Dominic Stone
Muscle fibres, or cells, are the basic unit of muscle. They are long, cylindrical, tubular cells with tapering conical ends and unbranching structures. Each skeletal muscle fibre is a skeletal muscle cell and is supplied by the axon branch of a somatic motor neuron, which signals the fibre to contract. Skeletal muscle fibres are made up of myofibrils, which are composed of actin (thin filaments), myosin (thick filaments), and support proteins. The actin filaments are bound by α-actinin, which acts as an anchor and maintains the constant length of the actin filaments in a sarcomere.
| Characteristics | Values |
|---|---|
| Role | Beta-actinin is responsible for maintaining the constant length of the actin filaments in a sarcomere |
| Composition | Actin filaments are composed of the actin protein |
| Structure | Actin filaments are asymmetric with distinguishable ends that are detectable by the way in which they interact with myosin |
| Location | Actin filaments are present in most cells but are especially abundant in muscle cells |
| Polymorphism | A common polymorphism (R577X) in the ACTN3 gene results in the complete absence of the Z-disc protein alpha-actinin-3 from fast-twitch muscle fibers in ~16% of the world's population |
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What You'll Learn
Alpha-actinin anchors actin filaments at the Z line
Muscle fibres, or myofibers, are the basic unit of muscle. They are long, cylindrical, tubular cells with tapering conical ends. Each skeletal muscle is an organ that consists of various integrated tissues, including muscle fibres, blood vessels, nerve fibres, and connective tissue.
Each muscle fibre is composed of several hundred to several thousand myofibrils. Myofibrils are composed of actin (thin filaments), myosin (thick filaments), and support proteins. The arrangement of actin and myosin gives skeletal muscle its microscopic striated appearance and creates functional units called sarcomeres.
The Z line, or Z disk, is the terminal boundary of the sarcomere, where alpha-actinin acts as an anchor for the actin filaments. The actin filaments extend their minus ends toward the centre of a sarcomere where they overlap with the thick myosin filaments. The Z line, therefore, is a highly organized arrangement of contractile, regulatory, and structural proteins.
Alpha-actinin, in association with CapZ, acts as an anchoring complex for thin filaments in the Z-line. CapZ is a widely distributed and highly conserved heterodimeric protein that nucleates actin polymerization and binds to the barbed ends of actin filaments, preventing the addition or loss of actin monomers. The affinity of CapZ for alpha-actinin was found to be in the microM range and independent of actin.
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Beta-actinin maintains the length of actin filaments
Muscle fibers are the basic unit of muscle. They are long, cylindrical, tubular cells with tapering conical ends. Each muscle fiber is composed of several hundred to several thousand myofibrils. Myofibrils are composed of actin (thin filaments), myosin (thick filaments), and support proteins. The arrangement of actin and myosin gives skeletal muscle its microscopic striated appearance and creates functional units called sarcomeres. The sarcomere is the smallest functional unit of a skeletal muscle fiber and is a highly organized arrangement of contractile, regulatory, and structural proteins.
The actin filaments in a sarcomere are bound with four filaments originating from the Z line material (Z line filaments). Each of the Z filaments runs diagonally through the Z line and joins the actin filament of the adjacent I band of the opposite sarcomere. This type of configuration makes up the Z line on the longitudinal section in a zigzag pattern, and actin filaments are bound to the Z line filaments alternatively. The density of the Z line matrix differs in different muscle fiber types: it is high in slow-contracting muscle fibers and has moderate density in fast-contracting muscle fibers.
Actin is the monomeric subunit of two types of filaments in cells: microfilaments, one of the three major components of the cytoskeleton, and thin filaments, part of the contractile apparatus in muscle cells. Actin can be present as either a free monomer called G-actin (globular) or as part of a linear polymer microfilament called F-actin (filamentous). Actin participates in many important cellular processes, including muscle contraction, cell motility, cell division, and cytokinesis.
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Skeletal muscle is made up of different fiber types
The different types of skeletal muscle fibers include slow oxidative (Type I or SO), fast oxidative (FO), and fast glycolytic (FG or Type II) fibers. Slow oxidative fibers are slow-twitching, small, and have low glycogen content, fatigue resistance, and slow contractile speed. They are best suited for endurance activities like maintaining posture and long-distance running. On the other hand, fast glycolytic fibers are used for rapid and forceful contractions, producing quick and powerful movements, but they fatigue quickly.
The visual appearance of skeletal muscle fibers can be categorized as red or white. Red muscle fibers, found in endurance muscles like those in the legs and shoulders, have a high concentration of myoglobin, giving them their red color. They also exhibit sustained contractile activity and high oxidative enzyme activity, resulting in a higher concentration of mitochondria. White muscle fibers, in contrast, have low myoglobin concentrations and are found in muscles such as porcine longissimus (loin) or poultry pectoralis major (breast) muscles.
The contractile properties of skeletal muscle fibers are influenced by the arrangement of actin and myosin filaments, which form functional units called sarcomeres. The sarcomere is the smallest functional unit of a skeletal muscle fiber and consists of contractile, regulatory, and structural proteins. The Z line, or Z disk, acts as an anchor for the actin filaments, while the M line is where the myosin filaments are anchored. The density of the Z line matrix varies across different muscle fiber types, with higher density in slow-contracting fibers and moderate density in fast-contracting fibers.
The β-actinin is responsible for maintaining the constant length of the actin filaments in the sarcomere. The thick and thin filaments of the sarcomere exhibit a highly regular pattern of overlap, forming a double hexagon in vertebrates. The speed of contraction in skeletal muscle fibers depends on how quickly myosin's ATPase hydrolyzes ATP, with fast fibers contracting approximately twice as rapidly as slow fibers.
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Skeletal muscle is responsible for voluntary movements
Skeletal muscles are responsible for voluntary movements. They are attached to the bones and allow us to perform a wide range of movements and functions. Skeletal muscles comprise 30 to 40% of our total body mass. Tendons, which are tough connective tissues, attach skeletal muscles to bones. Examples of skeletal muscles include shoulder muscles, hamstring muscles, and abdominal muscles.
Skeletal muscles consist of flexible muscle fibres that range from less than half an inch to just over three inches in diameter. Each muscle can contain thousands of fibres. These fibres contract, allowing the muscles to move bones and enabling us to perform various movements. Skeletal muscle fibres are red and white, and they have a striped appearance due to their structure, so they are often referred to as striated muscles.
The muscle fibres are organised into bundles called fascicles, which are surrounded by a layer of connective tissue called the perimysium. Each individual muscle fibre is encased in a thin layer of connective tissue called the endomysium. The outermost layer of tissue surrounding the entire muscle is called the epimysium.
Skeletal muscle fibres are composed of several hundred to several thousand myofibrils, which are made up of actin (thin filaments), myosin (thick filaments), and support proteins. The arrangement of actin and myosin gives skeletal muscles their striated appearance and creates functional units called sarcomeres. Beta-actinin is responsible for maintaining the constant length of the actin filaments in a sarcomere. The actin and myosin filaments overlap in a highly regular pattern, forming a double hexagon in vertebrates.
Skeletal muscles are under voluntary control, receiving neural inputs that allow conscious control of the muscles. The somatic nervous system sends signals to the skeletal muscles to make them function.
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Muscle cells are specialised for contraction
Muscle cells, also known as myocytes, are the smallest subunit of all muscular tissues and organs throughout the body. They are specialised for contraction, which allows for motion and facilitates bodily processes such as respiration and digestion.
There are three types of muscle cells in the human body: skeletal, smooth, and cardiac muscle. Skeletal muscles are attached to bones and move them relative to each other. They are under voluntary control, receiving neural inputs that allow for conscious control of muscles. Cardiac muscle, on the other hand, comprises the heart, which pumps blood through the vasculature. Cardiac contractions maintain blood pressure and cannot be consciously controlled. Smooth muscle does not contain sarcomeres but uses the contraction of filaments of actin and myosin to constrict blood vessels and move the contents of hollow organs in the body.
Skeletal and cardiac muscles are referred to as striated muscles because of the visible organisation of repeating units of contractile filaments, known as sarcomeres, into cylindrical bundles called myofibrils. Each skeletal muscle fibre is a skeletal muscle cell, and these cells are incredibly large, with diameters of up to 100 µm and lengths of up to 30 cm. The plasma membrane of a skeletal muscle fibre is called the sarcolemma, which is the site of action potential conduction that triggers muscle contraction.
Sarcomeres are the smallest functional unit of a skeletal muscle fibre and are composed of actin (thin filaments), myosin (thick filaments), and support proteins. The arrangement of actin and myosin gives skeletal muscle its microscopic striated appearance. The actin filaments are bound to the Z-line filaments, which run diagonally through the Z-line and join the actin filament of the adjacent I-band of the opposite sarcomere. This configuration creates a zigzag pattern. The Z-line material exhibits a strong affinity for isolated actin molecules, and its density differs in various muscle fibre types. β-Actinin is responsible for maintaining the constant length of the actin filaments in a sarcomere.
The sliding filament model describes sarcomere shortening through recurrent myosin/actin interactions. During each interaction, the myosin heads bring adjacent actin free ends closer to the centre of the sarcomere. This results in the shortening of both the H (containing only thick filaments) and I (containing only thin filaments) bands. The shortening of sarcomeres leads to the contraction of skeletal muscle fibres.
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Frequently asked questions
α-actinin acts as an anchor for actin filaments at the Z-line, or Z-disk, of the sarcomere. The Z-line is the terminal boundary of the sarcomere, which is the smallest functional unit of a skeletal muscle fiber.
The sarcomere is a highly organized arrangement of contractile, regulatory, and structural proteins. It is made up of thick and thin filaments, which are composed of the proteins myosin and actin, respectively.
The sarcomere is responsible for muscle contraction. As the sarcomeres contract, the myofibrils and muscle cells shorten, resulting in muscle movement.
The absence of α-actinin-3 impairs the mechanical stability of fast-twitch muscle fibers, making them prone to breakage when stretched. This leads to a faster decline in muscle function with age.
α-actinin helps to bundle and stabilize actin filaments, promoting their assembly into larger bundles. It also maintains the constant length of the actin filaments in the sarcomere.
