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Muscle tissue is classified into three types: striated (skeletal), smooth, and cardiac. Cardiac muscle, also called heart muscle or myocardium, 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 cells of cardiac muscle, known as cardiomyocytes, appear striated under a microscope. These striations are caused by lighter I bands composed mainly of actin, and darker A bands composed mainly of myosin.
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What You'll Learn
Cardiac muscle cells are striated and have a single nucleus
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 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).
Cardiac muscle cells (also called cardiomyocytes) are the contractile myocytes of the cardiac muscle. They are striated and have a single nucleus. When viewed through a microscope, cardiac muscle cells are roughly rectangular. Each cell contains myofibrils, specialized protein contractile fibers of actin and myosin that slide past each other. These are organized into sarcomeres, the fundamental contractile units of muscle cells. The regular organization of myofibrils into sarcomeres gives cardiac muscle cells a striped or striated appearance when viewed through a microscope, similar to skeletal muscle. These striations are caused by lighter I bands composed mainly of actin and darker A bands composed mainly of myosin.
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 joined at their ends by intercalated discs to form long fibers. Intercalated discs have both anchoring junctions and
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Striated muscle tissue has T-tubules that release calcium ions
Cardiac muscle, also known as myocardium, is one of the three types of muscle tissues, the other two being skeletal muscle and smooth muscle. It is an involuntary, striated muscle that forms the contractile walls of the heart.
Striated muscle tissue, including cardiac muscle, has T-tubules that release calcium ions. T-tubules, or transverse tubules, are invaginations of the plasma membrane (sarcolemma) of the muscle fibres. They are called "T-tubules" because they are shaped like a "T". These tubules lie over the junction between the A- and I-bands. The two terminal cisternae of the SR, along with their associated T-tubule, are known as a triad.
The T-tubules play a crucial role in the stimulation of the muscle fibre. When the muscle fibre is stimulated, a wave of depolarization passes down the T-tubule, causing the release of calcium ions into the sarcoplasm, the jelly-like substance found between the sarcolemma and the muscle fibre. This release of calcium ions is triggered by an electrical impulse called an action potential, which causes the ions to enter the cell through the T-tubule membrane.
The calcium ions then bind to a protein called troponin, found on the thin filaments. This binding allows myosin to bind to actin, enabling the thick and thin filaments to slide past each other, resulting in the muscle shortening. This process is known as excitation-contraction coupling and is fundamental to the functioning of striated, cardiac, and smooth muscles.
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Cardiomyocytes contract without external stimulation
Cardiac muscle, or myocardium, is one of three types of vertebrate muscle tissues, the others being skeletal muscle and smooth muscle. Cardiac muscle cells, also known as cardiomyocytes, are the contractile myocytes of the cardiac muscle. They are single cells with a single, centrally located nucleus and appear striated under a microscope.
The functional unit of cardiomyocyte contraction is the sarcomere, which consists of thick (myosin) and thin (actin) filaments. These filaments interact to form the basis of the sliding filament theory, where the myofilaments slide past each other during contraction. The released calcium attaches to troponin C, causing tropomyosin to detach from the myosin-binding sites on actin. Actin and myosin then form a cross-bridge, and contraction occurs. This process is known as excitation-contraction coupling (ECC).
Additionally, cardiomyocytes contain T-tubules, which are pouches of cell membrane that improve the efficiency of contraction. These T-tubules are involved in ECC, action potential initiation and regulation, maintaining the resting membrane potential, and signal transduction. They regulate cardiac ECC by concentrating voltage-gated L-type calcium channels and positioning them near calcium sense and release channels at the junctional membrane of the sarcoplasmic reticulum.
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Cardiac muscle cells are involuntary
Cardiac muscle cells are also known as cardiomyocytes and are the contractile myocytes of the cardiac muscle. They are surrounded by an extracellular matrix produced by supporting fibroblast cells. The cardiac muscle pumps blood through the body and is under involuntary control. The attachment junctions hold adjacent cells together across the dynamic pressure changes of the cardiac cycle.
Cardiac muscle cells contract in a coordinated manner to 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. This process requires a lot of energy, and therefore a constant flow of blood to provide oxygen and nutrients.
Specialised modified cardiomyocytes, known as pacemaker cells, set the rhythm of the heart contractions. They are located in the sinoatrial node (the primary pacemaker) and the atrioventricular node (secondary pacemaker). These pacemaker cells carry the impulses that are responsible for the beating of the heart. They are distributed throughout the heart and are responsible for several functions, including being able to spontaneously generate and send out electrical impulses.
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Cardiac muscle tissue cannot be regenerated by adult humans
Cardiac muscle, also known as myocardium, is an involuntary, striated muscle that forms the main tissue of the heart wall. It is composed of individual cardiac muscle cells or cardiomyocytes, which are joined by intercalated discs to form long fibres. These cells contain myofibrils, specialised protein contractile fibres of actin and myosin that give cardiac muscle its striated appearance.
Cardiac muscle tissue plays a vital role in maintaining circulation by pumping blood through the body. However, adult humans have a limited ability to regenerate cardiac muscle tissue following injury. This lack of regenerative capacity is a leading cause of heart failure and death worldwide. While some lower vertebrates, such as newts and zebrafish, can replace lost cardiac tissue, adult mammals, including humans, typically fail to regenerate the majority of lost cardiomyocytes.
Instead of regenerating functional cardiac muscle, adult humans tend to replace necrotic muscle tissue with scar tissue, which compromises the contractility of the myocardium and can lead to heart failure. This phenomenon of extensive scarring instead of regeneration is an active area of research in regenerative biology, with scientists seeking to understand the underlying mechanisms.
While the innate regenerative capacity of the adult mammalian heart is limited, recent studies have provided some hope. These studies indicate that mammals, including humans, may have some cardiac regeneration potential during development and shortly after birth. Additionally, advances in in vitro cardiomyocyte production and cell transplantation offer potential avenues for stimulating cardiac regeneration.
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Frequently asked questions
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.
Yes, cardiac muscle tissue is striated. The striations are caused by lighter I bands composed mainly of actin, and darker A bands composed mainly of myosin.
Striated means having a striped appearance. Under a microscope, the repeating functional units called sarcomeres are visible along muscle fibres, giving a striated appearance to the tissue.
Cardiac muscle is made of individual cardiac muscle cells, known as cardiomyocytes, joined by intercalated discs and encased by collagen fibres and other substances that form the extracellular matrix.
The physiological concept of muscle contraction is based on two variables: length and tension. The release of calcium from the cell's internal calcium store, the sarcoplasmic reticulum, triggers the cell's myofilaments to slide past each other in a process called excitation-contraction coupling.
