Dominic Stone
Muscle tissue is classified into three types according to structure and function: striated (skeletal), smooth, and cardiac. Striated muscle tissue is characterised by the presence of stripes or striations, visible under a microscope. Cardiac muscle, also known as myocardium, is the tissue that forms the contractile walls of the heart. Cardiac muscle cells are striated and have a single, centrally located nucleus. They are tightly connected by intercalated discs, which allow the cells to beat in synchrony.
| Characteristics | Values |
|---|---|
| Striated | Yes |
| Involuntary | Yes |
| Location | Walls of the heart |
| Appearance under a light microscope | Striated with many nuclei squeezed along the membranes |
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What You'll Learn
Cardiac muscle is striated due to the regular alternation of actin and myosin
Cardiac muscle, or 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 main tissue of the heart wall. Striated muscles are those that exhibit cross-striations or stripes, which are the result of the regular alternation of actin and myosin.
Each myofibril in the muscle is organized as a chain of contractile units called sarcomeres, which are responsible for the striated appearance of skeletal and cardiac muscle. The thick filaments are made from the protein myosin, which has one pair of heavy chains and two pairs of light chains. The thin filaments are made of actin. These filaments are arranged longitudinally in small units known as sarcomeres, which give the muscle a striated appearance under microscopy.
The contractile elements of the cytoskeleton are present in highly organized arrays that give rise to characteristic patterns of cross-striations. In striated muscle, each tropomyosin molecule is bound to troponin, which is a complex of three polypeptides: troponin C (Ca2+-binding), troponin I (inhibitory), and troponin T (tropomyosin-binding). The Ca ions produce attractive forces that cause the actin and myosin filaments to slide alongside each other, leading to the contractile process.
In the case of cardiac muscle, the myofilaments are arranged in a similar pattern to skeletal muscle, resulting in cross-striations. The fibers are crossed by linear bands called intercalated discs. These structures allow cardiac muscle tissue to function as a functional syncytium, where the contractile stimuli are propagated from one cell to the next, resulting in a synchronous contraction of the entire tissue section.
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Cardiac muscle is involuntary
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. Cardiac muscle is involuntary, meaning it is not under conscious control. It contracts rhythmically and continuously without our conscious input, enabling the heart to pump blood efficiently and constantly without us having to think about it.
The cardiac muscle constitutes the main tissue of the wall of the heart. It forms a thick middle layer between the outer layer of the heart wall (the pericardium) and the inner layer (the endocardium). The heart wall is a three-layered structure, with the myocardium sandwiched between the inner endocardium and the outer epicardium (also known as the visceral pericardium). The inner endocardium lines the cardiac chambers, covers the cardiac valves, and joins with the endothelium that lines the blood vessels connected to the heart.
The cardiac muscle is composed of individual muscle cells, known as cardiomyocytes, which are joined by intercalated discs and encased by collagen fibres and other substances that form the extracellular matrix. Cardiomyocytes are single cells with a single, centrally located nucleus. They contract on their own intrinsic rhythms without any external stimulation, attaching to one another with specialised cell junctions called intercalated discs. The intercalated discs have both anchoring junctions and gap junctions, with attached cells forming long, branching cardiac muscle fibres that are a mechanical and electrochemical syncytium, allowing the cells to synchronise their actions.
Specialised modified cardiomyocytes known as pacemaker cells set the rhythm of the heart contractions. These cells are only weakly contractile and are connected to neighbouring contractile cells via gap junctions. They are located in the sinoatrial node (the primary pacemaker) on the wall of the right atrium and in the atrioventricular node (secondary pacemaker). Pacemaker cells carry the impulses responsible for the beating of the heart and are distributed throughout the organ. They are responsible for generating and sending out electrical impulses and are crucial for the heart's automaticity.
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Cardiac muscle forms the contractile walls of the heart
The heart is made up of three layers: the pericardium, myocardium, and endocardium. The endocardium is not cardiac muscle and forms the inner lining of the heart chambers and valves. The pericardium is a fibrous sac surrounding the heart, consisting of the epicardium, pericardial space, parietal pericardium, and fibrous pericardium. The cardiac muscle, or myocardium, forms the thick middle layer of the heart, between the outer layer of the heart wall (the pericardium) and the inner layer (the endocardium).
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.
Cardiac muscle cells are striated, branched, and contain many mitochondria. They are under involuntary control. Each myocyte contains a single, centrally located nucleus surrounded by a cell membrane known as the sarcolemma. The sarcolemma of cardiac muscle cells contains voltage-gated calcium channels, specialised ion channels that skeletal muscle does not possess.
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.
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Cardiac muscle cells are called cardiomyocytes
Cardiac muscle cells, also called cardiomyocytes, are the contractile myocytes of the cardiac muscle. They are the individual cells that make up the myocardium, which is the thick middle layer of the heart wall. The myocardium forms the bulk of the heart and is surrounded by a thin outer layer called the epicardium (or visceral pericardium) and an inner endocardium. The cardiac muscle cells are located in the walls of the heart and are under involuntary control. They are responsible for the contractility and rhythmicity of the heart, which are essential for its pumping action.
Cardiomyocytes are tubular structures 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 made up of long proteins that organize into thick and thin filaments called myofilaments. These myofilaments slide past each other during muscle contraction and relaxation, forming "cross-bridges" that cause the heart to contract and generate force.
The outer surface of the cardiomyocyte is surrounded by a plasma membrane called the sarcolemma, which separates the extracellular and intracellular contents. Invaginations of the sarcolemma, called T-tubules, contain proteins such as L-type calcium channels, sodium-calcium exchangers, calcium ATPases, and beta-adrenergic receptors. These proteins facilitate the exchange of ions with the extracellular fluid surrounding the cell. The T-tubules run adjacent to enlarged areas of the sarcoplasmic reticulum, known as terminal cisternae, and together they form a diad.
Cardiomyocytes are connected to each other by intercalated discs, creating a syncytium of cardiac cells. These discs provide structural support and facilitate intercellular communication, allowing ions to move directly between neighbouring cardiomyocytes. The lateral side of the intercalated discs contains gap junctions that enable rapid transmission of action potentials, resulting in synchronized contractions of the cardiomyocytes.
Cardiac muscle cells typically have a single, centrally located nucleus, unlike skeletal muscle cells, which can have multiple nuclei. The primary function of cardiomyocytes is to contract, generating the pressure required to pump blood through the circulatory system. This rapid and involuntary contraction and relaxation of the cardiac muscle are vital for efficient blood circulation.
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Cardiac muscle is one of three types of muscle tissues
Muscle tissue is classified into three types according to structure and function: striated (skeletal), smooth, and cardiac. Cardiac muscle, also called heart muscle or myocardium, is one of these three types of vertebrate muscle tissues. It is an involuntary, striated muscle that constitutes the main tissue of the wall of the heart.
The cardiac muscle forms a thick middle layer between the outer layer of the heart wall (the pericardium) and the inner layer (the endocardium). The heart wall is a three-layered structure with a thick layer of myocardium sandwiched between the inner endocardium and the outer epicardium. The inner endocardium lines the cardiac chambers, covers the cardiac valves, and joins with the endothelium that lines the blood vessels that connect to the heart. On the outer aspect of the myocardium is the epicardium, which forms part of the pericardial sac that surrounds, protects, and lubricates the heart.
The cardiac muscle is composed of individual muscle cells, known as cardiomyocytes, that are joined by intercalated discs and encased by collagen fibres and other substances that form the extracellular matrix. Cardiomyocytes are single cells with a single centrally located nucleus. They contract on their own intrinsic rhythms without any external stimulation, allowing the heart to beat on its own without conscious input.
Cardiac muscle cells are located in the walls of the heart and are responsible for pumping blood through the body. They contract 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. This rise in calcium causes the cell's myofilaments to slide past each other in a process called excitation-contraction coupling, allowing the ventricle to squeeze in several directions simultaneously.
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Frequently asked questions
Cardiac muscles are one of the three types of muscle tissues, the others being skeletal muscle and smooth muscle. They are located in the walls of the heart and are under involuntary control.
Yes, cardiac muscles are striated. This is because the actin and myosin are arranged in sarcomeres, which gives the muscle a striated appearance under a microscope.
Cardiac muscles contract to pump blood through the body. They are the only type of muscle found in the heart.
