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The human body is made up of three types of muscle tissue: skeletal, cardiac, and smooth muscle. Skeletal muscles are made up of muscle fibres, which are composed of myofibrils. These myofibrils are responsible for muscle movement. There are two types of muscle fibres: type I (red muscle) and type II (white muscle). The colour of the muscle fibres depends on the presence of myoglobin, with red muscle having high levels of myoglobin and white muscle having relatively low levels.
| Characteristics | Values |
|---|---|
| Types of muscle tissue in vertebrates | Skeletal, cardiac, and smooth muscle |
| Skeletal muscle composition | Actin and myosin filaments, repeated in units called sarcomeres |
| Skeletal muscle appearance | Striated or striped |
| Skeletal muscle fibres | Type I (red) and Type II (white) |
| Type I muscle fibre characteristics | Slow-twitch, high levels of myoglobin, mitochondria, and capillaries |
| Type II muscle fibre characteristics | Fast-twitch, low levels of myoglobin, mitochondria, and capillaries |
| Muscle tissue function | Contraction, movement, and maintenance of body posture, temperature, and joint stability |
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What You'll Learn
Skeletal muscle is red due to myoglobin
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 to the bones of a skeleton by tendons. The cells of skeletal muscle are much longer than those of other types of muscle tissue. They are also known as muscle fibres.
Skeletal muscle is broadly classified into two fibre types: type I (slow-twitch) and type II (fast-twitch). Type I fibres are red due to high levels of myoglobin. They also have more mitochondria and greater local capillary density. These fibres are more resistant to fatigue because they use oxidative metabolism to generate ATP (adenosine triphosphate). Type II fibres are white due to relatively low myoglobin levels and a reliance on glycolytic enzymes.
Myoglobin is a single polypeptide chain of 154 amino acids. It contains a heme prosthetic group, which includes a porphyrin ring iron ion. The iron ion interacts with six ligands, one of which serves as the binding site for oxygen. This binding site can also bind other molecules, including CO and NO. The primary function of myoglobin is to supply oxygen to the muscle. It does this by releasing oxygen to the mitochondria that make up the respiratory chain, helping the myocytes to meet their high energy demands. Myoglobin also brings oxygen into the muscle from the cell membrane. It has a higher affinity for oxygen than haemoglobin, allowing it to efficiently extract oxygen from the blood.
Myoglobin is necessary for normal muscle development and function. Myoglobin knockout models have been created to understand its functions more clearly. Mice with mutated myoglobin exhibit several lethal cardiovascular defects. However, those that survive have adaptive responses to deal with the lack of myoglobin, such as a higher capillary density in the heart to enhance oxygen supply.
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Type II muscle fibres are white
The colour of muscles is determined by their myoglobin and mitochondria content. Myoglobin is a protein that supplies oxygen to muscles, and mitochondria are organelles that generate energy for cells. Depending on the levels of these components, muscles can be classified as red or white.
Type II muscle fibres are fast-twitch fibres that can be further categorised into Type IIa, Type IIx, and Type IIb. Type II muscle fibres are white due to their relatively low content of myoglobin and mitochondria. Type IIb fibres, in particular, have the lowest content of these components among the Type II fibres.
Type IIb fibres, also known as fast glycolytic fibres, rely on anaerobic metabolic processes to generate energy. They have a high capacity for glycolysis, which allows them to generate ATP quickly and produce high levels of tension. However, this comes at the cost of quick fatigue as they cannot sustain continuous energy supply to the skeletal muscles.
The white colour of Type IIb fibres is more prominent in small animals like rodents, where it is the major fast muscle type. This explains the pale colour of their flesh. In humans, Type IIb fibres are found in large numbers in the arm muscles and are responsible for powerful, rapid movements.
While Type II muscle fibres are generally white, there are exceptions. Type IIa fibres, also known as fast oxidative fibres, have higher levels of myoglobin and mitochondria compared to Type IIb fibres. As a result, they exhibit a red colour and are often deoxygenated, appearing darker in colour. These fibres are used during sustained power activities such as sprinting or repeated lifts.
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Muscle is a soft tissue, one of four basic animal tissue types
Muscle is a soft tissue, one of four basic types of animal tissue. There are three types of muscle tissue in vertebrates: skeletal muscle, cardiac muscle, and smooth muscle. Each type of muscle tissue has a distinct structure and function, contributing to the overall movement and stability of the body.
Skeletal muscle, also known as voluntary muscle, is responsible for voluntary movements of the body. These muscles are attached to the skeleton via tendons and work together with bones, tendons, and ligaments to support the body's weight and facilitate movement. Skeletal muscle tissue is striated, exhibiting a striped appearance due to the arrangement of sarcomeres, which are contractile units necessary for muscle contraction. Skeletal muscle fibres can be further classified into Type I (slow-twitch) and Type II (fast-twitch) fibres, with Type I fibres appearing red due to high levels of myoglobin and mitochondria, and Type II fibres ranging from red to white depending on their oxidative capacity.
Cardiac muscle, on the other hand, is an involuntary muscle found exclusively in the walls of the heart, known as the myocardium. It is also striated, but unlike skeletal muscle, cardiac muscle cells connect at irregular angles called intercalated discs. Cardiac muscle tissue contracts involuntarily, controlled by the autonomic nervous system, to pump blood through the cardiovascular system.
Smooth muscle is the third type of muscle tissue, and it is found within the walls of various organs and structures such as the oesophagus, stomach, intestines, blood vessels, and skin. Unlike skeletal and cardiac muscles, smooth muscle is non-striated and has an involuntary contraction, regulated by both the central nervous system and hormonal influences.
The different types of muscle tissues work together to enable a wide range of bodily functions, from movement and posture control to the vital pumping of blood by the heart. The unique characteristics of each muscle tissue type contribute to their specific roles, highlighting the importance of understanding muscle tissue composition and function in maintaining overall health and well-being.
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Cardiac muscle is striated
There are three types of muscle tissue in vertebrates: skeletal, cardiac, and smooth muscle. The cardiac muscle is striated and found only in the walls of the heart as myocardium. It is an involuntary muscle, meaning its movement is not under conscious control. Instead, it is controlled by the autonomic nervous system.
Cardiac muscle tissue is striated, like skeletal muscle, containing sarcomeres in highly regular arrangements of bundles. While skeletal muscles are arranged in regular, parallel bundles, cardiac muscle connects at branching, irregular angles known as intercalated discs. The intercalated discs have both anchoring junctions and gap junctions. Attached cells form long, branching cardiac muscle fibres that are, essentially, a mechanical and electrochemical syncytium allowing the cells to synchronize their actions.
Cardiac muscle cells are rectangular in shape and are joined at their ends by intercalated discs to form long fibres. Each cell contains myofibrils, composed of actin and myosin filaments called myofilaments. These are organized into sarcomeres, the fundamental contractile units of muscle cells. The regular organization of myofibrils into sarcomeres gives cardiac muscle cells a striped or striated appearance when viewed under a microscope. These striations are caused by lighter I bands composed mainly of actin, and darker A bands composed mainly of myosin.
Cardiac muscle cells, or cardiomyocytes, are the contractile myocytes of the cardiac muscle. They are surrounded by an extracellular matrix produced by supporting fibroblast cells. Cardiomyocytes are single cells with a single centrally located nucleus. They contract on their own intrinsic rhythms without any external stimulation.
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Smooth muscle is involuntary
There are three types of muscle tissue in vertebrates: skeletal muscle, cardiac muscle, and smooth muscle. Smooth muscle is involuntary, meaning it contracts and relaxes without conscious intervention. It is controlled by the autonomic nervous system.
Smooth muscle is found in the walls of various organs and structures in the body, including the esophagus, stomach, intestines, bronchi, liver, pancreas, uterus, urethra, bladder, blood vessels, and the arrector pili in the skin. It is also present in the tracts of the respiratory, urinary, and reproductive systems.
Smooth muscle differs from skeletal muscle in several ways, one of the most significant being its ability to be contracted and controlled involuntarily. The nervous system regulates many of the body's subsystems through smooth muscle, without any conscious thought or effort from the individual. For example, a person does not need to actively think about their blood pressure for it to adjust to the increased oxygen demands during exercise.
Smooth muscle can be divided into two main subgroups: single-unit and multi-unit smooth muscle. Most smooth muscle is of the single-unit type, and it is found in the walls of most internal organs, blood vessels (except large elastic arteries), the urinary tract, and the digestive tract. Single-unit smooth muscle contracts as a syncytium, with each cell in a bundle innervated by an autonomic nerve fiber. In contrast, multi-unit smooth muscle is found in the trachea, the iris of the eye, and lining the large elastic arteries.
Smooth muscle cells are spindle-shaped myocytes with a wide middle and tapering ends. They contain large amounts of the proteins actin and myosin, which are responsible for muscle contraction. Smooth muscle, unlike skeletal muscle, does not contain myofibrils or sarcomeres, resulting in a non-striated or homogeneous appearance under a microscope.
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Frequently asked questions
Muscles can be red or white. Type I muscle fibers appear red due to high levels of myoglobin, and Type II muscle fibers are white due to relatively low myoglobin and a reliance on glycolytic enzymes.
Type II muscle fibers are further divided into three types: IIa, IIx, and IIb. Type IIa is red, Type IIx is red when deoxygenated, and Type IIb is white. The type of muscle fiber varies by animal. Type IIb is the major fast muscle type in small animals like rodents, which explains the pale color of their flesh.
Red muscle fibers tend to have more mitochondria and greater local capillary density. They are better suited for endurance and are slow to fatigue because they use oxidative metabolism to generate ATP. White muscle fibers can contract more quickly and with greater force, but they can only sustain short, anaerobic bursts of activity.
