Dominic Stone
There are three types of muscle tissue in vertebrates: skeletal, cardiac, and smooth muscle. Skeletal muscles are made up of muscle fibers, which are composed of myofibrils. These myofibrils are then made up of actin and myosin filaments, which are repeated in units called sarcomeres. The sarcomeres give skeletal muscle its striped appearance. There are two types of skeletal muscle fibers: Type I (red muscle) and Type II (white muscle). The color of the muscle fiber corresponds to its function, with red muscles being slow-twitch and rich in mitochondria and myoglobin, and white muscles being fast-twitch with a lower concentration of myoglobin.
| Characteristics | Values |
|---|---|
| Type | Three: skeletal, cardiac, and smooth muscle |
| Location | Skeletal: body and limbs; Cardiac: heart; Smooth: stomach, intestines, blood vessels, bronchi, uterus, urethra, bladder, skin, etc. |
| Function | Movement, maintaining body posture, controlling body temperature, stabilising joints, generating heat, enabling breathing, circulation, and digestion |
| Composition | Actin and myosin filaments (myofilaments), repeated in units called sarcomeres |
| Colour | Red or white, depending on the concentration of myoglobin and mitochondria |
| Contraction | Skeletal muscles: voluntary; Smooth and cardiac muscles: involuntary |
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What You'll Learn
Skeletal muscles are red due to high myoglobin levels
Muscle is a soft tissue and one of the four basic types of animal tissue. There are three types of muscle tissue in vertebrates: skeletal muscle, cardiac muscle, and smooth muscle. Skeletal muscle is commonly referred to as muscle and is part of the voluntary muscular system. It is attached to the bones of the skeleton by tendons. The skeletal muscle cells are much longer than in the other types of muscle tissue and are also known as muscle fibers.
Skeletal muscles are substantially composed of multinucleated contractile muscle fibers (myocytes). The tissue of a skeletal muscle is striated, having a striped appearance due to the arrangement of the sarcomeres. A skeletal muscle contains multiple fascicles or bundles of muscle fibers. Each individual fiber and each muscle is surrounded by a type of connective tissue layer of fascia.
Skeletal muscle comprises about 35% of the body of humans by weight. The functions of skeletal muscle include producing movement, maintaining body posture, controlling body temperature, and stabilizing joints. Skeletal muscle is also an endocrine organ.
Skeletal muscle is broadly classified into two fiber types: type I (slow-twitch) and type II (fast-twitch). Type I, slow-twitch, slow oxidative, or red muscle is dense with capillaries and is rich in mitochondria and myoglobin, giving the muscle tissue its characteristic red color. Type I fibers appear red due to the high levels of myoglobin. Myoglobin is a protein in your skeletal muscles that helps them get the oxygen they need to keep moving. It transports oxygen from your bloodstream to your muscles. Myoglobin is like a tiny delivery vehicle. It is made of amino acids, iron, and other molecules that work together to hold onto oxygen.
Myoglobin is a dark red cytoplasmic hemoprotein found only in cardiac myocytes and oxidative skeletal muscle fibers. It is present predominantly in the sarcoplasm of skeletal and cardiac muscles. It serves as a carrier and store for oxygen in muscle cells of the body. Myoglobin has more affinity for oxygen compared to hemoglobin. As a result, it can acquire oxygen from hemoglobin, hence transferring it from the blood to the muscle tissues.
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Type II muscle fibres are white
The colour of muscles depends on their myoglobin and mitochondria content. Myoglobin is a protein that supplies oxygen to muscles, giving them a red colour. Mitochondria, on the other hand, are the powerhouses of the cell and play a crucial role in energy production, with a higher number of mitochondria resulting in a greater capacity for endurance.
There are three types of muscle tissue in vertebrates: skeletal, cardiac, and smooth muscle. Skeletal muscles, which are under voluntary control, are composed of muscle fibres that can be further classified into Type I (slow-twitch) and Type II (fast-twitch) fibres. Type I muscle fibres, also known as red muscle fibres, have high levels of myoglobin and mitochondria, making them well-suited for endurance activities.
Type II muscle fibres, on the other hand, have relatively low levels of myoglobin and mitochondria, resulting in their white colour. Type II fibres can be further subdivided into three major types: Type IIa, Type IIx (also known as Type IId or Type IIb), and Type IIb. Type IIx and Type IIb have the lowest levels of myoglobin and mitochondria, making them the fastest muscle types in humans. These fibres are responsible for quick, powerful movements but can only sustain short, anaerobic bursts of activity before fatigue sets in.
The difference in colour between Type I and Type II muscle fibres is not just visual but also reflects their distinct functional roles in the body. Type I fibres, with their higher oxidative capacity, are built for endurance and can withstand prolonged periods of activity without fatigue. In contrast, Type II fibres are designed for short, intense bursts of energy and are more susceptible to fatigue due to their rapid energy expenditure.
While the colour of muscle fibres is primarily determined by their myoglobin and mitochondria content, it's worth noting that muscles also contain other proteins and enzymes that contribute to their overall function and performance. Additionally, the specific nomenclature of Type II fibres can vary, with some sources using different labels for the subtypes, further complicating the understanding of muscle fibre types.
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Smooth muscles are involuntary
Muscles are predominantly powered by the oxidation of fats and carbohydrates, but anaerobic chemical reactions are also used. They are one of the four basic types of animal tissue. There are three types of muscle tissue in vertebrates: skeletal muscle, cardiac muscle, and smooth muscle. Smooth muscles are involuntary muscles, meaning they contract and are controlled involuntarily, without conscious intervention. They are activated by the interaction of the central nervous system and innervation from the peripheral plexus or endocrine (hormonal) systems.
Smooth muscle tissue is non-striated and is found within the walls of organs and structures such as the oesophagus, stomach, intestines, bronchi, uterus, urethra, bladder, blood vessels, liver, pancreas, and the arrector pili in the skin that control the erection of body hair. It is also found throughout the urinary system, where it helps rid the body of toxins and works in electrolyte balance. In the cardiovascular system, it plays a vital role in regulating blood pressure and tissue oxygenation.
Smooth muscle cells contract uniformly in a spiral corkscrew fashion. The actin and myosin form continuous chains within the smooth muscle cell, which are anchored at the dense bodies. The intermediate and thin filaments formed by the actin and myosin chains can then stretch to dense bodies located on adjacent smooth muscle cells, forming a mesh-like network.
Smooth muscle is distinct from skeletal muscle, which is striated and arranged in regular, parallel bundles of myofibrils. Skeletal muscle is attached to the bones of the skeleton and is under voluntary control. It is responsible for producing movement, maintaining body posture, controlling body temperature, and stabilizing joints.
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Skeletal muscles are striated
Skeletal muscles are composed of multinucleated contractile muscle fibres, or myocytes, which are typically very large, measuring about 2-3 cm in length and 100 μm in diameter. The mononuclear cells in skeletal muscles, such as endothelial cells, macrophages, and neutrophils, are much smaller in comparison.
The tissue of skeletal muscles is arranged in regular, parallel bundles of myofibrils, which contain sarcomeres. This arrangement gives skeletal muscles their distinctive striated appearance. Skeletal muscles are also wrapped in epimysium, a connective tissue that provides structural integrity during contractions. The perimysium, another type of connective tissue, helps to organise the muscle fibres into fascicles, or bundles of muscle fibres.
The striated appearance of skeletal muscles is unique to vertebrates and is not present in invertebrates. Within the three types of muscle tissue in vertebrates (skeletal, cardiac, and smooth), only skeletal and cardiac muscles are striated. Smooth muscles, found in the walls of organs and structures such as the oesophagus, stomach, intestines, and blood vessels, have a non-striated appearance due to the absence of sarcomeres.
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Muscle fibres are composed of myofibrils
Muscle colour depends on the type of muscle tissue. There are three types of muscle tissue in vertebrates: skeletal muscle, cardiac muscle, and smooth muscle. Skeletal muscle is commonly referred to as muscle and is part of the voluntary muscular system. It is attached to bones by tendons and is striated, having a striped appearance due to the arrangement of the sarcomeres.
Skeletal muscle is composed of multinucleated contractile muscle fibres (myocytes) and is formed from the fusion of developmental myoblasts in a process known as myogenesis. These muscle fibres are, in turn, composed of myofibrils. Myofibrils are long contractile fibres, composed of actin and myosin filaments called myofilaments. These myofilaments are repeated in units called sarcomeres, which are the basic functional, contractile units of the muscle fibre necessary for muscle contraction.
The thick and thin myofilaments give the muscle its striped appearance. The thick filaments are composed of strands of the protein myosin, and the thin filaments are made of strands of the protein actin. There are also two other muscle regulatory proteins, tropomyosin and troponin. The actin and myosin filaments form a contractile compound, actomyosin, which is required for muscle contraction. The sarcomeres are the repeating units that form the myofibrils, and the shortening of the individual sarcomeres leads to the contraction of the individual muscle fibres, resulting in muscle contractions.
The colour of the muscle fibres depends on the content of myoglobin, mitochondria, and myosin ATPase. Type I muscle fibres are red due to high levels of myoglobin, and Type II fibres are white due to relatively low myoglobin levels and a reliance on glycolytic enzymes.
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Frequently asked questions
Muscles can be red or white.
Type I or slow-twitch muscle fibers are red due to high levels of myoglobin. They also have more mitochondria and greater local capillary density.
Type II or fast-twitch muscle fibers are white due to relatively low levels of myoglobin and a reliance on glycolytic enzymes.
The myocardium or cardiac muscle tissue found in the walls of the heart is an example of red muscle tissue.
Smooth muscles found in the stomach, intestines, blood vessels, and hollow organs are examples of white muscle tissue.
