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Skeletal muscles are the most common type of muscle in the human body, making up 30-40% of our total body mass. They are located throughout the body and are attached to the bones, allowing us to perform a wide range of movements and functions. In recent years, skeletal muscle has been identified as a secretory organ, producing and releasing various substances, including myokines, which are believed to be responsible for the health benefits of exercise. This discovery has provided new insights into how skeletal muscles, as organs, communicate with other organs in the body.
| Characteristics | Values |
|---|---|
| Type of muscle | Skeletal muscle is the most common type of muscle in the body |
| Control | Skeletal muscles are voluntary muscles, meaning you control how and when they work |
| Location | Throughout the body, located between bones |
| Function | Producing movement, maintaining body posture, controlling body temperature, stabilising joints and protecting internal organs |
| Composition | Skeletal muscle fibres, blood vessels, nerve fibres and connective tissue |
| Connective tissue layers | Three layers: epimysium, perimysium and endomysium |
| Mechanical activity | Required for posture, movement and breathing |
| Secretory function | Identified as a secretory organ, producing, expressing and releasing several hundred secreted peptides, or myokines |
| Size | The largest organ in the body, comprising 30-40% of total body mass |
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What You'll Learn
Skeletal muscles are organs
Each skeletal muscle has three layers of connective tissue called "mysia" that enclose and provide structure to the muscle. The outermost layer is the epimysium, which is a sheath of dense and irregular connective tissue that allows the muscle to contract and move powerfully while maintaining its structural integrity. The epimysium also separates the muscle from other tissues and organs, allowing independent movement. The middle layer is the perimysium, which surrounds bundles of muscle fibres called fascicles. The innermost layer is the endomysium, which surrounds individual muscle fibres and plays a role in transferring force produced by the muscle fibres to the tendons.
Skeletal muscles are also characterised by their mechanical activity, which is required for posture, movement, and breathing. They contract to allow for various movements and functions, such as swallowing, urination, and defecation. These muscles also protect internal organs by acting as a shield against external trauma and supporting the weight of the organs.
Furthermore, skeletal muscle has been identified as a secretory organ, producing, expressing, and releasing various substances. It secretes myokines, which are cytokines or other peptides that act as signalling molecules. Myokines are believed to mediate the health benefits of exercise and are released into the bloodstream after muscle contraction. Interleukin 6 (IL-6) is the most studied myokine, and other identified myokines include BDNF, FGF21, and SPARC.
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Skeletal muscle functions
Skeletal muscles are the most common type of muscle in the human body, comprising 30 to 40% of total body mass. They are voluntary muscles, meaning an individual can control how and when they move and work. They are attached to bones by tendons, allowing for a wide range of movements and functions.
Skeletal muscles serve a variety of functions, including:
Chewing and Swallowing
The first parts of the digestion process are facilitated by skeletal muscles.
Expanding and Contracting the Chest Cavity
Allowing for inhalation and exhalation at will.
Maintaining Body Posture
Skeletal muscles are responsible for maintaining body posture and producing movement.
Moving the Bones
Skeletal muscles are located between bones and consist of flexible muscle fibres that contract, allowing for a wide range of movements.
Protecting and Stabilising Joints
Skeletal muscles also play a role in protecting joints and holding them in place.
Controlling Body Temperature
This is a vital function of skeletal muscles, which produce body heat through muscle contraction, responsible for 85% of the body's heat.
Communicating with Other Organs
Skeletal muscle has been identified as a secretory organ, producing and releasing myokines (a type of cytokine) that communicate with other organs, such as adipose tissue, liver, pancreas, bones and the brain.
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Skeletal muscle composition
Skeletal muscles are located throughout the body and are attached to bones by tendons, allowing for a wide range of movements and functions. They make up about 30 to 40% of an average human's total body mass, although this can vary from person to person. Men tend to have about 36% more skeletal muscle mass than women, and taller or overweight people also tend to have higher muscle mass. Skeletal muscles are composed of flexible muscle fibres that range from less than half an inch to just over three inches in diameter. These fibres, called myofibers, contract to enable movement and can contain thousands of smaller units called myofibrils.
Myofibrils are composed of actin and myosin filaments, or myofilaments, which are repeated in units called sarcomeres. The interaction of these two proteins results in muscle contraction. Sarcomeres are attached to other organelles, such as the mitochondria, by intermediate filaments in the cytoskeleton. The costamere attaches the sarcomere to the sarcolemma, which is the cell membrane. The cytoplasm, or the area inside the cell membrane, is called the sarcoplasm, and this is where the myofibrils are found. The myofibrils are long protein bundles about one micrometer in diameter.
Each muscle fibre is composed of several hundred to several thousand myofibrils. The arrangement of actin and myosin gives skeletal muscle its microscopic striated appearance. Skeletal muscle fibres are red and white and are often called striated muscles. The skeletal muscle is surrounded by different types of sheaths, or coverings. The outermost layer of tissue surrounding the entire muscle is called the epimysium. The perimysium is the middle layer, surrounding bundles of muscle fibres. The endomysium is the innermost layer, surrounding individual muscle fibres.
Skeletal muscle also acts as a storage source of amino acids that different organs of the body can use for synthesising organ-specific proteins. Skeletal muscle plays a central role in maintaining thermostasis and acts as an energy source during starvation. Skeletal muscle also functions as an endocrine organ by secreting myokines, which are signalling molecules that mediate the health benefits of exercise.
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Skeletal muscle diseases
Skeletal muscles are a type of voluntary muscle, meaning that we can control how and when they work. They are attached to bones by tendons, allowing us to move our limbs and perform a wide range of movements and functions. Skeletal muscles make up about 30-40% of our total body mass.
Muscular Dystrophies
This group of diseases causes progressive degeneration and weakness of skeletal muscle fibres due to an abnormal gene, which can be inherited. There are various types of muscular dystrophies, including Duchenne muscular dystrophy.
Myasthenia Gravis (MG)
MG is an autoimmune neuromuscular disease that interferes with nerve signals to the muscles, resulting in severe muscle weakness and fatigue. It can affect various functions, including movement, walking, speaking, chewing, seeing, and breathing.
Rhabdomyolysis
Rhabdomyolysis is a life-threatening condition that causes the breakdown of skeletal muscle tissue, releasing proteins and other substances into the blood, which can lead to serious organ damage, especially in the kidneys.
Sarcopenia
Sarcopenia is a muscle disease that can be primary or secondary. It causes a loss of skeletal muscle mass and strength, which can be noticed as muscle weakness. This disease is often associated with ageing.
Myopathies
Myopathies are diseases that affect the skeletal muscles, causing the muscle fibres to function improperly and resulting in muscular weakness.
Polymyositis (PM)
PM is a rare type of myositis, an inflammatory muscle disease, that is characterised by muscle inflammation and weakness. It primarily affects individuals over the age of 20, with a higher prevalence in females. While there is no cure, the condition is treatable.
Amyotrophic Lateral Sclerosis (ALS)
ALS, also known as Lou Gehrig's disease, affects the muscles responsible for voluntary movement. It worsens over time, and while there is no cure, treatments can help manage symptoms.
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Skeletal muscle as a secretory organ
Skeletal muscles are located throughout the body and are attached to bones by tendons. They allow us to perform a wide range of movements and functions. Skeletal muscles are also the most common type of muscle in the body, comprising 30 to 40% of our total body mass.
During the past decade, skeletal muscle has been identified as a secretory organ. It has the capacity to produce, express, and release several hundred secreted peptides, known as myokines. These myokines are cytokines or other peptides that are produced, expressed, and released by muscle fibres. They exert autocrine, paracrine, or endocrine effects. The muscle secretome, consisting of several hundred secreted peptides, provides a new paradigm for understanding how muscles communicate with other organs, such as adipose tissue, the liver, the pancreas, bones, and the brain.
The secretion of many of these proteins is accelerated in response to exercise and muscle contraction and can regulate the functions of several organs via autocrine, paracrine, and endocrine routes. This is referred to as the myokine theory. Habitual exercise leads to various health benefits, such as metabolic improvement, anti-inflammation, and muscle building, which are at least partly caused by myokines, including specific interleukins.
Some of the identified myokines are myostatin, LIF, IL-6, IL-7, BDNF, IGF-1, FGF-2, FSTL-1, and irisin. Myokines may also balance and counteract the effects of adipokines. Myostatin, LIF, IL-6, and IL-7 are involved in muscle hypertrophy and myogenesis, while BDNF and IL-6 are involved in AMPK-mediated fat oxidation. IL-6 also appears to have systemic effects on the liver, adipose tissue, and the immune system, and mediates crosstalk between intestinal L cells and pancreatic islets.
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Frequently asked questions
Yes, each skeletal muscle is an organ that consists of various integrated tissues, including skeletal muscle fibers, blood vessels, nerve fibers, and connective tissue.
Skeletal muscles are the most common type of muscle in your body. They are voluntary muscles, meaning you control how and when they work. They are located throughout your body and are attached to your bones, allowing you to perform a wide range of movements and functions.
Skeletal muscles contain connective tissue, blood vessels, and nerves. They have three layers of connective tissue: epimysium, perimysium, and endomysium. Inside each skeletal muscle, muscle fibers are organized into bundles called fascicles.
The primary function of skeletal muscles is contraction, which allows for movement and the stopping of movement, such as maintaining posture and stabilizing joints. Skeletal muscles also control internal movement, generate heat, and protect internal organs.
Skeletal muscles have been identified as secretory organs that produce, express, and release several hundred secreted peptides called myokines. Myokines are cytokines or other peptides produced by muscle fibers, and they play a role in mediating the health benefits of exercise.
