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The human body is an intricate system, with over 600 muscles that help us move, breathe, pump blood, and support vital organs. These muscles are made up of thousands of elastic fibres bundled together, forming the skeletal, smooth, and cardiac muscles. Skeletal muscles, attached to bones, enable voluntary movements like walking and writing. Smooth muscles, found in the stomach and bladder, work involuntarily to push food through the body and control urine release. Cardiac muscles, unique to the heart, contract and relax to pump blood, working without conscious control. While each muscle has a specific role, they all work together to keep us alive and moving.
| Characteristics | Values |
|---|---|
| Muscle type | Skeletal, cardiac, and smooth muscle |
| Muscle formation | Myogenesis |
| Muscle composition | Actin, myosin, troponin, tropomyosin |
| Muscle function | Contraction, movement, posture, body temperature, storing nutrients, stabilizing joints |
| Muscle appearance | Striated |
| Muscle performance | Affected by physical training, disuse, endurance exercise |
| Muscle injuries | Strains, muscle dystrophies, infections, spasms |
| Muscle pain | Occasional aches are normal, but chronic soreness or weakness may indicate serious issues |
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What You'll Learn
Muscle formation in vertebrate embryos
The development of the musculoskeletal system begins at the fourth week of gestation. The first step in the formation of bone and muscle is the condensation of mesenchymal cells, which are loosely organized embryonic connective tissue. The paraxial mesoderm then differentiates into somites, which are blocks of mesoderm cells. Somites form in the occipital region of the embryo, at the base of the head, and continue to form cranio-caudally, with about three pairs forming each day. Each somite undergoes a split, with cells from the ventral portion forming sclerotomes, which create the vertebrae and ribs, and cells from the dorsomedial lip mixing with cells from the ventrolateral lip to form dermomyotomes. The dermomyotomes give rise to the derm of the back and the skeletal muscles of the body and limbs. The first muscle mass to form is the myotome, which has epaxial and hypaxial components that are subsequently integrated into the trunk musculature. Myoblasts, which originate from the mesoderm, fuse together to form muscle fibres, which are attached by collagenous connective tissues.
The development of the skeletal muscle involves the differentiation of myotome cells into myoblasts. In limbs, myoblasts migrate to the limb buds and surround the primordial limb bones. The pattern of muscle formation is dictated by the same mesenchymal cells that give rise to the bones. In the mammalian case, it appears that the progenitor cells for some muscle masses migrate first, and the regulatory environment changes over time. Distal muscles, such as those of the hand, may be formed by an earlier migratory population than more proximal muscles.
Several genes play a role in muscle formation. Pax3 and its orthologue Pax7 are important in adult muscle regeneration, and Pax3 is also involved in maintaining the proliferative phase of muscle precursor cells. In the early embryo, all developing muscle masses are derived from Pax. Msx1 is present in migrating muscle progenitor cells and has been shown to keep cultured myoblasts dividing.
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The role of myoblasts
Myoblasts are precursor cells that play a crucial role in the formation and regeneration of skeletal muscles through a process called myogenesis. Myogenesis is the formation of skeletal muscular tissue, particularly during embryonic development. It involves the fusion of myoblasts to form multinucleated muscle fibres called myotubes, which further mature into myofibers, the basic unit of skeletal muscle.
During myogenesis, myoblasts can either proliferate or differentiate into myotubes. In cell culture, myoblasts proliferate when growth factors like fibroblast growth factor (FGF) are present in the surrounding medium. When the growth factors are depleted, myoblasts stop dividing and undergo terminal differentiation into myotubes.
The differentiation of myoblasts into myotubes occurs in stages. The first stage involves the exit from the cell cycle and the expression of specific genes. The second stage involves the alignment of myoblasts with each other. Myoblast fusion starts with a recognition and adhesion phase, involving surface receptors and proteins involved in endocytosis and membrane repair pathways. Actin dynamics also play a crucial role, with fusion-competent myoblasts producing actin-based podosome-like structures that invade the muscle founder cell, forming pores for the transfer of cytoplasm and nucleus.
Several proteins have been implicated in myoblast fusion, including Myomaker, Minion–Myomerger, and Myomerger–Minion. These proteins are necessary for muscle formation, and their absence can lead to severe defects or death at birth due to a lack of skeletal muscle. In addition, the cytokine IL4 controls myoblast fusion, acting downstream of NFATC2 to regulate the fusion of myoblasts to myotubes.
Myoblasts also have satellite cells, which are crucial for muscle repair and regeneration in adult organisms. These satellite cells can differentiate into bone or fat and play an important role in muscle development and maintenance throughout adulthood.
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The three types of muscle tissue
The human body is an intricate machine, with over 600 muscles that help us move, breathe, and perform a host of other functions. These muscles are made of thousands of small fibres woven together, and they come in three distinct types, each with unique characteristics and roles.
The first type is skeletal muscle, which is attached to bones and helps produce movement, maintain posture, and store nutrients. These muscles are composed of long, multinucleate fibres, or myofibers, that run the entire length of the muscle. They are called skeletal muscles because they are always connected to the skeleton. Most skeletal muscles are attached to two bones through tendons, which are tough connective tissues. Skeletal muscles contract in response to stimuli, and their movement is controlled voluntarily by the nervous system.
The second type is cardiac muscle, which is found only in the heart. These muscles are striated, like skeletal muscles, but they have distinct ends called intercalated discs. Cardiac muscle fibres are mononucleate, with only one nucleus per fibre, and they can sometimes be branched. The sarcolemma, a tubular sheath encasing each muscle fibre, acts as a conductor for electrochemical signals that stimulate muscle cells.
The third type is smooth muscle, found in the walls of internal organs such as the digestive tract and blood vessels. Smooth muscle fibres have tapered edges, and no striations are visible under a microscope. They often wrap around the organ they are associated with, making it challenging to find an entire smooth muscle fibre in a tissue slice.
Each of these three types of muscle tissue plays a crucial role in the body's movement and function, working together to keep us alive and mobile.
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Muscle function and movement
The human body is made up of more than 600 muscles, which help us do everything from moving our bodies to breathing and staying alive. Muscles are pieces of soft tissue that are found throughout the body. They are made of thousands of small fibres woven together, and these fibres stretching and pressing together is what moves our organs and bodies.
There are three types of muscle tissue in the body: skeletal, smooth, and cardiac. Skeletal muscles are attached to bones through tendons and work together with bones and joints to form lever systems. The muscle acts as the effort force, the joint acts as the fulcrum, the bone that the muscle moves acts as the lever, and the object being moved acts as the load. Skeletal muscles are under voluntary control, receiving neural inputs that allow conscious control of muscles. They contract in response to a stimulus and are responsible for producing movement, sustaining body posture and position, maintaining body temperature, storing nutrients, and stabilizing joints.
Smooth and cardiac muscles, on the other hand, are involuntary muscles that contract without conscious control. They are responsible for functions such as digesting food, pumping blood through the heart and blood vessels, and giving birth.
To contract, skeletal muscles shorten their length, pulling on tendons and moving bones closer to each other. This movement is made possible by the contraction of muscle fibres, which are composed of proteins called myosin and actin. Myosin is the protein that causes muscles to contract, while actin forms a helical structure that makes up the bulk of the thin filament mass. These filaments slide past each other during contraction, allowing muscles to shorten and lengthen and ultimately resulting in movement.
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Skeletal muscle vs. other muscles
The human body has more than 600 muscles, which help us move, breathe and stay alive. There are three types of muscles: skeletal, smooth and cardiac.
Skeletal Muscles
Skeletal muscles are the most common type of muscle in the human body. They are attached to the bones and allow us to perform a wide range of movements and functions. They are also known as striated muscles because they appear striped or striated. These muscles are under voluntary control, meaning we can control how and when they move. Skeletal muscles make up between 30% and 40% of our total body mass. They are essential for critical movements, such as bending our arms or squatting down.
Smooth Muscles
Smooth muscles are found in the walls of hollow visceral organs, such as the liver, pancreas and intestines. They are under involuntary control, meaning they work without us having to think about them.
Cardiac Muscles
Cardiac muscles are located in the walls of the heart and help pump blood throughout the body. They are also striated and under involuntary control. Cardiac muscles, along with skeletal muscles, can be affected by muscular dystrophies, which cause permanent muscle weakness.
In summary, skeletal muscles are the most prevalent type of muscle in our bodies, and they work in tandem with our bones and joints to enable movement. Smooth and cardiac muscles, on the other hand, function involuntarily to support vital processes in our organs and heart.
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