary, striated muscle that makes up the thick middle layer of 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. Cardiac muscle is responsible for the heart's contractility and pumping action.
Cardiac muscle cells, or cardiomyocytes, are striated, branched, and contain many mitochondria. They are under involuntary control and have their own autorhythmicity, which means they can initiate an electrical potential at a fixed rate that spreads rapidly from cell to cell, triggering the contractile mechanism. This is in contrast to smooth and skeletal muscle, which require neural input for contraction.
Cardiac muscle cells contain branched fibers that are connected via intercalated discs, which help support the synchronized contraction of the muscle. These discs contain gap junctions and desmosomes, which allow the passage of ions between the cells and help synchronize the contraction. The intercalated discs also provide structural support through fascia adherens and desmosome connections.
The functional unit of cardiomyocyte contraction is the sarcomere, which consists of thick (myosin) and thin (actin) filaments. These filaments slide past each other during contraction and relaxation, causing the formation of "cross-bridges" and resulting in the generation of force. The concentration of calcium in the myocyte determines the amount of force generated with each contraction.
The primary function of cardiac muscle is to pump blood into circulation by generating sufficient force through contraction and relaxation. This rapid and involuntary process is vital for pumping blood throughout the cardiovascular system.
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They don't have voluntary control
The cardiac muscle, also called the myocardium, is one of three types of muscle tissues found in the human body, 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.
Cardiac muscle is under involuntary control. This is one feature that differentiates it from skeletal muscle, which you can control. It does this through specialised cells called pacemaker cells, which control the contractions of the heart. The nervous system sends signals to the pacemaker cells, prompting them to either speed up or slow down the heart rate.
The cardiac muscle contracts in a similar manner to skeletal muscle, although with some important differences. Electrical stimulation in the form of a cardiac action potential triggers the release of calcium from the cell's internal calcium store, the sarcoplasmic reticulum. The rise in calcium causes the cell's myofilaments to slide past each other in a process called excitation-contraction coupling.
Cardiac muscle cells have baroreceptors that detect high blood volume through increased stretch in the atrium wall. In response, the cells in the atrium wall generate a hormone, atrial natriuretic peptide, which functions to decrease the blood volume by increasing sodium excretion in the kidneys.
Cardiac muscle tissue is only found in the heart. It performs involuntary, coordinated contractions that allow the heart to pump blood through the circulatory system.
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They don't have more than three layers
The heart wall is made up of three layers, and does not have more than three layers. These layers are the outer epicardium (also known as the visceral pericardium) , the
The outer layer of the heart wall is the pericardium, a fibrous sac surrounding the heart, consisting of the epicardium, pericardial space, parietal pericardium, and fibrous pericardium. The pericardium serves to protect the heart. The middle myocardium is composed of cardiac muscle fibers, which enable the heart to contract and pump blood. The myocardium is the thickest layer, with its thickness varying in different parts of the heart. The left ventricle has the thickest myocardium, as this ventricle is responsible for generating the power needed to pump oxygenated blood from the heart to the rest of the body. The inner endocardium is not cardiac muscle and is comprised of simple squamous epithelial cells. It forms the inner lining of the heart chambers and valves, and covers the cardiac valves.
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 force developed by the cardiac muscle cells, as well as the frequency with which they are activated (rhythmicity), determine the amount of blood pumped by the heart per minute (the cardiac output).
The heart is an extraordinary organ, about the size of a clenched fist and weighing about 10.5 ounces. It is located in the chest cavity just posterior to the breastbone, between the lungs, and superior to the diaphragm.
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Frequently asked questions
Unlike skeletal muscles, cardiac muscles are under involuntary control. They also do not have multiple nuclei.
Smooth muscles require neural input for contraction, unlike cardiac muscles, which have their own pacemaker cells.
Cardiac muscles have minimal regeneration capabilities, so any insult or injury can have grave consequences.
