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Muscle fibres, or myofibres, are cylindrical cells that are part of skeletal muscle, one of the three types of vertebrate muscle tissue. Skeletal muscle is the voluntary muscular system that is attached to bones or skin and controls locomotion and any movement that can be consciously controlled. Each muscle fibre is a single large cell with several hundred cell nuclei located at regular intervals near its surface. The muscle fibres are bundled together and wrapped in connective tissue.
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What You'll Learn
Skeletal muscle is cylindrical
Skeletal muscle is one of the three types of vertebrate muscle tissue, the others being cardiac muscle and smooth muscle. They are part of the voluntary muscular system and are attached by tendons to the bones of the skeleton. Skeletal muscle comprises about 35% of the body of humans by weight and is considered an organ of the muscular system. Each skeletal muscle is an organ that consists of various integrated tissues, including skeletal muscle fibres, blood vessels, nerve fibres, and
Skeletal muscle cells (fibres) are long and cylindrical, and they are commonly referred to as muscle fibres or myofibers. Each skeletal muscle fibre is a single cylindrical muscle cell. An individual skeletal muscle may be made up of hundreds, or even thousands, of muscle fibres bundled together and wrapped in a connective tissue covering. Each muscle is surrounded by a connective tissue sheath called the epimysium.
Bundles of muscle fibres, called fascicles, are covered by the perimysium, and each individual muscle fibre is encased in a thin connective tissue layer of collagen and reticular fibres called the
The smallest contractile unit of skeletal muscle is the muscle fibre or myofiber, which is a long cylindrical cell that contains many nuclei, mitochondria, and sarcomeres. Each muscle fibre is surrounded by a thin layer of connective tissue called the endomysium. Approximately 20–80 of these muscle fibres are grouped together in a parallel arrangement called a muscle fascicle or fibre bundle, which is encapsulated by a perimysium.
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Muscle fiber constituents
Muscle fibres are cylindrical in shape and are composed of four fundamental constituents. These constituents are:
- The sarcolemma, which is the equivalent of the cell membrane.
- The fibrils, which are the structural elements responsible for contraction. Fibrils are composed of filaments, which come in two main varieties: thin and thick filaments. The major constituent of thin filaments is actin, while the main component of thick filaments is myosin.
- The sarcosomes, which are the mitochondria and contribute to the supply of energy for muscular contraction.
- The sarcoplasm, which is the ground cytoplasmic substance in which the other structures of the muscle fibre are embedded.
- The sarcoplasmic reticulum.
In addition, there is a specialised region of the fibre called the motor end plate, which is where the motor nerve fibre makes a junction with the sarcolemmal membrane, initiating the contraction of the fibre.
Muscle fibres can be further classified into three types: slow oxidative (SO), fast oxidative (FO), and fast glycolytic (FG). These different types of muscle fibres allow for a wide variety of capabilities in human muscles. For example, FG fibres are used for rapid, forceful contractions to produce quick, powerful movements, but they fatigue quickly. On the other hand, SO and FO fibres have higher densities of energy-generating mitochondria, making them more fatigue-resistant.
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Muscle fascicles
The muscle fascicles are then grouped together to form an individual muscle, which is enclosed within an outer layer of connective tissue called the epimysium. The epimysium is the fascia surrounding the whole muscle and is contiguous with the perimysial septa separating the fascicles. The epimysium is also penetrated by nerve cells and blood vessels, with an artery and at least one vein accompanying each nerve that penetrates it. The epimysium, perimysium, and endomysium extend beyond the fleshy part of the muscle to form a thick, ropelike tendon or a broad, flat sheet-like aponeurosis. These tendons and aponeuroses form indirect attachments from muscles to bones or to the connective tissue of other muscles.
The fascicles of skeletal muscles are macroscopically visible and can be arranged into four basic structural patterns: circular, parallel, convergent, and pennate. The orientation of the muscle fascicles contributes to the functional capabilities of skeletal muscles, including the types of movements, the force that can be produced, and the range of motion. Ultrasound examinations can be used to evaluate the structure and function of muscle fascicles, with the ultrasound appearance of the muscle being affected by factors such as the angulation of the ultrasound beam, the state of contraction of the muscle, and the ratio of perimysium to fascicles.
In most skeletal muscles, individual fascicles are composed of two or more types of muscle fibres, including slow-twitch (type I) and fast-twitch (type IIa and IIb) fibres. These different types of muscle fibres have distinct characteristics, such as differences in oxidative enzymes, myoglobin and glycogen content, the rate of force development, capillary densities, and fatigability.
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Muscle contractions
Skeletal muscle contractions are neurogenic as they require synaptic input from motor neurons. Skeletal muscles are attached to bones and give the body structure and strength. Skeletal muscle cells (fibres) are soft and fragile, and they are protected by connective tissue coverings. Before a skeletal muscle fibre can contract, it has to receive an impulse from a nerve cell.
Cardiac muscle tissue is a striated muscle fibre under involuntary control by the body's autonomic nervous system (ANS). Cardiac muscle comprises the walls of the heart, allowing blood to be pumped through the vasculature.
Smooth muscle fibres do not contain sarcomeres but use actin and myosin contraction to constrict blood vessels and move the contents of hollow organs in the body. These fibres are under involuntary control by reflexes and the body's ANS.
The muscle fibre is composed of four fundamental constituents: the sarcolemma, the fibrils, the sarcosomes, and the sarcoplasm. The sarcolemma is the equivalent of the cell membrane, and the fibrils represent the structural elements responsible for contraction. The sarcosomes are the mitochondria and contribute to the supply of energy for muscular contraction. The sarcoplasm is the ground cytoplasmic substance in which the other structures of the muscle fibre are embedded.
The smallest contractible unit of skeletal muscle is the muscle fibre or myofibre, which is a long cylindrical cell that contains many nuclei, mitochondria, and sarcomeres. Each muscle fibre is surrounded by a thin layer of connective tissue called the endomysium.
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Muscle fiber types
Muscle fibres are cylindrical in shape and are composed of four fundamental constituents: the sarcolemma, the fibrils, the sarcosomes, and the sarcoplasm. The sarcolemma is the cell membrane of the muscle fibre. The fibrils are the structural elements responsible for contraction. The sarcosomes are the mitochondria that contribute to the supply of energy for muscular contraction. The sarcoplasm is the ground cytoplasmic substance in which the other structures of the muscle fibre are embedded.
Muscle fibres can be classified into different types based on various criteria, including histochemical, biochemical, morphological, or physiological characteristics. One common classification is based on the contraction speed of the fibres, with slow-twitch and fast-twitch fibres being the two main categories. Slow-twitch fibres, also known as type I fibres, are fatigue-resistant, rely on oxidative metabolism, and contain myoglobin, giving them a red colour. On the other hand, fast-twitch fibres, or type II fibres, can be further divided into type IIa and type IIb. Type IIa fibres share some features with slow-twitch fibres, such as fatigue resistance and the presence of myoglobin, but they also contain abundant glycogen and more mitochondria. Type IIb fibres, on the other hand, have lower myoglobin content, rely on the energy stored in glycogen and phosphocreatine, and are more easily fatigued.
Another classification system categorizes muscle fibres into three types: slow oxidative (SO), fast oxidative (FO), and fast glycolytic (FG). Most skeletal muscles contain all three types, but in varying proportions. SO fibres are similar to slow-twitch fibres, while FO and FG fibres exhibit characteristics of fast-twitch fibres. FG fibres are used for rapid and powerful movements but fatigue quickly.
The different types of muscle fibres allow for a wide variety of capabilities in human muscles. They can also adapt to changing demands by altering their size or composition, which forms the basis for many physical therapy interventions aimed at improving patients' force development or endurance. Additionally, muscle fibre type composition may play a role in certain impairments and disabilities observed in patients with prolonged inactivity, limb immobilization, or muscle denervation.
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Frequently asked questions
Yes, muscle fibers are cylindrical.
Myofibrils are cylindrical.
Muscle fibers have a diameter of up to 100 μm.
Muscle fibers can be up to 30 cm long.
Muscle fibers are composed of myofibrils, which are made up of actin and myosin filaments called myofilaments.
