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Muscle fibres are a type of muscle tissue that consists of a single muscle cell. They help to control the physical forces within the body and facilitate organised movement when grouped together. There are several types of muscle fibres, each with different characteristics. These include slow oxidative (SO), fast oxidative (FO), and fast glycolytic (FG) fibres. Skeletal muscle fibres are classified into two types: Type 1 and Type 2, with the latter further divided into subtypes. Cardiac muscle fibres, on the other hand, are striated and unique to the heart, with their own rhythm and interconnected structure. Smooth muscle fibres are involuntary and found in organs, with a uniform, oblong shape.
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What You'll Learn
Muscle fiber types: slow oxidative, fast oxidative, and fast glycolytic
Muscle fibers are broadly divided into two major categories: fast-glycolytic and slow-oxidative. However, there is a third intermediate category, fast oxidative, which exhibits characteristics of both slow oxidative and fast glycolytic fibers. The primary metabolic pathway used by a muscle fiber determines whether it is classified as oxidative or glycolytic.
Slow oxidative fibers (also called slow twitch or Type I) contract relatively slowly and use aerobic respiration (oxygen and glucose) to produce ATP. They contain a large number of mitochondria and can contract for long periods because they can produce large quantities of ATP, which makes them resistant to fatigue. Their ability to function for long periods without fatiguing makes them useful in maintaining posture, producing isometric contractions, and stabilizing bones and joints. They are not used for powerful, fast movements that require high amounts of energy.
Fast oxidative fibers (also called fast twitch or Type IIa) have relatively fast contractions and primarily use aerobic respiration to generate ATP. They are sometimes called intermediate fibers because they possess characteristics that are intermediate between slow oxidative and fast glycolytic fibers. They produce ATP relatively quickly and can thus produce relatively high amounts of tension. But because they are oxidative, they do not fatigue quickly.
Fast glycolytic fibers (also called fast twitch or Type IIx) have relatively fast contractions and primarily use anaerobic glycolysis as their ATP source. They have a large diameter and possess large volumes of glycogen, which is used in glycolysis to generate ATP quickly. Because of their reliance on anaerobic metabolism, they do not possess a substantial number of mitochondria, resulting in a white coloration for muscles that contain large numbers of these fibers. They fatigue quickly and can only be used for short periods.
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Skeletal muscle fibers: type 1 and type 2
Skeletal muscles are composed of four different types of muscle fibers: type 1, type 2A, type 2X, and type 2B. However, type 2B fibers are absent in humans. Each bundle of muscle fibers is called a fasciculus and is surrounded by a layer of connective tissue called the perimysium. Skeletal muscle cells (fibers) are soft and fragile, and the connective tissue covering provides support and protection, allowing them to withstand the forces of contraction.
Type 1 and type 2 muscle fibers can be further distinguished by their characteristics, such as movement rates, response to neural inputs, and metabolic styles. Type 1 fibers are slow-twitch fibers, while type 2 fibers are fast-twitch fibers. Type 1 fibers are rich in mitochondria, have high oxidative capacity, and are resistant to fatigue. On the other hand, type 2 fibers exhibit high glycolytic metabolism and fatigue and are the fastest to twitch.
The subtypes of type 2 fibers, namely type 2A, type 2X, and type 2B, exhibit a range of characteristics. Type 2A fibers are sometimes called intermediate fibers as they possess a mix of characteristics from both fast and slow fibers. They produce ATP relatively quickly through aerobic metabolism and have high amounts of mitochondria, resulting in higher tension production and fatigue resistance compared to type 1 fibers. Type 2B fibers, found in rodents but not in humans, primarily use anaerobic glycolysis as their energy source and exhibit the fastest twitch speed. Type 2X fibers fall between type 2A and type 2B in terms of contractile characteristics and are superior in terms of glycolysis.
The proportions of type 1 and type 2 fibers vary across different muscle groups and individuals. For example, the human soleus leg muscle is predominantly composed of type 1 fibers, while the triceps arm muscle has a higher proportion of type 2 fibers. These proportions are not static and can change due to factors such as endurance exercise training, disease states, and age-related reinnervation processes.
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Cardiac muscle fibers: striated, involuntary, and branched
Muscle fibres are the individual muscle cells that make up a muscle. There are three types of muscle tissue: cardiac, smooth, and skeletal. Cardiac muscle fibres, also known as myocardium, are located in the walls of the heart and are responsible for the heart's contractility. These muscle fibres are striated, involuntary, and branched.
Cardiac muscle fibres are striated, which means they appear striped or ridged under a microscope. This is due to the arrangement of thick and thin filaments within the muscle fibres, which allow for contraction. The thick and thin filaments interact to form cross-bridges, resulting in muscle contraction.
These muscle fibres are involuntary, meaning they are not under voluntary control. The generation of a cardiac action potential is involuntary and occurs through a process called excitation-contraction coupling (ECC). Action potentials travel along the sarcolemma, which is the plasma membrane of cardiomyocytes, and into the t-tubules, initiating muscle contraction.
Cardiac muscle fibres are also described as branched. These branched fibres are vital in maintaining the structural integrity of the heart. Gap junctions between adjacent cardiomyocytes allow for coordinated electrical coupling, ensuring the synchronized contraction of the heart muscle.
In summary, cardiac muscle fibres are striated, involuntary, and branched. Their unique structure and properties enable the heart to contract effectively and pump blood to meet the metabolic demands of the entire body.
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Smooth muscle fibers: involuntary, non-striated, and shorter
Smooth muscle fibres are involuntary, non-striated, and shorter than skeletal muscle fibres. Smooth muscle is found in the walls of hollow visceral organs, such as the liver, pancreas, intestines, and other areas like the iris and blood vessels. It is also present in the urinary bladder, uterus, and vaginal wall. Smooth muscle is responsible for involuntary body activities, such as the churning of the stomach or the constriction of arteries. Unlike skeletal muscles, smooth muscles do not have visible striations or T-tubules. Smooth muscle contractions are relatively slow, but they can contract over a much greater range of lengths than striated muscles.
Smooth muscle fibres are shorter and smaller than skeletal muscle fibres, typically measuring 200 µm or less in length and 5 to 6 µm in diameter. They are spindle-shaped and have a unique appearance due to their irregular physical arrangement. Smooth muscle uses actin and myosin filaments for contraction, but the absence of sarcomeres means they lack the troponin complex required for skeletal muscle contraction. Smooth muscle contractions are controlled by the autonomic nervous system, hormones, and intrinsic factors in the organ.
The two types of smooth muscle cells are single-unit and multi-unit. Single-unit smooth muscle cells are found in the gut and blood vessels, while multi-unit smooth muscle cells are found in the muscles of the eye and at the base of hair follicles. Single-unit smooth muscle cells are linked by gap junctions, allowing them to contract as a functional unit, while multi-unit smooth muscle cells are controlled by the nervous system, allowing for fine control and gradual response. Smooth muscle exhibits spontaneous electrical activity, primarily generated by K channels, resulting in oscillatory behaviour of membrane potentials.
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Muscle fiber structure: thin, elongated cylinders with rounded ends
Muscle fibres, or myofibers, are long and cylindrical cells, commonly reaching lengths of up to 30 cm and diameters of up to 100 μm. They are multinucleated and have a striated appearance due to the arrangement of thick and thin myofilaments within each sarcomere. The sarcomere is the functional unit of contraction, extending from one Z-disc to the next. The Z-discs, also known as Z-lines or Z-bands, are composed of thin filaments that are firmly attached to the elastic fibres in the 'I' band centre.
The thick filaments, composed mainly of myosin, form the dark striated A-band and are held together in the middle by the M-line, a thin fibrous membrane. The lighter I-band regions contain thin actin filaments that overlap with the thick filaments and are anchored at the Z-discs. The cytoplasm of muscle fibres, called the sarcoplasm, contains many myofibrils—bundles of filaments arranged longitudinally parallel to the long axis of the muscle fibre. These myofibrils are tiny cylinders, approximately 1 μm in diameter, that exhibit cross-striations due to the regular overlap of thick and thin filaments.
The sarcolemma, or plasma membrane of muscle fibres, surrounds the sarcoplasm and can be penetrated by holes that are the ends of caveolae intracellulares. The sarcoplasm also contains the sarcoplasmic reticulum, a specialized form of endoplasmic reticulum that stores, releases, and retrieves calcium ions. Additionally, muscle fibres are surrounded by connective tissue layers called the endomysium, perimysium, and epimysium, which provide support, protection, and pathways for blood vessels and nerves.
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Frequently asked questions
Muscle fibers are single muscle cells that help to control the physical forces within the body. They are responsible for causing movement in the body.
There are three types of muscle fibers: slow oxidative (SO), fast oxidative (FO), and fast glycolytic (FG). Each type has different characteristics and functions within the body.
Slow oxidative (SO) fibers have a high density of mitochondria and are dark in color. Fast oxidative (FO) fibers have a lower density of mitochondria and are light in color. Fast glycolytic (FG) fibers have the lowest density of mitochondria and appear white. FG fibers produce rapid and powerful movements but fatigue quickly.
Muscle fibers receive impulses from nerve cells, which lead to a complex chain reaction within the fibers, resulting in a release of energy and subsequent muscle contraction.
