Logan Steele
The human body has over 600 muscles, which are used constantly, even without our conscious thought. These muscles are responsible for moving our body and supporting our organs. Skeletal muscles, which make up 30-40% of our body mass, are attached to bones and allow us to perform a wide range of movements. Cardiac muscles are found only in the heart, helping to pump blood throughout the body. Smooth muscles line our organs, blood vessels, and digestive tract, performing essential functions like moving waste through the intestines and helping our lungs expand when we breathe. Beyond these basic functions, recent research has revealed that skeletal muscle also acts as a secretory organ, producing and releasing various peptides and proteins, including myokines, which play a role in muscle-organ communication and the multiple health benefits of exercise. This discovery has led to new hypotheses about the role of muscle-derived factors in health and disease, including the potential secretion of a hormone called Musculin that may influence insulin sensitivity and appetite regulation.
| Characteristics | Values |
|---|---|
| Number of muscles in the body | More than 600 |
| Types of muscles | Skeletal, Cardiac, Smooth |
| Skeletal muscle composition | Flexible muscle fibres ranging from less than half an inch to just over three inches in diameter |
| Skeletal muscle functions | Connect to bones and allow a wide range of movements and functions |
| Skeletal muscle composition | 30-40% of total body mass |
| Cardiac muscle function | Squeezes and relaxes to pump blood through the cardiovascular system |
| Smooth muscle function | Line the inside of some organs, performing essential jobs like moving waste through intestines and helping lungs expand when breathing |
| Skeletal muscle secretory functions | Produces, expresses and releases several hundred secreted peptides, or myokines, which mediate muscle-organ crosstalk |
| Muscle secretome functions | Plays a role in health and disease, including diabetes, obesity, and exercise |
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What You'll Learn
Skeletal muscle as a secretory organ
Skeletal muscle is the largest organ in the human body. They are primarily characterised by their mechanical activity, which is required for posture, movement, and breathing, and depends on muscle fibre contractions. However, skeletal muscle is not just a component in our locomotor system.
During the past decade, skeletal muscle has been identified as a secretory organ. Skeletal muscle has the capacity to produce, express, and release several hundred secreted peptides, known as myokines. This finding provides a new way of understanding the role of skeletal muscle in organ crosstalk, including muscle–liver and muscle–adipose tissue crosstalk. Cytokines and other peptides that are produced, expressed, and released by muscle fibres and exert either autocrine, paracrine, or endocrine effects should be classified as myokines. The discovery that the muscle secretome consists of several hundred secreted peptides provides a new way of understanding how muscles communicate with other organs, such as adipose tissue, liver, pancreas, bones, and brain.
Many proteins produced by skeletal muscle are dependent upon contraction; therefore, physical inactivity probably leads to an altered myokine response, which could provide a potential mechanism for the association between sedentary behaviour and many chronic diseases. In response to muscle contractions, both type I and type II muscle fibres express the myokine IL-6, which subsequently exerts its effects both locally within the muscle and, when released into the circulation, peripherally in several organs in a hormone-like fashion. Specifically, in skeletal muscle, IL-6 acts in an autocrine or paracrine manner to signal through a gp130Rβ/IL-6Rα homodimer, resulting in the activation of AMP‐kinase and/or PI3‐kinase to increase glucose uptake and fat oxidation. IL-6 is also known to increase hepatic glucose production during exercise or lipolysis in adipose tissue. Other myokines include the osteogenic factors IGF-1 and FGF-2; FSTL-1, which improves the endothelial function of the vascular system; and the PGC-1α-dependent myokine irisin, which drives brown-fat-like development.
The study of the mechanisms by which regular exercise improves overall health and influences the activity of tissues, organs, and systems is an important area of research. The finding that muscle produces and releases myokines provides a conceptual basis for understanding some of the molecular mechanisms that link physical activity to protection against premature mortality.
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Muscles and exercise
The human body has over 600 muscles, which are used constantly, whether consciously or not. Some muscles are visible and can be felt, especially after a workout, while others, like the heart and those that line some organs, work silently to keep us alive.
There are two types of muscle movement: voluntary and involuntary. Voluntary movements are actions that we choose to perform, using our nervous system to control our muscles to move our bodies. Involuntary movements are those that our bodies perform without our input, such as the beating of the heart.
Exercise is an important way to keep muscles healthy and strong. Different types of exercise work different muscles, so it is important to vary physical activity to ensure all muscles are worked. For example, aerobic exercise involves extended periods of activity at levels below maximal contraction strength, using a high percentage of Type I slow-twitch muscle fibres. This type of exercise includes activities such as walking, jogging, swimming, and biking. On the other hand, anaerobic exercise involves short, high-intensity activity that utilizes little or no oxygen and leads to increased levels of lactic acid. This type of exercise includes sprinting and weightlifting, which rely on Type II fast-twitch muscle fibres for short, sharp bursts of movement.
It is important to warm up and cool down when exercising to reduce the risk of muscle strain and injury. Warm-up exercises, such as stretching and light jogging, increase muscle temperature and flexibility, while cool-down exercises help to loosen the muscles that have tightened during exercise.
In addition to keeping muscles strong and healthy, exercise also has an impact on muscle secretions. Skeletal muscle has been found to produce, express, and release several hundred secreted peptides, known as myokines. This discovery has provided new insight into how muscles communicate with other organs, such as the liver, adipose tissue, pancreas, bones, and brain.
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Muscles and obesity
Skeletal muscle has been identified as a secretory organ, producing, expressing, and releasing several hundred secreted peptides, known as myokines. These myokines play a role in muscle-fat crosstalk and have an impact on multiple organs in the body, including adipose tissue, the liver, the pancreas, bones, and the brain.
Obesity has been shown to have a negative impact on skeletal muscle, with a decline in contractile function, reducing mobility and promoting health risks. Obesity leads to a shift in muscle fibre types, from slow to fast, and can cause a decrease in force and power produced per muscle mass. This results in a reduced muscle performance when normalized to body mass. Obese individuals may also experience a relative weakness in their muscles due to reduced mobility, neural adaptations, and changes in muscle morphology.
Research has indicated that obesity is associated with a higher risk of poor muscle quality, with age, sex, and BMI being independent predictors. Poor muscle quality can lead to functional incapacity and an increased risk of mortality. The prevalence of obesity is a growing public health concern, with associated increased risks of developing conditions such as diabetes, cardiovascular disease, coronary heart disease, hypertension, stroke, and cancer.
The link between obesity and skeletal muscle function is a complex area of study, with various factors influencing muscle performance and quality. Further research is needed to fully understand the effects of obesity on muscle function and the potential for reversibility.
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Muscles and diabetes
Skeletal muscle has been identified as a secretory organ, producing, expressing, and releasing several hundred secreted peptides, known as myokines. These myokines are involved in muscle–liver and muscle–adipose tissue crosstalk, and have clinical implications in mediating the health benefits of exercise.
The role of the muscle secretome in health and disease is an area of active research, with studies investigating the impact of muscle-derived factors on insulin secretion in healthy individuals and patients with type 2 diabetes. Evidence suggests that muscle-derived factors may suppress glucose-induced insulin secretion, indicating a potential skeletal muscle-pancreas crosstalk.
In terms of muscle mass and diabetes prevalence, studies have found an association between lower muscle mass and greater diabetes prevalence in young adults, independent of body fat distribution. This association was observed in both men and women under 50 years old. Additionally, older adults with type 2 diabetes exhibit excessive loss of appendicular lean mass and trunk fat mass compared to their non-diabetic counterparts, with older women at a particularly high risk for loss of skeletal muscle mass.
The impact of muscle strength and cardiorespiratory fitness on diabetes risk has also been explored. Research suggests that moderate muscle strength and good cardiorespiratory fitness are associated with a reduced risk of developing diabetes. Specifically, individuals with medium strength were found to have a 32% reduced risk of developing diabetes compared to those with low strength. Furthermore, cardiorespiratory fitness, as measured by oxygen uptake during exercise, was significantly associated with a lower risk of developing diabetes over a 20-year period in a study of Japanese men.
Overall, the relationship between muscles and diabetes is a complex one, involving the study of muscle secretome, muscle mass, muscle strength, and cardiorespiratory fitness. Further research is needed to fully understand the mechanisms underlying the association between muscles and diabetes.
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Muscles and insulin secretion
Skeletal muscle has been identified as a secretory organ, producing, expressing, and releasing several hundred secreted peptides, or myokines. These myokines provide a new understanding of how muscles communicate with other organs, such as the pancreas, liver, adipose tissue, bones, and brain.
Skeletal muscle can secrete interleukin-6 (IL-6), interleukin-8 (IL-8), and interleukin-15 (IL-15), as well as irisin, myonectin, and myostatin. These myokines are regulated by exercise and contraction and can either increase or decrease obesity, inflammation, and insulin resistance. For example, interleukin-6 (IL-6) signalling in liver-parenchymal cells suppresses hepatic inflammation and improves systemic insulin action.
In the context of insulin resistance and glucose uptake, skeletal muscle plays a crucial role. Insulin resistance in skeletal muscle is characterised by decreased insulin-stimulated glucose uptake, resulting from impaired insulin signalling and various intracellular defects. This can lead to high blood glucose levels, which can further contribute to the development of insulin resistance and type 2 diabetes.
Additionally, chronic inflammation caused by obesity is a significant contributor to the development of insulin resistance and type 2 diabetes. Obesity-induced inflammation can lead to the release of specific cytokine hormones, known as adipo-myokines, from both adipose tissue and skeletal muscle. These adipo-myokines, such as leptin, can become dysregulated in obese and diabetic patients, impacting insulin resistance and glucose uptake.
Furthermore, skeletal muscle is the primary tissue for insulin-stimulated glucose disposal and plays a pivotal role in whole-body glycemic control. Skeletal muscle responds uniquely to muscle contraction or exercise by increasing its sensitivity to subsequent insulin stimulation. This response highlights the complex interplay between skeletal muscle and insulin secretion, contributing to our understanding of how insulin promotes glucose uptake in muscle.
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Frequently asked questions
Muscles are pieces of soft tissue that are found throughout your body. There are more than 600 muscles in the human body, and they help you do everything from holding your body still to performing complex movements.
Muscles perform two types of movements: voluntary and involuntary. Voluntary movements are actions you control, such as reaching for a book on a shelf, which involves the use of skeletal muscles in your neck, arm, and shoulder. Involuntary movements are those that happen without your input, such as your heart beating or your lungs expanding when you breathe.
Yes, skeletal muscles have been identified as secretory organs, producing, expressing, and releasing several hundred secreted peptides, known as myokines. These myokines play a role in muscle communication with other organs, such as the liver, adipose tissue, pancreas, bones, and brain. Additionally, there is a hypothesis that skeletal muscle also secretes an unidentified hormone called Musculin, which may play a role in regulating insulin secretion and appetite responses to exercise.
