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598 Unit 5: Human Biology
VOCABULARYmuscular system p. 599skeletal muscle p. 600voluntary
muscle p. 600smooth muscle p. 600involuntary muscle p. 600cardiac
muscle p. 600
BEFORE, you learned• There are different types of
bone tissue• The human skeleton has two
separate divisions• Joints function in several
different ways
NOW, you will learn• About the functions of muscles• About the
different types of
muscles and how they work• How muscles grow and heal
KEY CONCEPT
The muscular system makes movement possible.
Muscles perform important functions.Every movement of your
body—from the beating of your heart, to themovement of food down
your throat, to the blinking of your eyes—occurs because of
muscles. Some movements are under your control,and other movements
seem to happen automatically. However, mus-cles do more than
produce movement. They perform other functionsas well. Keeping body
temperature stable and maintaining posture aretwo additional
functions of muscles.
Check Your Reading What are three functions that muscles
perform?
EXPLORE Muscles
How do muscles change as you move?
PROCEDURE
Sit on a chair with your feet on the floor.
Place your hand around your leg. Straighten one leg as shown in
the photograph.
Repeat step 2 several times.
WHAT DO YOU THINK?• How did your muscles change during
the activity?• Record your observations.• What questions do you
have about the
muscular system?
3
2
1
MAIN IDEA WEBMake a web for the mainidea: Muscles
performimportant functions.
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Chapter 17: Systems, Support, and Movement 599
MovementThe works with the skeletal system to allow move-ment.
Like all muscles, the muscles that produce movement are made upof
individual cells called muscle fibers. These fibers contract and
relax.
Most of the muscles involved in moving the body work in pairs.
Asthey contract, muscles shorten, pulling against bones. It may
surpriseyou to know that muscles do not push. Rather, a muscle on
one sideof a bone pulls in one direction, while another muscle
relaxes. Musclesare attached to bones by stretchy connective
tissue.
Maintaining Body TemperatureEarlier you read that processes
within the body requirecertain conditions, such as temperature and
the rightamount of water and other materials. The balance of
con-ditions is called homeostasis. One of the functions of
themuscular system is related to homeostasis. Muscles func-tion to
maintain body temperature.
When muscles contract, they release heat. Without thisheat from
muscle contraction, the body could not main-tain its normal
temperature. You may have observed theway your muscles affect your
body temperature when youshiver. The quick muscle contractions that
occur when youshiver release heat and raise your body
temperature.
Check Your Reading How do muscles help maintain homeostasis?
Maintaining PostureHave you ever noticed that you stand up
straight withoutthinking about it, even though gravity is pulling
your bodydown? Most muscles in your body are always a little
bitcontracted. This tension, or muscle tone, is present evenwhen
you are sleeping. The muscles that maintain posture relax
completely only when you are unconscious.
Try standing on the balls of your feet for a few moments, or
onone leg. When you are trying to balance or hold one position for
anylength of time, you can feel different muscles contracting and
relaxing.Your muscles make constant adjustments to keep you sitting
or stand-ing upright. You don’t have to think about these tiny
adjustments;they happen automatically.
muscular system
Muscles contract duringshivering, raising bodytemperature.
RESOURCE CENTERCLASSZONE.COM
Discover more aboutmuscles.
http://www.classzone.com/redirect_science/hbm06_pg23_muscles.html
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600 Unit 5: Human Biology
Your body has different types of muscle.Your body has three
types of muscle. All three types of muscle tissueshare certain
characteristics. For example, each type of muscle con-tracts and
relaxes. Yet all three muscle types have different functions,and
different types of muscle are found in different locations.
Skeletal MuscleThe muscles that are attached to your skeleton
are called
Skeletal muscle performs voluntary movement—that is, movement
that you choose to make. Because they are involvedin voluntary
movement, skeletal muscles are also called
Skeletal muscle, like all muscle, is made of long fibers. The
fibersare made up of many smaller bundles, as a piece of yarn is
made up ofstrands of wool. One type of bundle allows your muscles
to moveslowly. Those muscles are called slow-twitch muscles.
Another type ofbundle allows your muscles to move quickly. These
are called fast-twitch muscles. If you were a sprinter, you would
want to developyour fast-twitch muscles. If you were a long
distance runner, youwould develop your slow-twitch muscles.
Check Your Reading What does it mean that skeletal muscles are
voluntary muscles?
Smooth Muscleis found inside some organs, such as the
intestines
and the stomach. Smooth muscles perform automatic movement
andare called In other words, smooth muscles workwithout your
knowing it. You have no control over their movement.For example,
smooth muscles line your stomach wall and push foodthrough your
digestive system. Smooth muscle fibers are not as longas skeletal
muscle fibers. Also, unlike skeletal muscles, smooth musclesare not
fast-twitch. Smooth muscles contract slowly.
Cardiac MuscleYour heart is made of Like smooth muscle,
cardiacmuscle moves without conscious control. Each cardiac muscle
cell hasa branched shape. The cells of the heart connect in a
chain. Thesechains form webs of layered tissue that allow cardiac
cells to contracttogether and make the heart beat. Just like the
smooth muscle cells,the cardiac muscle cells contract slowly,
except in emergencies.
Check Your Reading Compare and contrast the three types of
muscle described: skeletal, smooth, and cardiac.
cardiac muscle.
involuntary muscles.
Smooth muscle
voluntary muscles.
skeletal muscles.
VOCABULARYRemember to add foursquares for involuntarymuscles and
voluntary mus-cles to your notebook. Note differences in the
twodiagrams.
reading tip
The root of the word volun-tary comes from the Latinroot vol-,
meaning “wish.”In the word involuntary theprefix in- suggests
themeaning “unwished for.”Involuntary movementmeans movement you
can’tcontrol.
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Chapter 17: Systems, Support, and Movement 601
The marchers in this band are using all three different typesof
muscle tissue.
Muscle Tissue
Skeletal muscle movesthe legs of these marchers.
Smooth muscle in theair passages of the lungsallows the band
membersto breathe as they play their instruments.
Cardiac muscle allows thehearts of the band membersto pump blood
as they marchto the beat of the music.
Which movements of these band members are voluntary, and which
are involuntary?
250�
360�
150�
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602 Unit 5: Human Biology
Skeletal muscles and tendons allow bonesto move.
Skeletal muscles are attached to your bones by strong tissues
calledtendons. The tendons on the end of the muscle attach firmly
to thebone. As the fibers in a muscle contract, they shorten and
pull the ten-don. The tendon, in turn, pulls the bone and makes it
move.
You can feel your muscles moving your bones. Place your left
arm,stretched out flat, in front of you on a table. Place the
fingers of yourright hand just above your left elbow. Bend your
elbow and raise andlower your left arm. You are contracting your
biceps. Can you feel themuscle pull on the tendon?
The dancers in the photograph are using many sets of muscles.The
diagrams show how muscles and tendons work together to movebones.
Muscles are shown in red. Notice how each muscle crosses ajoint.
Most skeletal muscles do. One end of the muscle attaches to
onebone, crosses a joint, then attaches to a second bone. As the
musclecontracts, it pulls on both bones. This pulling produces
movement—in the case of these dancers, very exciting movement.
musclecontracts
tendon
musclerelaxes
bones tendons
Skeletal muscles contract by shorten-ing and pulling the
tendon.
musclerelaxes
tendon
musclecontracts
bones tendons
Muscle fibers can return to their origi-nal length after being
shortened.
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Muscles grow and heal.Developing Muscles An infant’s muscles
cannot do very much. Ababy cannot lift its head, because the neck
muscles are not strongenough to support it. For the first few
months of life, a baby needsextra support, until the neck muscles
grow strong and can hold up thebaby’s head.
The rest of the skeletal muscles also have to develop and
strengthen.During infancy and childhood and into adolescence,
humans developmuscular coordination and become more graceful in
their movements.Coordination reaches its natural peak in
adolescence but can be furtherimproved by additional training.
Exercise and Muscles When you exercise regularly, your muscles
mayget bigger. Muscles increase in size with some types of
exercise,because their cells reproduce more rapidly in response to
theincreased activity. Exercise also stimulates growth of
individual musclecells, making them larger.
You may have experienced soremuscles during or after
exercising.During exercise, chemicals can build upin the muscles
and make them crampor ache. The muscle soreness you feel aday or so
after exercise is caused bydamage to the muscle fibers. The mus-cle
fibers have been overstretched ortorn. Such injuries take time to
heal,because the body must remove injuredcells, and new ones must
form.
KEY CONCEPTS1. What are the three main func-
tions of the muscular system?
2. Make a rough outline of ahuman body and label placeswhere you
could find each ofthe three types of muscles.
3. Explain why you may be soreafter exercise.
CRITICAL THINKING4. Apply You are exercising and
you begin to feel hot. Explainwhat is happening in
yourmuscles.
5. Analyze Describe what hap-pens in your neck muscleswhen you
nod your head.
CHALLENGE6. Infer The digestive system
breaks down food and trans-ports materials. How are theshort
length and slow move-ment of smooth muscle tissuesin the stomach
and intestinesrelated to the functions ofthese organs?
Chapter 17: Systems, Support, and Movement 603
Stretching your musclesbefore exercise helps prevent injury.
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