HUMAN BIOLOGY

72 Chapter 4

What is muscle tissue?

• Muscle tissue helps move the body and its parts. It is built of
  cells that can contract.


• Muscle tissue can contract (shorten) when it is stimulated by an
  outside signal.


• Skeletal muscle attaches to bones.


• Smooth muscle is found in internal organs.


• Cardiac muscle makes up the walls of the heart.

taKe-hoMe Message

muscle tissue Tissue built
of cells that can contract.

Muscle tissue: Movement

Smooth muscle cells taper at both ends (Figure 4.5B). They

are bundled inside a connective tissue sheath. This type

of muscle tissue is specialized for ongoing contraction. It

is found in the walls of internal organs—including blood

vessels, the stomach, and the intestines. The contraction of

smooth muscle is “involuntary” because we usually cannot

make it contract just by thinking about it (as we can with

skeletal muscle).

Cardiac muscle (Figure 4.5C) is found only in the wall of

the heart and its sole function is to pump blood. As you

will read in Chapter 7, special junctions fuse the plasma

membranes of cardiac muscle cells. In places, communica-

tion junctions allow the cells to contract as a unit. When

one cardiac muscle cell is signaled to contract, the cells

around it contract, too.

n    Cells in muscle tissue can contract, a property that enables

muscles to move body parts.

The cells in muscle tissue contract, or shorten, when they

are stimulated by an outside signal. Then they relax and

lengthen. Muscle tissue has long,

cylindrical cells lined up in parallel.

This shape is why muscle cells are

often called “muscle fibers.” Muscle

layers and muscular organs contract and relax in a coordi-

nated way. This is how the action of muscles maintains and

changes the positions of body parts, movements that range

from leaping to blinking your eyes. The three types of mus-

cle tissue are skeletal, smooth, and cardiac muscle tissues.

Skeletal muscle is the main tissue of muscles that attach

to your bones (Figure 4.5A). Skeletal muscle cells are

unusual in that they have more than one nucleus. In a

typical muscle, the cells line up in parallel bundles and

look striped, or striated. The bundles, called fascicles,

are enclosed by a sheath of dense connective tissue.

This arrangement of muscle and connective tissue

makes up the organs we call “muscles.” Because we can

exert conscious control over our skeletal muscles, their

contractions are said to be “voluntary.” The structure

and functioning of skeletal muscle tissue are topics we

consider in Chapter 6.

Figure 4.5 Animated! All types of muscle tissue consist of cells that can contract. (A, C: Ed Reschke; B: Biophoto Associates/Science Source)

A Skeletal muscle

• Long, multinucleated, cylindrical cells
  with conspicuous striping (striations)


• Interacts with bone to bring about
  movement, maintain posture

C Cardiac muscle

• Striated cells attached end to
  end, each with a single nucleus


• Found only in the heart wall

B Smooth muscle

• Cells with a single nucleus, tapered
  ends, and no striations


• Found in the walls of some blood
  vessels, the digestive tract, the
  reproductive tract, the bladder, and
  other hollow organs

nucleus nucleus

adjoining

ends of

abutting

cells

vOLunTAry invOLunTAry

4.3

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