What Are Muscles?

The muscles in the human body can be divided into three main
networks of specialized muscle tissues. These networks move our
bones, blood vessels, internal organs, and more, twenty-four hours a day.
Within the three systems, more than eight hundred muscles generate
movement and heat when they contract—or tighten—during use, or
relax when not in use.
The three main muscle systems are the skeletal muscle system,
the smooth muscle system, and the cardiac muscle system. The skeletal
muscle system holds our bones in place so that we remain upright. This
voluntary system responds when we tell our bodies to use muscles to do
things like clench a fist or run. The smooth muscle system lines many

• Different types of muscle fibers are responsible for many

voluntary and involuntary actions in the body.

The seemingly simple act of throwing something is actually a complex process that involves the muscles and other body systems, such as the nervous and skeletal systems. internal organs, such as the liver and kidneys. It helps to push substances, such as blood, food, and waste, through the body. The cardiac muscle system helps the heart pump blood throughout the body. We cannot tell the smooth or cardiac muscle systems what to do. These two involuntary systems work automatically. Muscles give us the power to smile, frown, speak, chew, jump, climb, throw a ball, type, digest the food we eat, read these words, and much more. With all these jobs, it is no wonder that muscles in the three muscle systems make up the highest percentage of body weight in an averagesized person. That is more than the weights of bones, fat, blood, orother tissues. When healthy muscles are well fed, exercised, and rested, they can literally make us jump for joy. The flexibility and support muscles provide make it possible for us to participate in life.

Muscles are found throughout the body and help to protect organs and other parts, all while

allowing us to move and balance

WHAT MUSCLES DO

Muscles perform several major jobs. They take energy from nutrients in the food we eat and use it to move our bodies. Muscles work in pairs to create movement. When one muscle contracts, the muscle it is paired
with relaxes. Healthy muscles maintain muscle tone— or structure—because they are always somewhat tightened. Muscle tone means that muscles are working, keeping us upright and ready to move. Even during sleep, muscles remain slightly contracted. Muscles help to keep our body healthy. Muscle contractions create heat and keep the body at its ideal temperature. If cold air starts to lower body temperature, tiny muscles—goose bumps at the base of each hair—contract to hold in body heat. When outside conditions are hot,
these same tiny muscles expand to let out heat and cool us down. Healthy muscles also providea layer of protective tissues over organs, such as the liver and kidneys, and other structures inside the body.

Goose bumps are the result of tiny muscles

tightening and making fine body hairs

stand up.

MUSCLE STRUCTURE

The organization of muscle structures makes
them strong. Imagine layers of stretchy cylinders
9
Muscles are made up of smaller fibers, which give them great
strength and flexibility.
inside other stretchy
cylinders then stacked into
bundles. Such bundles would
resemble muscles. They are
packed, multi-layered, and
hard, yet flexible.
Even the largest,
firmest muscle on a body
builder is made up of delicate
threadlike structures
called muscle fibers. These
fibers are actually cells,
the smallest basic unit
in an organism. Under a
microscope, a single muscle
cell, or fiber, looks thin and
fragile. Yet each fiber is
packed with many filaments.
Myofibril filaments are
coated cylinders. Within them are thick and thin myofilaments. The thicker ones are made of a chemical protein substance called actin. The thin myofilaments contain myosin proteins. Groups of myofilament cylinders are bundled into units called sarcomeres. Inside sarcomeres, actin and myosin proteins, slide past each other. This sliding makes muscles move. It is hard to imagine that tiny muscle fibers could possibly contain even more structures, but they do. Muscle spindles inside muscle fibers react to muscle stretching. They send messages to the brain that one muscle is stretched out. The brain then causes electrical and chemical changes to relax the paired muscle. Muscle spindles also communicate with the brain about where
muscles are located. They tell the brain things like how an arm is bent or if a leg is up or down. The brain then adjusts the movements of other body parts for balance. Even if you close your eyes, muscle spindles and the brain’s messengers (called neurons) work together to tell you your position. All muscle fiber structures are individually coated with connective tissue, mainly made up of collagen. This natural protein substance
strengthens everything it encloses. Bundles of filaments, which are called the fascicles, are also covered with connective tissue. These wrapped, coated bundles form the muscle itself and make each one incredibly strong. Muscles do not just float around loosely inside the body. Cord-like connective tissues called tendons attach skeletal muscles to bones, skin, or to other muscles. Muscles are also threaded with networks of tiny blood vessels called capillaries and tubules. These carry nutrients and oxygen in blood into the muscles. When muscles are being used, they release chemicals that cause the heart to direct more blood to the muscles from

Collagen can be found in different parts of the body. This connective tissue strengthens muscles.

The pink and red tubes shown here are capillaries found in muscles. The blood vessels’ many loops and

bends allow them to adjust in size and length as the muscles contract and expand.

the other parts of the body. This extra blood gives the muscles an extra

boost of oxygen and nutrients so they can move faster and longer.

MUSCLE CHEMISTRY

Living muscles are like laboratories where quick, chemical reactions make energy. Mitochondria, which are microscopic structures inside cells, produce the chemical adenosine triphosphate (ATP). ATP is the major
chemical that produces the energy muscles need to move. However, muscle fibers only have a small amount of available ATP. They must get ATP more to keep moving.

This is the molecular structure of ATP. The human body stores and uses ATP to create the energy

needed for everyday life-sustaining activities.

When muscles use up stored ATP, the body breathes harder to take

in more oxygen. The heart pumps faster to send more oxygenated blood

to muscle cells to make more ATP. Increased oxygen in the blood stimulates

the liver to convert glucose sugars from nutrients in food. The liver

then releases the glucose into muscle cells, which builds up the ATP levels.

With more ATP, thick myofilaments can detach themselves from thin myofilaments

so that they can move.

Another chemical reaction begins when electrical signals from the

brain cause the release of acetylcholine. This neurotransmitter triggers

electrical activity in muscle cells. The cells release stored calcium ions.

In turn, calcium makes it possible for thin and thick muscle filaments to

slide past each other. When that happens, muscles move.

FIGHT OR FLIGHT?

A car races out of a driveway inches in front of you, or something crashes in another room while you are home alone.In an instant, your heart pounds, your neck prickles, and you breathe harder. You may get goosebumps. These physical reactions, which happen automatically, are a reaction to danger. In humans and many other organisms, muscles undergo immediate changes. These begin after the brain
floods the body with dozens of emergency chemicals, called hormones, which set off a chain reaction of muscle activities. When certain hormones are released, heart muscles pump more oxygenated blood to muscles in the arms and legs. These muscles tighten to gather energy that may be needed to fly from the danger or fight it directly. Capillaries in the skin constrict so that blood will go to the muscles instead of the skin. The loss of blood near the skin causes chills and goose bumps in scary or stressful situations.
Eye muscles expand. As a result, widened pupils take in more light so that you can see better. Muscles near the lungs relax to let in more oxygen so that breathing speeds up. At the same time, digestive muscles slow down. This increases energy and blood flow to the muscles needed in an emergency. The flight or fight response is an instinct that helps all animals, including humans, to survive dangerous situations.

TWITCHY MUSCLES

Athletes hope for it and train for it. It’s that sudden burst of energy at a critical moment when they can sprint ahead in a competition. At the muscular level, such energy bursts come from fast-twitch muscle fibers. They make and release small amounts of energy quickly, without using oxygen. Fast-twitch muscles fire their energy during rapid movements, such as sprints, fast breaks, and the quick moves we make in daily life. Blinking is one of those fast moves, so eye muscles are made up of fast-twitch fibers. Slow-twitch muscle fibers, on the
other hand, are our everyday workhorses. They make energy slowly and steadily from oxygen. Because oxygen fuels the energy they make, slow-twitch muscles contain many more blood vessels than fast-twitch fibers. Slow-twitch muscles are active during long-distance walking, running, swimming, and other physical

activities that require a constant, steady supply of energy. They do not tire as quickly as fast-twitch muscle fibers do. Many experts believe that people inherit the number of slow-twitch and fasttwitch muscles they have. If that is true, then that means certain individuals are better suited to certain sports that involve more fast-twitch muscle fibers, such as sprinting. However, some scientific studies that show an individual who trains intensively in sports requiring fast motion can develop more fast-twitch muscles.