Rebecca L. Johnson - Your Muscular System

Your

Muscular
System

Rebecca L. Johnson

For Mum

Copyright 2013 by Rebecca L. Johnson
All rights reserved. International copyright secured. No part of this book may be
reproduced, stored in a retrieval system, or transmitted in any form or by any means
electronic, mechanical, photocopying, recording, or otherwisewithout the prior written
permission of Lerner Publishing Group, Inc., except for the inclusion of brief quotations in
an acknowledged review.
Lerner Publications Company
A division of Lerner Publishing Group, Inc.
241 First Avenue North
Minneapolis, MN 55401 U.S.A.
Website address: www.lernerbooks.com
Library of Congress Cataloguing-in-Publication Data
Johnson, Rebecca L.
Your muscular system / by Rebecca L. Johnson.
p. cm. (Searchlight booksTMhow does your body work?)
Includes index.
ISBN 9780761374497 (lib. bdg. : alk. paper)
1. 1. Musculoskeletal systemJuvenile literature. I. Title.
QP301.J586 2013
612.7dc23 2011044166
Manufactured in the United States of America
1 DP 7/15/12
eISBN: 978-1-4677-0103-7

Contents

BODY
MOVERS ... page

HOW MUSCLES
WORK ... page

Chapter

BODY
MOVERS

Every time you take a
step, blink your eyes,
or smile, you can thank
your muscles. Muscles
make you move. Some
movements are large. You
leap over a puddle. You swing
a bat. Other movements are
small. You raise an eyebrow.
You tap your finger.

What do muscles
make it possible for
you to do?

You can see some muscles at work. Try making a
muscle with your arm. Can you see the muscle bulging
just under your skin?

You cant see other muscles in your body. When you
swallow, muscles push food into your stomach. Muscles
in the stomach wall mix your food. Muscles in your heart
move blood through your body.

You can see some
muscles just under
your skin.

Muscles help you
do all the things
you do.

Working with Other Systems

You need your muscles for almost everything you do. So
its not surprising that you have hundreds of them. All
these muscles make up your muscular system. The
muscular system works with other body systems to
keep you alive and healthy. It helps you do all the things
you do.

Muscle Control

You can control
some muscles.
You can make
them move
just by thinking.
Suppose you
want to pick an
apple. You reach
out and curl your
fingers around
the apple. You
pull it down. You
decide you want
to do this. And
your muscles
make it happen.

WHEN YOU PICK AN APPLE, YOU USE
MUSCLES YOU CAN CONTROL.

There are other muscles that you cannot control, no
matter how hard you try. You cant make food travel to
your stomach more quickly. You cant make your heart
beat more slowly. These kinds of muscles do their work
automatically.

As hard as you try, you cant
control the muscles that make
goose bumps on your skin.

Contracting Muscles

All muscles are alike in one important way. They all can
contract, or shorten. When a muscle contracts, it moves
whatever body part its attached to. You can see muscles
contract in your arm. The big muscle in your upper arm
is called the biceps. When your biceps contracts, your
lower arm moves up.

Put your right hand on
your left biceps. Bend
your left elbow. Can
you feel your biceps
contracting as your
lower arm moves up?

Chapter

HOW MUSCLES
WORK

Muscles move body parts by
contracting. But how do muscles
contract?

Muscles are made up of cells.
Each muscle cell is called a muscle fiber.
A muscle fiber is thinner than one of the
hairs on your head.

Muscles are made up of
many muscle fibers. How
big are muscle fibers?

Strands

Inside each muscle fiber are hundreds of strands.
These strands are made up of even thinner strands.
The thinnest strands overlap one another. They look
like fingers laced together.

The thinnest strands
of muscle fiber
overlap like fingers
laced together.

Nerves

Your brain controls your body. It sends messages all
the time. These messages go to every part of your
body. They travel along nerves. Nerves reach into every
muscle. They touch every muscle fiber.

NERVES CARRY MESSAGES
TO MUSCLE FIBERS.

When a message reaches a muscle fiber, the tiny
strands inside it move. The strands slide toward one
another. So the muscle fiber gets shorter.

As muscle fibers get shorter, the whole muscle
contracts. It pulls on the body parts that it is attached to.
It makes the body parts move.

thin strand

thick strand

relaxed
strands
(longer muscle
fibers)

contracted
strands
(shorter muscle
fibers)

When the tiny strands in a
muscle f iber slide together,
the f iber gets shorter.

Chapter

MUSCLES
YOU MOVE

You have three
different kinds of
muscles in your
body. All three kinds
are made up of muscle
fibers that contract.
But each kind of muscle
does a special job.

Skeletal muscles are all
over your body. Can
you name one of your
skeletal muscles?