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Cardiac muscle, also known as myocardium, is a type of muscle tissue that is found only in the heart. It is one of three types of muscle tissues in the body, including skeletal and smooth muscle. Cardiac muscle is made up of cardiomyocytes, which are rectangular, branching cells that contract to generate the pressure needed to pump blood through the circulatory system. These cells are connected by intercalated discs, which allow the cardiac muscle cells to contract in a wave-like pattern. The branching nature of cardiac muscle fibres is a distinct feature that differentiates them from skeletal muscle fibres, which are typically not branched.
| Characteristics | Values |
|---|---|
| Cardiac muscle cell shape | Branched |
| Cardiac muscle cell structure | Tubular |
| Cardiac muscle cell composition | Chains of myofibrils |
| Myofibril composition | Sarcomeres |
| Sarcomere composition | Thick and thin filaments (myofilaments) |
| Thick filaments | Polymerised myosin type II protein |
| Thin filaments | Alpha actin |
| Intercalated discs | Allow cardiac muscle cells to contract in a wave-like pattern |
| Pacemaker cells | Control heart rate |
| Contractions | Strong, continuous, rhythmic |
| Contractility | Altered by autonomic nervous system and hormones |
| Tissue type demands | High metabolic, energy, and vascular |
| Cardiac muscle fibre composition | Many mitochondria and myoglobin |
| Energy production | ATP produced primarily through aerobic metabolism |
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What You'll Learn
Cardiac muscle cells are single-nucleated
Cardiac muscle cells, also known as cardiomyocytes, are single-nucleated. They are one of three types of muscle in the body, along with skeletal and smooth muscle. Cardiac muscle cells are found only in the heart and are specialised to pump blood powerfully and efficiently throughout a person's lifetime. They are shorter than skeletal muscle fibres and usually contain only one nucleus, which is located in the central region of the cell.
The individual cardiac muscle cell 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. The thick filaments are composed of polymerised myosin type II protein, while the thin filaments consist of polymers of the protein alpha actin. The myofilaments slide past each other as the muscle contracts and relaxes, producing the formation of "cross-bridges" which cause the contraction of the heart and the generation of force.
The contractility of cardiac muscle cells can be altered by the autonomic nervous system and hormones. This tissue type has high metabolic, energy, and vascular demands. The myofibrils and many mitochondria in cardiac muscle cells provide them with great strength and endurance to pump blood throughout a lifetime. The rapid, involuntary contraction and relaxation of the cardiac muscle are vital for pumping blood throughout the cardiovascular system.
Cardiac muscle fibres are extensively branched and are connected to one another at their ends by intercalated discs, which allow the cardiac muscle cells to contract in a wave-like pattern so that the heart can work as a pump. Intercalated discs are part of the sarcolemma and contain two structures important in cardiac muscle contraction: gap junctions and desmosomes. A gap junction forms channels between adjacent cardiac muscle fibres that allow the depolarising current produced by cations to flow from one cardiac muscle cell to the next.
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Cardiac muscle cells are striated
Cardiac muscle tissue, or myocardium, makes up the thick middle layer of the heart. It is one of three types of muscle tissue, the other two being skeletal and smooth muscle. Cardiac muscle cells are striated, meaning they appear striped.
Cardiac muscle cells, or cardiomyocytes, are rectangular, branching cells that typically contain a single nucleus located centrally. This is in contrast to skeletal muscle cells, which often contain multiple nuclei. The branching nature of cardiac muscle cells is important in allowing them to form a functional syncytium, or a network of electrically connected cardiac muscle cells that contracts in a coordinated manner. This contraction is essential for the heart to work as a pump and generate the pressure required to pump blood through the circulatory system.
The striated appearance of cardiac muscle cells is due to the arrangement of their myofibrils, which are rod-like units within the cells. Myofibrils consist of repeating sections of sarcomeres, which are the fundamental contractile units of the muscle cells. Sarcomeres are composed of long proteins that organize into thick and thin filaments called myofilaments, which are primarily composed of myosin and actin, respectively. The thick and thin myofilaments overlap within the sarcomere of the cell, creating a striated pattern that is visible under microscopy.
Additionally, the myofilaments of cardiac muscle are arranged in a similar pattern to skeletal muscle, resulting in cross-striations. The thick myofilaments are composed of polymerised myosin type II protein and are attached to the M-line in the middle of the sarcomere. The thin myofilaments, on the other hand, consist of polymers of the protein alpha-actin and are attached to the Z-lines. The Z-lines, along with the A-band, appear darker in electron microscopy due to their electron-rich nature. In contrast, the I-band and H-band appear lighter as they represent regions consisting of only thin or thick filaments, respectively.
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Intercalated discs allow cardiac muscle cells to contract in a wave-like pattern
Cardiac muscle fibres are branched cells that are connected to one another at their ends by intercalated discs. Intercalated discs are complex structures that connect adjacent cardiac muscle cells. They are composed of three types of cell junctions: desmosomes, fascia adherens junctions, and gap junctions.
Desmosomes are cell structures that anchor the ends of cardiac muscle fibres together, preventing separation during contraction. Gap junctions connect the cytoplasms of neighbouring cells electrically, allowing cardiac action potentials to spread between cardiac cells by permitting the passage of ions between cells, producing depolarization of the heart muscle. This allows the quick transmission of action potentials and the coordinated contraction of the entire heart.
The coordinated contraction of the heart allows it to work as a unit, called a functional syncytium. This wave of contraction begins with specialized cardiac muscle cells called pacemaker cells that directly control heart rate. Pacemaker cells are self-excitable and able to depolarize to threshold and fire action potentials on their own, a feature called autorhythmicity. They do this at set intervals that determine heart rate.
By connecting cardiac muscle cells with intercalated discs, the depolarization can be transferred to other cardiac muscle fibres in a manner that allows the heart to contract in a coordinated, wave-like pattern. This allows the heart to work as a pump.
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Cardiac muscle cells are joined by intercalated discs
The three types of cell junction recognised as making up an intercalated disc are desmosomes, fascia adherens junctions, and gap junctions. Desmosomes are cell structures that anchor the ends of cardiac muscle fibres together so that the cells do not pull apart during the stress of individual fibres contracting. Fascia adherens are anchoring sites for actin and connect to the closest sarcomere. Gap junctions connect the cytoplasms of neighbouring cells electrically, allowing cardiac action potentials to spread between cardiac cells by permitting the passage of ions between cells, producing depolarization of the heart muscle. This joining is called electric coupling, and in cardiac muscle, it allows the quick transmission of action potentials and the coordinated contraction of the entire heart.
The intercalated disc provides the electrochemical and mechanical connection between neighbouring cardiac muscle cells. It is a highly specialised structure that connects cardiomyocytes via mechanical and electrical junctions. It is not the sum of isolated molecular silos but an interacting molecular complex, an "organelle" where components work in concert to bring about electrical and mechanical synchrony. This synchrony is important for the normal conduction of cardiac action potentials and the impact on the pathophysiology of arrhythmias.
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Cardiac muscle cells are controlled by pacemaker cells
Cardiac muscle, or myocardium, is one of three major categories of muscles in the human body, along with smooth and skeletal muscle. The heart is made up of three layers: the pericardium, myocardium, and endocardium. The cardiac muscle is responsible for the contractility of the heart and, therefore, the pumping action.
Cardiac muscle cells, also known as cardiomyocytes, are striated, branched, and contain many mitochondria. They are under involuntary control, meaning that their contractions cannot be consciously controlled. However, the rate and force of contraction can be altered by the autonomic nervous system and hormones. The contractility of the heart is essential for pumping oxygenated blood around the body, which the heart must do in a coordinated fashion.
The contractions of the heart are controlled by specialized cardiac muscle cells called pacemaker cells, which directly control heart rate. Pacemaker cells are highly specialized myocardial cells with an intrinsic ability to depolarize rhythmically and initiate an action potential. They are self-excitable and able to fire action potentials on their own, a feature called autorhythmicity. This autorhythmicity is due to the presence of funny current channels that allow sodium ions to leak continuously into the cell, slowly raising the membrane potential until a certain threshold is reached, causing depolarization.
Pacemaker cells are located primarily in the sinoatrial (SA) and atrioventricular (AV) nodes, with some cells also in the bundle of His and Purkinje fibers. The SA node is the pacemaker of the heart and reaches the threshold faster than any other component of the conduction system. The wave of contraction that allows the heart to work as a unit, called a functional syncytium, begins with these pacemaker cells. They are connected to surrounding muscle fibers and the specialized fibers of the heart's conduction system by gap junctions, which allow the depolarization to be transferred to other cardiac muscle fibers in a coordinated manner. This results in the contraction of the heart as a functional unit.
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Frequently asked questions
Yes, cardiac muscle fibres are branched. They are shorter than skeletal muscle fibres and usually contain only one nucleus, which is located in the central region of the cell.
Intercalated discs are part of the sarcolemma and contain two structures important in cardiac muscle contraction: gap junctions and desmosomes. A gap junction forms channels between adjacent cardiac muscle fibres that allow the depolarizing current produced by cations to flow from one cardiac muscle cell to the next. A desmosome is a cell structure that anchors the ends of cardiac muscle fibres together so the cells do not pull apart during the stress of individual fibres contracting.
The primary function of cardiomyocytes is to contract, which generates the pressure needed to pump blood through the circulatory system.
Skeletal muscle is the most common and widely distributed muscle tissue in the body, making up around 40% of the body's total mass. It is the only muscle tissue under the direct conscious control of the cerebral cortex of the brain. Cardiac muscle, on the other hand, is considered involuntary tissue as it is controlled unconsciously by regions of the brain stem and hypothalamus.
