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The human body is made up of four basic types of tissues, one of which is muscle tissue. There are three types of muscle tissue: skeletal, cardiac, and smooth. Skeletal muscles are attached to the bones and allow body movements. Smooth muscles, on the other hand, are found in the walls of hollow visceral organs like the intestines, liver, and pancreas. Cardiac muscles are located in the walls of the heart and are under involuntary control. They appear striped or striated due to the arrangement of contractile units called sarcomeres. This means that cardiac muscle does have striations.
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What You'll Learn
- Cardiac muscle cells are striated and located in the walls of the heart
- Striated muscle tissue has repeating functional units called sarcomeres
- Skeletal muscle is striated and under voluntary control, unlike cardiac muscle
- Cardiac muscle cells are joined by intercalated discs, which are visible under a light microscope
- Calcium ions are released from the sarcoplasmic reticulum during cardiac muscle contraction
Cardiac muscle cells are striated and located in the walls of the heart
Cardiac muscle tissue, also known as myocardium, is a structurally and functionally unique subtype of muscle tissue located in the heart. It is one of three types of vertebrate muscle tissues, the others being skeletal muscle and smooth muscle. Striated muscle tissue features repeating functional units called sarcomeres, which are visible along muscle fibres under a microscope, giving a striated appearance to the tissue.
Cardiac muscle tissue is a striated muscle fibre under involuntary control by the body's autonomic nervous system (ANS). Skeletal muscle tissue, on the other hand, is a striated muscle fibre under voluntary control. Smooth muscle tissue, unlike skeletal and cardiac muscle tissue, is not striated since it lacks sarcomeres. Smooth muscle fibres are located in the walls of hollow visceral organs such as the liver, pancreas, and intestines, but not in the heart.
The two types of cardiac muscle cells are autorhythmic and contractile. Autorhythmic cardiac cells do not contract; they set the pace of contraction for other cardiac muscle cells, which the ANS can modulate. Contractile cardiac cells, or cardiomyocytes, constitute the majority of the heart muscle and can contract. The growth of individual cardiomyocytes occurs during normal heart development and in response to extensive exercise, heart disease, or heart muscle injury. A healthy adult cardiomyocyte has a cylindrical shape that is approximately 100 μm long and 10–25 μm in diameter.
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Striated muscle tissue has repeating functional units called sarcomeres
Cardiac muscle tissue, also known as myocardium, is a structurally and functionally unique subtype of muscle tissue located in the heart. It appears striped (striated) under a microscope. The striations in cardiac muscle tissue are due to repeating functional units called sarcomeres.
Striated muscle tissue, which includes skeletal and cardiac muscle, features repeating functional units called sarcomeres. Under the microscope, sarcomeres are visible along muscle fibres, giving a striated appearance to the tissue. The sarcomeres give skeletal and cardiac muscle their striated appearance, which was first described by Van Leeuwenhoek. A sarcomere is defined as the segment between two neighbouring Z-lines (or Z-discs).
Sarcomeres are composed of actin and myosin filaments. The interaction between actin and myosin filaments in the A-band of the sarcomere is responsible for muscle contraction. The concentration of calcium within muscle cells controls the contraction of sarcomeres. Calcium ions bind with troponin C molecules, which are dispersed throughout the tropomyosin protein, and alter the structure of the tropomyosin, forcing it to reveal the cross-bridge binding site on the actin.
The structure of the sarcomere affects its function. The length of the actin and myosin filaments (taken together as sarcomere length) affects force and velocity – longer sarcomeres have more cross-bridges and generate more force. Arthropods show tremendous variation in sarcomere length, both between species and between muscles in a single individual.
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Skeletal muscle is striated and under voluntary control, unlike cardiac muscle
There are three types of muscle tissue in the human body: cardiac, smooth, and skeletal. Skeletal muscle is attached to the skeleton and is striated in appearance. It is under voluntary control, unlike cardiac muscle, which is under involuntary control.
Skeletal muscle is composed of skeletal muscle fibers, blood vessels, nerve fibers, and connective tissue. It is wrapped in epimysium, which allows the muscle to maintain its structural integrity despite contractions. The perimysium organizes the muscle fibers, which are encased in collagen and endomysium, into fascicles. Each muscle fiber contains sarcolemma, sarcoplasm, and sarcoplasmic reticulum. The functional unit of a skeletal muscle fiber is called a sarcomere, which is composed of actin and myosin myofilaments. These myofilaments are responsible for the striated appearance of skeletal muscle, as they form repeating units that can be seen under a microscope.
The striations in skeletal muscle tissue are due to the presence of sarcomeres, which are absent in smooth muscle tissue. Skeletal muscle cells have many nuclei, in contrast to the single nucleus found in both cardiac and smooth muscle cells. The contractions in skeletal muscle enable essential functions such as breathing, movement, and posture maintenance.
Unlike skeletal muscle, cardiac muscle cells are located in the walls of the heart and are under involuntary control. They exhibit strong, continuous, and rhythmic contractions that are automatically generated. These contractions are responsible for pumping blood throughout the body. Cardiac muscle cells are striated and branched, containing many mitochondria. Each cardiac muscle cell contains a single, centrally located nucleus surrounded by a cell membrane called the sarcolemma. The sarcolemma contains voltage-gated calcium channels, which are specialized ion channels absent in skeletal muscle cells.
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Cardiac muscle cells are joined by intercalated discs, which are visible under a light microscope
Cardiac muscle tissue, also known as myocardium, is a structurally and functionally unique subtype of muscle tissue located in the heart. It is made up of cardiac muscle cells, also known as cardiomyocytes, which are joined by intercalated discs. These discs are easily visible under a light microscope as thin lines dividing adjacent cardiac muscle cells and running perpendicular to the direction of muscle fibres.
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 prevent the separation of cells during contraction by binding intermediate filaments and anchoring the cell membrane to the intermediate filament network. Gap junctions connect the cytoplasms of neighbouring cells electrically, allowing cardiac action potentials to spread between cardiac cells and producing depolarization of the heart muscle. Fascia adherens junctions are anchoring sites for actin, and connect to the closest sarcomere.
The intercalated discs serve two important functions. Firstly, they provide attachment points that give the tissue its characteristic branched pattern. Secondly, they allow cardiac muscle tissue to function as a functional syncytium, enabling cardiomyocytes to contract together synchronously so that the heart can work like a pump.
Cardiac muscle cells are long, branched cells, shaped like cylinders joined end-to-end, with one or two nuclei located centrally. The myofilaments of cardiac muscle are arranged in a similar pattern to skeletal muscle, resulting in cross-striations. These striations are visible under a microscope as repeating functional units called sarcomeres, which give the tissue a striated appearance.
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Calcium ions are released from the sarcoplasmic reticulum during cardiac muscle contraction
Cardiac muscle tissue, also known as myocardium, is a structurally and functionally unique subtype of muscle tissue located in the heart. It is made up of cardiac muscle cells that appear striped (striated) and are under involuntary control. Striated muscle tissue has repeating functional units called sarcomeres, which are visible under a microscope as striations along muscle fibres.
The sarcoplasmic reticulum is a membrane-bound system of tubules and cisterns that surrounds the myocytes. It is an intracellular structure that stores and releases calcium ions, which are essential for muscle contraction and relaxation. The sarcoplasmic reticulum is not as well-developed in cardiac muscle as in skeletal muscle, but it still plays a crucial role in transmitting electrical impulses and controlling calcium ion levels.
During cardiac muscle contraction, calcium ions are released from the sarcoplasmic reticulum through a process called excitation-contraction coupling. This release is triggered by an electrical impulse (action potential) that causes calcium ions to enter the cell through a T-tubule membrane. These calcium ions then bind to and activate ryanodine receptors (RyR), specifically RyR2 in cardiac muscle, leading to a larger increase in intracellular calcium. The increased calcium concentration drives the movement of myosin and actin filaments, resulting in muscle contraction.
The release of calcium ions from the sarcoplasmic reticulum is carefully regulated to prevent excessive calcium levels within the cell, which can lead to hardening (calcification) of intracellular structures and cell death. This regulation is achieved through mechanisms such as the Na+/Ca2+ exchanger (NCX) and sarco/endoplasmic reticulum calcium ATPase (SERCA), which control calcium efflux and influx, respectively. SERCA, for example, mediates smooth, cardiac, and skeletal muscle relaxation by regulating intracellular free calcium levels.
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Frequently asked questions
Yes, cardiac muscle is an involuntary, striated muscle.
Striations are repeating functional units called sarcomeres that are visible along muscle fibers, giving a striated appearance to the tissue.
Skeletal muscle is a voluntary muscle that acts on the skeletal system by pulling on bones and allowing body movements. Cardiac muscle is involuntary and forms the walls of the heart, allowing blood to be pumped through the vasculature.
The main function of striated muscle tissue is to create force and contract. In cardiac muscle, these contractions pump blood throughout the body.
Cardiac muscle contracts through excitation-contraction coupling (ECC), which involves the conduction of calcium ions into the cardiomyocyte, leading to muscle contraction.
