Skeletal muscle

Skeletal muscle

Skeletal muscle, often just called muscle, is one of the three types of vertebrate muscle tissue. The other two types are cardiac muscle and smooth muscle. Skeletal muscle belongs to the voluntary muscular system. It is attached to the bones of a skeleton by things called tendons.

What makes skeletal muscle special is its long, thin cells called muscle fibers. These fibers look striped because of their internal structure. The muscle fibers are made of tiny units called sarcomeres. These are the building blocks that help us move.

When we move, walk, or even stand still, skeletal muscles are working. They help us move our bodies, stay upright, control our temperature, and support our joints. Skeletal muscle makes up about 35% of a person's body weight. Muscles also act like an endocrine organ, releasing substances that help the body work properly.

Structure

There are more than 600 skeletal muscles in the human body. They make up about 40% of body weight in healthy young adults. Most muscles come in pairs on both sides of the body. Muscles are connected to bones by tendons, which help us move.

Skeletal muscle cells, also called muscle fibers, are longer than other muscle cells. They have many nuclei and can get bigger when we exercise. Muscles have different shapes and names depending on their size, location, and what 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 looks special when you see it close up. This look comes from two important proteins called myosin and actin. These proteins are arranged in parts called sarcomeres, which help the muscle get smaller.

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

Development

Main article: Myogenesis

All muscles start from a special part of the body called paraxial mesoderm. When a baby is growing inside its mother, this part splits into pieces called somites. These pieces line up along the body like segments of a worm.

Each somite then splits into three parts: one helps form the spine, one helps form the skin, and one helps form muscles.

Muscle cells begin as tiny building blocks called myoblasts. Some stay close to the spine to make back muscles. Others move out to make muscles in the arms and legs. These building blocks follow chemical signals to find the right spot. There, they 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 act like an endocrine organ. They release special molecules called myokines into the blood. These help make exercise better for our health. One myokine is Interleukin 6. Others like BDNF, FGF21, and SPARC are also released after we move our muscles.

Muscles also help keep us warm. When we are very cold, our muscles can shake or shiver. This creates heat and helps keep our body temperature stable.

Contraction

Muscles contract because of special cells called muscle fibers. Groups of fibers are called motor units. These fibers get signals from the brain through nerves. When a nerve signal reaches a muscle fiber, it makes tiny changes. This lets the muscle shorten and pull on bones, which creates movement.

Muscle movement

Our brain controls most of our muscle movements. Signals go from the brain through the spinal cord to the muscles. They tell the muscles when to contract. Some movements, like reflexes, happen faster and don’t always need to check with the brain first.

Proprioception

Muscles have sensors called muscle spindles. These tell the brain how long and stretched the muscles are. This helps us know where our body parts are without looking. It 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 hard activity, muscles can quickly turn glycogen into energy. Even when we are resting, muscles use energy to keep us stable and ready to move.

Muscle strength

Muscle strength depends on 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 push harder, but other things matter too.

Signal transduction pathways

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

Exercise

Main article: Exercise

Exercise is really good for your body! It helps you move better, makes your muscles and bones stronger, and keeps your joints healthy. When you exercise, your muscles can grow 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 slower but last longer. Your body uses oxygen and foods like fats and carbs for 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. Scientists think this is 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. 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. 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 has many different types of cells. These include nuclei from the muscle cells and other special cells called mononuclear cells. Scientists have found nine main types of these mononuclear cells in muscle tissue.

These cells include ones 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 genes. These genes help the cell do its special job. When a small piece of muscle is taken, all these different cells are there together.

Endocrine functions of skeletal muscle

Skeletal muscle can act like an endocrine organ because it releases special molecules to help other parts of the body. These molecules are called cytokines and peptides. They are made by the muscle and can help the body in many ways.

When people exercise regularly, like through running or lifting weights, the types of molecules released by their muscles change. This can help many body systems.

Having more muscle mass, especially when people are older, may help protect some mental skills, like planning and organizing. Walking more can also help lower the risk of dying early for people over 60. Exercise changes the genes in muscle cells. This leads 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, scientists found that exercise changes how genes work in our muscles. These changes are linked to something called epigenetic mechanisms.

Exercise can add or remove small chemical groups to DNA, especially at special spots called CpG sites. This can make the DNA tighter or looser, which changes which genes are active.

Exercise can also change 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. Regular exercise helps muscles become stronger and healthier.