Skeletal muscle

Skeletal muscle, commonly referred to simply as muscle, is one of the three types of vertebrate muscle tissue, along with cardiac muscle and smooth muscle. It belongs to the voluntary muscular system and is attached by tendons to the bones of a skeleton. What makes skeletal muscle special is its long, thin cells called muscle fibers, which give the tissue a striped appearance due to their internal structure.

These muscle fibers are made up of tiny units called sarcomeres, which are the building blocks for movement. When we move, walk, or even stand still, skeletal muscles are at work. They help us move our bodies, keep us upright, control our temperature, and support our joints. In fact, skeletal muscle makes up about 35% of a person's body weight. Beyond movement, muscles also act like an endocrine organ, releasing useful substances that help the body function properly.

Structure

There are more than 600 skeletal muscles in the human body, making up around 40% of body weight in healthy young adults. Most muscles occur in pairs on both sides of the body. Muscles are attached to bones by tendons, which allow movement.

Skeletal muscle cells, also called muscle fibers, are longer than other types of muscle cells. They contain many nuclei and can grow larger when exercised. Muscles have different shapes and names based on their size, location, and the movements they help with. For example, the biceps has two heads, and the gluteus maximus is one of the largest muscles in the body.

Main article: Muscle architecture

Fiber types

There are two main types of muscle fibers in skeletal muscles: Type I, which are slow, and Type II, which are fast. Type II fibers have two sub-types: type IIA, which uses oxygen for energy, and type IIX, which uses stored energy and tires quickly.

These fibers differ in color, speed, and how they make energy. Type I fibers are red and good for long, steady activities, while Type II fibers are white and better for quick, powerful movements but tire faster. Different muscles in your body have different mixes of these fibers, depending on what they need to do. For example, leg muscles used for walking have more Type I fibers, while muscles used for quick actions have more Type II fibers.

Muscle fiber type evolution

Almost all animals need muscles to move. Most have two main types of muscle fibers: slow-twitch and fast-twitch. The mix of these fibers can change depending on what the animal needs — whether it's for quick bursts of speed or longer, steady movement.

Different animals show this variation in interesting ways. For example, lobsters have three types of fibers, including ones that can hold their shape for longer periods. In zebrafish, the first muscles to form are slow-twitch fibers. Turtles have muscles in their necks with different mixes of fast and slow fibers, depending on the muscle's job. Chimpanzees have more fast-twitch fibers than humans, which helps them perform powerful movements, while humans are better at activities needing steady energy, like walking.

Microanatomy

Skeletal muscle has a special pattern when you look at it through a microscope. This pattern is created by two important proteins called myosin and actin. These proteins are arranged in repeating parts called sarcomeres, which help the muscle shrink or contract.

Inside each muscle cell, everything is organized to help the muscle work. The cell’s outer layer is called the sarcolemma, and the inside part is called sarcoplasm. The sarcoplasm contains long protein bundles called myofibrils. There are also special structures like mitochondria that give the muscle energy and sarcoplasmic reticulum that stores calcium ions needed for contraction. These parts work together to make our muscles move!

Development

Main article: Myogenesis

All muscles come from a special part of the body called paraxial mesoderm. As a baby grows inside its mother, this mesoderm splits into pieces called somites, which line up along the body like segments of a worm. Each somite then splits into three parts: one that helps form the spine, one that helps form the skin, and one that helps form muscles.

Muscle cells start as tiny building blocks called myoblasts. Some stay close to the spine to form back muscles, while others move out to form muscles in the arms and legs. These building blocks follow chemical signals to find just the right spot, where they then join together to create the long muscle cells we use to move.

Function

The main job of skeletal muscle is to contract. When muscles contract, they also act like an endocrine organ, releasing special signaling molecules called myokines into the bloodstream. These myokines help boost the health benefits of exercise. One well-known myokine is Interleukin 6, but others like BDNF, FGF21, and SPARC are also released after muscle activity.

Muscles also help keep our body warm. When we’re very cold, our muscles can shake or shiver, creating heat as a by-product of moving. This shaking helps keep our body temperature stable.

Contraction

Muscles contract thanks to special cells called muscle fibers and groups of fibers called motor units. These fibers get signals from the brain through nerves. When a nerve signal reaches a muscle fiber, it causes tiny changes that allow the muscle to shorten and pull on bones, creating movement.

Muscle movement

Our brain controls most muscle movements. Signals travel from the brain through the spinal cord to the muscles, telling them when to contract. Some movements, like reflexes, happen faster and don’t always need to check in with the brain first.

Proprioception

Muscles contain sensors called muscle spindles that tell the brain how long and stretched the muscles are. This helps us know where our body parts are without looking, which is important for balance and coordination.

Energy consumption

Muscles need a lot of energy to work. They store energy in different forms, like glycogen (a type of glucose) and fats. During intense activity, muscles can quickly convert glycogen into energy. Even at rest, muscles use a fair amount of the body’s energy to keep us stable and ready to move.

Muscle strength

Muscle strength comes from three main things: the size of the muscle, how strongly the brain tells the muscle to contract, and the angle at which the muscle pulls on the bones. Bigger muscles can usually exert more force, but other factors also play a role.

Signal transduction pathways

Different signals in the body help determine what kind of muscle fibers we develop. These signals can change how muscles use energy and how they grow, helping them adapt to different activities and conditions.

Exercise

Main article: Exercise

Exercise is really good for your body! It helps improve how well you move, makes your muscles and bones stronger, and keeps your joints working well. When you exercise, your muscles can get bigger and stronger. This happens because of more muscle fibers or bigger muscle parts.

There are two main types of exercise: aerobic and anaerobic. Aerobic exercises, like running long distances, are lower intensity but last longer. Your body uses oxygen and foods like fats and carbs to give you energy during these activities. Anaerobic exercises, like sprinting or lifting weights, are short and intense. Your muscles work hard quickly, but they don’t need much oxygen. Some activities, like soccer or rock climbing, use both types of energy.

After hard exercise, you might feel some muscle soreness a day or two later. This isn’t because of lactic acid anymore; scientists now think it’s because tiny tears in your muscle fibers need to heal.

Clinical significance

Muscle disease

Main articles: Myopathy and Neuromuscular disease

Diseases that affect skeletal muscles are called myopathies, while problems with nerves are called neuropathies. Both can make muscles weak or cause pain. Some myopathies happen because of changes in the proteins that help muscles work.

Neuromuscular diseases can affect how nerves control muscles, sometimes causing muscles to stiffen or stop moving. Symptoms of these diseases can include weakness, stiffness, sudden jerking movements, and pain. Doctors can test for these problems by checking certain chemicals in the blood, measuring electrical activity in muscles, or sometimes taking a small piece of muscle for closer examination.

Hypertrophy

Main article: Muscle hypertrophy

Muscles can grow larger because of different factors like hormones, development, exercise, or certain diseases. While exercise can make muscles stronger, it doesn’t increase the number of muscle fibers. Instead, muscles grow by adding more protein and using special cells that help them expand.

Hormones, especially during puberty, can speed up muscle growth. Men often find it easier to build muscle than women because of higher levels of certain hormones. Some people use extra hormones or steroids to grow muscles more quickly, but this can be harmful.

Atrophy

Main article: Muscle atrophy

Every day, a small amount of muscle breaks down and is rebuilt. However, inactivity, poor nutrition, illness, and aging can cause muscles to shrink, a condition called muscle atrophy. As people get older, they may naturally lose muscle mass, which can make them feel weaker.

Long periods without movement, like during space travel, can also make muscles weaker. Some serious illnesses can lead to muscle loss as well.

Research

Researchers study muscles in many ways. They use special tests to see how muscles work and contract. They also look at how muscles generate force and get tired. Some scientists are even working on creating artificial muscles using special materials.

Mononuclear cells of skeletal muscle

Skeletal muscle contains many different types of cells, including nuclei from muscle cells themselves and other special cells called mononuclear cells. Scientists have found nine main types of these mononuclear cells in muscle tissue. These include cells that line tiny blood vessels, cells that can become fat or connective tissue, cells that support muscle fibers, and immune cells that help fight infections. Each type of cell in muscle has its own unique set of genes, which helps it perform its specific role. When a small sample of muscle is taken, all these different cell types are present together.

Endocrine functions of skeletal muscle

Skeletal muscle can act like an endocrine organ because it releases special molecules that help other parts of the body. These molecules, called cytokines and peptides, are made by the muscle and can have helpful effects far away from the muscle itself. When people exercise regularly, such as through endurance or resistance training, the types of molecules released by their muscles change, which can influence many body systems.

Research shows that having more muscle mass, especially in older adults, may help protect certain mental skills, like planning and organizing. Studies also suggest that walking more can lower the risk of dying early, especially for people over 60. Exercise changes the genes in muscle cells, leading to the release of molecules that support heart health, thinking skills, kidney function, and blood clotting.

Exercise-trained effects are mediated by epigenetic mechanisms

Between 2012 and 2019, many reports showed that changes in how genes work in our muscles after exercise are linked to something called epigenetic mechanisms. These changes often happen by adding or removing small chemical groups to DNA, especially at special spots called CpG sites. This can make the DNA tighter or looser, affecting which genes are active.

Exercise can also change how genes work by altering the tiny "tails" on proteins that wrap around DNA, called histones. When these tails are changed through a process called acetylation or deacetylation, it can make genes more or less active for a long time. Studies have shown that after people exercise regularly, many genes in their muscles change how they work, which helps the muscles get stronger and healthier.