Childrens Illustrated Encyclopedia

EVERY MOVEMENT YOU MAKE is powered by muscles. Muscles are controlled

by nerve signals from the brain. There are three main types of muscles—skeletal,

smooth, and cardiac. Skeletal muscle is also called striated muscle, and it covers

the bones of the skeleton. It is attached to the bones by cords called tendons.

When the muscle contracts, or shortens, it moves the bone. Skeletal muscles are

also called voluntary muscles because they can be controlled at will. Smooth

muscle is found in the digestive system, bladder, and blood vessels. It is called

involuntary muscle because it works automatically, even when you are asleep.

Cardiac muscle is found only in the heart. All muscles need energy in order to

work properly. Blood carries oxygen and glucose (sugar) to muscles to provide

them with fuel. As a muscle works harder, it needs more fuel, so the heart pumps

faster to supply it with

more blood.

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Muscles and movement

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Brain and nerves

Heart and blood

Human body

Lungs and breathing

Skeletons

KEEpINg IN SHApE

If skeletal muscles are not

used regularly they gradually

waste away. Regular exercise

is an important part of

staying healthy. Taking part

in a sport, or exercising two

to three times a week, helps

keep a person fit.

INSIdE A MUSCLE

Each muscle consists of a bundle

of thin fibers. Each of these fibers

is made up of even smaller myofibrils.

Myofibrils contain long, interlocking

groups of molecules called actin and

myosin. Actin and myosin molecules

slide past each other in a ratchet fashion

in order to make the muscle contract.

BICEpS ANd

TRICEpS MUSCLES

Muscles can pull, but they

cannot push. Many, such as the

biceps and triceps muscles in

the upper arm, are arranged

in opposing pairs. The biceps

muscle in the arm contracts to

pull on the forearm bones and

bend the elbow. The triceps

muscle in the arm contracts

to straighten the elbow.

HUMAN MUSCLES

A simple movement such as lifting your arm

involves dozens of muscles, acting together in

sequence with split-second timing. About 650

muscles move the various parts of the skeleton. Muscles

work in teams. The largest muscle is called the gluteus

maximus, in the buttock. The smallest muscles are the

tiny muscles of the small bones inside the ear. Altogether,

muscles make up to half of an adult’s body weight.

Flexors

move

fingers.

Biceps femoris

(hamstring) moves

knees and hips.

Myofibril

Biceps

relaxes and

lengthens.

Biceps

contracts and

shortens.

Pectoralis

moves shoulder

and helps deep

breathing.

Deltoid raises arm.

Fascicle

(bundle of

fibers)

Epimysium

(muscle

sheath)

Elbow

straightens.

Elbow

bends.

Triceps

contracts

and shortens.

Triceps

relaxes.

Gastrocnemius bends

foot downward.

Rectus abdominis

strengthens front

of abdomen.

Muscle

fibre

Trapezius

pulls shoulder

back and up.

Gluteus maximus,

used in walking

and climbing

Blood

vessel

Smooth muscle has

no stripes under

the microscope.

Skeletal muscle

looks striped under

the microscope.

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