Dominic Stone
Muscle fibres are long, cylindrical cells that give skeletal muscles their striped appearance. They are made of contractile proteins called actin and myosin, which interact to cause movement. Actin, a thin filament, is twisted around strands of another protein called tropomyosin. Thick filaments, on the other hand, are made of myosin. These contractile proteins are responsible for the movement of muscles, including the skeletal, cardiac, and smooth muscle tissues found in vertebrates. The different types of muscle fibres, such as slow-twitch and fast-twitch, vary in their contraction speeds and resistance to fatigue.
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What You'll Learn
Skeletal muscle is made up of three types of fibres
Muscle fibres are indeed protein. Proteins are the basic material of tissue structure and are the most important component of striated skeletal muscle. Skeletal muscle comprises approximately 40% of the human body weight and contains 50 to 75% of all body proteins.
Slow oxidative fibres use aerobic metabolism to produce low-power contractions over long periods and are slow to fatigue. They are the smallest fibre type and have a low glycogen content, a low rate of fatigue, a slow contractile speed, and low myosin ATPase activity. They are best suited for endurance types of contraction, such as maintaining posture and marathon running.
Fast oxidative fibres are fast-twitch fibres. They are larger in diameter due to their high density of actin and myosin proteins. They contain few mitochondria and are termed white fibres due to their low myoglobin content. They obtain ATP primarily from anaerobic glycolysis, have a high myosin ATPase activity, and have a fast rate of fatigue. They are best suited for short-duration, intense movements such as sprinting and weight-lifting.
The three types of muscle fibres allow for the wide variety of capabilities that human muscles display. Muscle fibres can adapt to changing demands by changing size or fibre type composition. This plasticity serves as the basis for numerous physical therapy interventions designed to increase a patient's force development or endurance.
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Actin and myosin are the most abundant muscle proteins
Muscle fibres are composed of myofibrils, which are made up of actin and myosin filaments. Actin and myosin are the most abundant proteins in muscle tissue and are directly involved in the ability of muscles to contract and relax. They are found in all three types of muscle tissue: smooth, cardiac, and skeletal.
Actin is a protein that produces thin contractile filaments within muscle cells. These filaments are attached at their plus ends to the Z disc, which includes the crosslinking protein α-actinin. The Z disc is the terminal boundary of the sarcomere, which is the functional unit created by the arrangement of actin and myosin filaments. The I bands of the sarcomere contain only actin filaments, while the A bands contain both actin and myosin filaments. The H band, which is within the A band, contains only myosin filaments.
Myosin is a protein that produces thick contractile filaments within muscle cells. Myosin constitutes as much as 35% of the total protein in muscle tissue. The myosin filaments are anchored together at the M line, which is the central-most line of the sarcomere. The A band contains the entirety of the myosin filaments and includes regions of actin and myosin overlap.
Actin and myosin work together to produce muscle contractions and, therefore, movement. A motor neuron delivers an electrical signal to the muscle cell from the brain, triggering the release of acetylcholine. Acetylcholine causes calcium ions to be released from the sarcoplasmic reticulum, which is a structure surrounding the actin and myosin filaments. The calcium ions then bind to the protein troponin, which is attached to another protein, tropomyosin, found between the actin filaments. This changes the shape of troponin, moving tropomyosin from the myosin-binding sites on the actin filament and allowing the myosin heads to bind to the actin filament. This results in the shortening of the sarcomere, leading to muscle contraction.
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Muscle protein synthesis is triggered by exercise and protein ingestion
Muscle fibers are composed of myofibrils, which are made up of actin (thin filaments), myosin (thick filaments), and support proteins. These proteins are essential for muscle contraction and relaxation. Human skeletal muscle is composed of three major fiber types: type 1, 2A, and 2X fibers, also known as slow type 1, fast 2A, and fast 2X fibers.
Muscle protein synthesis (MPS) is the body's adaptive response to exercise and nutrition. It is triggered by exercise and protein ingestion, working in synergy when protein consumption occurs before or after resistance exercise. MPS is the process of building and repairing muscle tissue, leading to muscle hypertrophy or growth. The body can only utilize a limited amount of essential amino acids (EAAs) at a time, and any excess is broken down and excreted by the liver. Therefore, the anabolic effect of exercise, which lasts at least 24 hours, is crucial for maximizing MPS.
Resistance exercise, in particular, has been shown to increase myofibrillar protein synthesis, leading to muscle hypertrophy. This is evident in studies where individuals performed resistance exercises, such as weight-lifting or cycling, and displayed increased myofibrillar protein synthesis rates. Additionally, protein ingestion after resistance-type exercise stimulates MPS rates, with higher protein doses resulting in higher MPS responses. For example, ingesting 40 grams of whey protein after whole-body resistance exercise resulted in a 20% higher MPS rate compared to a 20-gram dose.
To maximize MPS and promote muscle growth, it is recommended to consume 1.4-2.0 grams of protein per kilogram of body weight per day. This can be achieved through whole foods or supplementation. For athletes aiming to maximize anabolism, ingesting 50 grams of protein per meal is suggested. However, it is important to note that the ideal protein intake may vary depending on individual factors such as biological sex and DNA.
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Muscle fibres are multinucleated cells
Muscle fibres are indeed protein in nature. Human skeletal muscle is composed of three major fibre types, referred to as type 1, 2A, and 2X fibres. These fibres are defined by the presence of MYH7 (myosin heavy chain 7), MYH2, and MYH1, respectively, as well as hybrid fibres containing multiple MYHs.
Skeletal muscle comprises approximately 40% of the human body weight and contains 50 to 75% of all body proteins. Each muscle fibre is 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 muscle its microscopic striated appearance and creates functional units called sarcomeres.
Skeletal muscle fibres are multinucleated cells, ranging from 10 to 100 micrometers in diameter and many centimetres long. This multinucleated condition results from multiple myoblasts fusing to produce each muscle fibre, with each myoblast contributing a nucleus to the newly formed muscle cell or myotube. The fusion depends on muscle-specific proteins known as fusogens, called myomaker and myomerger.
The presence of multiple nuclei in skeletal muscle cells is unique compared to other types of muscle cells, such as smooth and cardiac muscle cells, which typically have a single nucleus. This multinucleated structure allows for the large size and specialised function of skeletal muscle cells, which are responsible for voluntary movements and make up a significant portion of the human body's protein content.
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Muscle fibres are striated, giving them a banded appearance
Muscle fibres are composed of myofibrils, which are made up of actin and myosin proteins. The arrangement of these proteins gives skeletal muscle its striated appearance.
Skeletal muscle is a highly organised tissue composed of bundles of muscle fibres called myofibers, which contain several myofibrils. Each myofibril is made up of smaller structures called myofilaments, which are divided into thick and thin filaments. Thick filaments are composed of myosin proteins and occur only in the A band of a myofibril. Actin, on the other hand, forms the thin filaments, which attach to a protein called alpha-actinin in the Z disc and extend across the I band and partially into the A band.
The dense appearance of the region where thick and thin filaments overlap is due to the minimal space between the filaments. Thin filaments do not extend all the way into the A bands, leaving a central region of the A band that only contains thick filaments. This central region, known as the H zone, appears slightly lighter than the rest of the A band.
The striated appearance of skeletal muscle is caused by the regular arrangement of contractile proteins, specifically actin and myosin. The repeating band pattern indicates that all the sarcomeres, which are regions from one Z line to the next Z line, are of the same length. Additionally, all the A bands, located in the middle of the sarcomeres, are of uniform length. The sarcomeres are arranged longitudinally and include structures such as the M line, Z disc, H band, A band, and I band.
The Z line, or Z disc, serves as the terminal boundary of the sarcomere, with alpha-actinin acting as an anchor for the actin filaments. The M line, or the central-most line, is where the myosin filaments are anchored together through binding sites. The H band encompasses the M line and is the central region containing only myosin filaments. The A band, which is slightly lighter in the centre due to the absence of thin filaments, includes regions of actin and myosin overlap. Lastly, the I band is composed of thin filaments attached to the Z disc.
The discovery of the striated appearance of skeletal muscle can be traced back to the observations of the Dutch microscopist Antony van Leeuwenhoek in the 1700s. However, it was not until the 1970s that the molecular understanding of the striations emerged with the discovery of the gigantic protein titin.
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Frequently asked questions
Muscle fibers are composed of myofibrils, which are made up of actin (thin filaments) and 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.
Muscle fibers can be broken down into three groups: Type I or slow oxidative fibers, Type IIb or fast glycolytic fibers, and Type II or fast-twitch fibers. Type I fibers are slow-twitching, small in diameter, and have a low glycogen content and low rate of fatigue. Type IIb fibers are the opposite, being the largest in diameter with a high density of actin and myosin proteins, and a fast rate of fatigue. Type II or fast-twitch fibers are further divided into 2A and 2X fibers.
Proteins are the basic material of tissue structure and are the most important component of striated skeletal muscle. About 40% of the body weight of a healthy human adult is muscle, and muscle proteins make up about 20% of that muscle. Actin and myosin are the most abundant proteins in muscle and are directly involved in the ability of muscles to contract and relax.
