HUMAN BIOLOGY

104 Chapter 6

cardiac muscle The heart’s
muscle tissue.

muscular system Organ
system that consists of
skeletal muscles, which
partner with bones to move
body parts.

skeletal muscle Muscle
tissue of the skeletal
system.

smooth muscle Muscle
tissue in the walls of hollow
organs and tubes.

What types of muscle tissue occur in

the body?

• The body’s muscle tissue includes skeletal, smooth, and
  cardiac muscle tissue.


• Skeletal muscle makes up the muscular system, which
  partners with the skeleton to move body parts.


• Smooth muscle is found in the walls of organs and of tubes,
  such as blood vessels. Cardiac muscle is the muscle of
  the heart.

takE-homE mEssagE

the Body’s three kinds of muscle

cardiac muscle is found only in the heart (Figure 6.1C).

It looks striated, like skeletal muscle. Unlike skeletal and

smooth muscle, however, cardiac muscle can contract with-

out stimulation by signals from the nervous system. Special

junctions between its cells allow the contraction signals to

pass between them so fast that for all intents and purposes

the cells contract as a single unit.

We do not have conscious control over contractions of

cardiac muscle and smooth muscle, so they are said to be

“involuntary” muscles. We can control many of our skel-

etal muscles, so they are “voluntary” muscles. Figure 6.2

shows the major skeletal muscles in the body. Some are

close to the surface, others deep in the body wall. Some,

such as facial muscles, attach to the skin. The trunk has

muscles of the thorax (chest), spine, abdominal wall, and

pelvic cavity. And of course, other muscle groups attach

to limb bones.

When we speak of the body’s muscular system, we’re

talking about skeletal muscle—the focus of the rest of this

chapter. Those movements range from delicate adjustments

that help you keep your balance to the cool moves you

might execute on a dance floor. Our skeletal muscles also

help stabilize joints and generate body heat.

n    in all three kinds of muscle in the body, groups of cells

contract to produce movement.

n    Links to Muscle tissue 4.3, Nervous tissue 4.4,

Tissue membranes 4.7

the three kinds of muscle have different

structures and functions

In Chapter 4 we introduced the three basic kinds of mus-

cle tissue—skeletal muscle, smooth muscle, and cardiac

muscle. Smooth and cardiac muscle occur with other tis-

sues that form organs such as the bladder and the heart.

Skeletal muscle makes up the muscular system, which

interacts with the skeleton to move body parts. Together

these three types of muscle tissue make up about 50 per-

cent of the body. In all of them, cells specialized to contract

bring about some type of movement.

Most of the body’s muscle tissue is skeletal muscle.

Its long, thin cells are often called muscle “fibers” (Figure

6.1A). And unlike other body cells, skeletal muscle fibers

have more than one nucleus. As you may remember from

Section 4.3, the internal structure of

muscle fibers gives them a striated,

or striped, appearance, and bundles

of them form skeletal muscles.

Smooth muscle occurs in the

walls of hollow organs and of tubes,

such as blood vessels (Figure 6.1B).

Its cells are smaller than skeletal

muscle cells, and they do not look

striped—hence the “smooth” name

for this muscle tissue. Junctions link

smooth muscle cells, which often are

organized into sheets.

Figure 6.1 Animated! Muscle tissue in the human body includes skeletal muscle, smooth muscle, and cardiac (heart) muscle.

6.1

A Skeletal muscle

One skeletal muscle fiber

B Smooth muscle

Smooth muscle fibers

Cardiac muscle

fibers

Voluntary

muscle

Involuntary

muscle

C Cardiac muscle

© Cengage Learning

Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).