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The cardiac muscle, also known as the myocardium, is a specialised type of muscle tissue that exists only in the heart. It is responsible for the rhythmic contractions that pump blood into circulation by generating sufficient force. The cardiac muscle is one of three major categories of muscles found within the human body, the other two being smooth muscle and skeletal muscle. Unlike skeletal muscle, which is under voluntary control, the cardiac muscle contracts and releases involuntarily.
| Characteristics | Values |
|---|---|
| Type of Muscle | Cardiac muscle is one of three major categories of muscles found within the human body, the others being smooth muscle and skeletal muscle. |
| Location | Cardiac muscle is found only in the heart. |
| Function | Cardiac muscle is responsible for the contractility of the heart and, therefore, the pumping action. |
| Involuntary Control | Unlike skeletal muscle, cardiac muscle contracts and releases involuntarily. |
| Structure | Cardiac muscle cells form a highly branched cellular network in the heart. |
| Composition | Cardiac muscle cells contain mitochondria and are striated or striped under a microscope. |
| Electrical Coupling | Gap junctions between adjacent cardiomyocytes allow for the propagation of coordinated action potentials from one cell to the next. |
| Cardiac Output | The amount of blood pumped by the heart per minute (the cardiac output) varies to meet the metabolic needs of the body. |
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What You'll Learn
Cardiac muscle cells
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.
The rate at which the heart contracts and the synchronization of atrial and ventricular contraction required for the efficient pumping of blood depend on the electrical properties of the cardiac muscle cells and on the conduction of electrical information from one region of the heart to another. The action potential (activation of the muscle) is divided into five phases. Each phase is caused by time-dependent changes in the permeability of the plasma membrane to potassium, sodium, and calcium ions.
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Contractions
Cardiac muscle, also called the myocardium, is one of three major categories of muscles in the human body, the other two being smooth muscle and skeletal muscle. The myocardium is the only type of muscle that develops in the heart. It is responsible for the contractility of the heart and its pumping action.
The contractility of the heart is determined by the contractile forces of the cardiac muscle and the frequency at which they are activated. This is termed cardiac output and is defined as heart rate multiplied by stroke volume. The contractile functions of the heart require adenosine triphosphate (ATP), which powers the contraction of the cardiac muscle.
Cardiac muscle cells (cardiomyocytes) are striated, branched, and contain many mitochondria. They are under involuntary control, meaning their contractions are automatic and a person cannot control them. 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 specialised ion channels that skeletal muscle does not possess.
The functional unit of cardiomyocyte contraction is the sarcomere, which consists of thick (myosin) and thin (actin) filaments. The interactions between these filaments form the basis of the sliding filament theory of muscle contraction. When a cardiac muscle cell contracts, the myosin filament pulls the actin filaments towards each other, causing the cell to shorten. This is known as the power stroke.
Calcium plays a crucial role in the contraction of cardiac muscle. Calcium ions enter the cardiomyocyte through voltage-operated calcium channels (VOCCs) in the T-tubule system. Calcium then binds to cardiac troponin-C, which moves the troponin complex away from the actin-binding site. This frees the actin to be bound by myosin and initiates contraction.
Once calcium is bound to troponin-C and the conformational change of tropomyosin has occurred, the myosin heads can bind to actin. ADP and inorganic phosphate are released from the myosin head, allowing the power stroke to occur. The myosin head then pivots and bends, pulling on the actin and causing muscle contraction. After this, a new molecule of ATP binds to the myosin head, causing it to detach from the actin. The ATP is then hydrolysed into ADP and inorganic phosphate, and the cycle can begin again for further contraction.
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Cardiac muscle tissue
The human body contains three kinds of muscle tissue: skeletal, smooth, and cardiac. Cardiac muscle tissue, also called myocardium, is a type of muscle tissue that forms the heart. It is one of three major categories of muscles found within the human body. The heart is the only organ that is also a muscle and the only place in the body that has cardiac tissue.
Cardiac muscle is made up of sarcomeres, which allow for contractility. The heart consists mostly of cardiac muscle cells (cardiomyocytes) that are striated, branched, and contain many mitochondria. Each cardiomyocyte contains a single, centrally located nucleus surrounded by a cell membrane known as the sarcolemma. The sarcolemma is the cardiomyocyte plasma membrane containing transverse tubules (t-tubules). These t-tubules are highly branched invaginations of the cardiomyocyte sarcolemma that function in excitation-contraction coupling (ECC), action potential initiation and regulation, maintaining the resting membrane potential, and signal transduction. The contractile functions of the heart require adenosine triphosphate (ATP), which can be obtained through various substrates, including fatty acids, carbohydrates, proteins, and ketones.
Cardiac muscle cells form a highly branched cellular network in the heart. They are connected end-to-end by intercalated discs and are organized into layers of myocardial tissue that are wrapped around the chambers of the heart. The contraction of individual 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. The heart begins as a single chamber, but four separate chambers are created through the growth of various septa. The muscular ventricular septum originates from the bottom of the ventricle, with a membranous septum forming shortly after, joining with the aortic-pulmonary septum as it twists down and fuses. The endocardial cushions also appear at this time and separate the left and right atria and ventricles.
The primary function of cardiac muscle is to pump blood into circulation by generating sufficient force. The rate at which the heart contracts and the synchronization of atrial and ventricular contraction required for the efficient pumping of blood depend on the electrical properties of the cardiac muscle cells and on the conduction of electrical information from one region of the heart to another. The sinoatrial node of the heart, which serves as the heart’s pacemaker, regulates the rhythmic contraction of cardiac muscle.
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Cardiomyocytes
The cardiac muscle, also called the myocardium, is one of three major categories of muscles in the human body, the other two being smooth muscle and skeletal muscle. The cardiac muscle is made up of cardiac muscle cells, also known as cardiomyocytes. Cardiomyocytes are striated, branched, and contain many mitochondria. Each cardiomyocyte contains a single, centrally located nucleus surrounded by a cell membrane called the sarcolemma.
Specialized modified cardiomyocytes known as pacemaker cells set the rhythm of heart contractions. These cells are only weakly contractile and are connected to neighbouring contractile cells via gap junctions. Pacemaker cells generate electrical impulses, or action potentials, that tell cardiac muscle cells to contract and relax, controlling heart rate and determining how fast the heart pumps blood.
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Cardiac muscle and the heart
The heart is the only organ that is also a muscle. It is made of a special type of muscle tissue called cardiac muscle. This tissue is found only in the heart and constitutes the main tissue of the wall of the heart.
Cardiac muscle, also called myocardium, is one of three major categories of muscles found within the human body, the other two being skeletal muscle and smooth muscle. The heart consists mostly of cardiac muscle cells (cardiomyocytes or myocardium) that form a highly branched cellular network in the heart. These cells are striated, branched, and contain many mitochondria. Each cell contains a single, centrally located nucleus surrounded by a cell membrane known as the sarcolemma.
The primary function of cardiac muscle is to pump blood into circulation by generating sufficient force. The heart begins as a single chamber, but four separate chambers are created through the growth of various septa. The muscular ventricular septum originates from the bottom of the ventricle, with a membranous septum forming shortly after, joining with the aortic-pulmonary septum as it twists down and fuses. The endocardial cushions also appear at this time and separate the left and right atria and ventricles.
The contractile functions of the heart require adenosine triphosphate (ATP), which can be obtained through various substrates, including fatty acids, carbohydrates, proteins, and ketones. The cardiac muscle cells contract in a coordinated manner, allowing 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.
Cardiac muscle tissue gets its strength and flexibility from its interconnected cardiac muscle cells, or fibers. Gap junctions between adjacent cardiomyocytes allow for the propagation of coordinated action potentials from one cell to the next in a phenomenon known as electrical coupling.
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Frequently asked questions
Cardiac muscle is one of three major categories of muscles found within the human body, the other two being smooth muscle and skeletal muscle. It is also called myocardium.
Cardiac muscle is 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.
Cardiac muscle is made up of sarcomeres that allow for contractility. Cardiac muscle cells (or cardiomyocytes) form a highly branched cellular network in the heart.
Cardiac muscle is found only in the heart.
