138
UNIT 1
Organization of the Body
4
cell types. Tere are three kinds of muscle tissue: skeletal, car-
diac, and smooth.
Skeletal Muscle
Skeletal muscle
tissue is packaged by connective tissue sheets
into organs called
skeletal muscles
that are attached to the
bones of the skeleton. Tese muscles form the flesh of the body,
and as they contract they pull on bones or skin, causing body
movements.
Skeletal muscle cells, also called
muscle fibers
, are long, cy-
lindrical cells that contain many peripherally located nuclei.
Teir obvious banded, or striated, appearance reflects the pre-
cise alignment of their myofilaments
(Figure 4.9a)
.
Cardiac Muscle
Cardiac muscle is found only in the walls of the heart. Its con-
tractions help propel blood through the blood vessels to all parts
of the body. Like skeletal muscle cells, cardiac muscle cells are
striated. However, cardiac cells differ structurally in that they are
Generally uninucleate (one nucleus) with the nucleus situ-
ated centrally
Branching cells that fit together tightly at unique junctions
called
intercalated discs
(in-ter
9
kah-la
0
ted) (Figure 4.9b)
Smooth Muscle
Smooth muscle
is so named because its cells have no visible
striations. Individual smooth muscle cells are spindle shaped
and contain one centrally located nucleus (Figure 4.9c). Smooth
muscle is found mainly in the walls of hollow organs other than
the heart (digestive and urinary tract organs, uterus, and blood
vessels). It squeezes substances through these organs by alter-
nately contracting and relaxing.
Because skeletal muscle contraction is under our conscious
control, skeletal muscle is o±en referred to as
voluntary mus-
cle
, and the other two types are called
involuntary muscle
be-
cause we do not consciously control them. We describe skeletal
muscle and smooth muscle in detail in Chapter 9, and cardiac
muscle in Chapter 18.
Check Your Understanding
16.
You are looking at muscle tissue through the microscope
and you see striped branching cells that connect with one
another. What type of muscle are you viewing?
17.
Which muscle type(s) is voluntary? Which is injured when
you pull a muscle while exercising?
For answers, see Appendix H.
(a)
Skeletal muscle
Description:
Long, cylindrical, multinucleate
cells; obvious striations.
Function:
Voluntary movement; locomotion;
manipulation of the environment; facial
expression; voluntary control.
Location:
In skeletal muscles attached to
bones or occasionally to skin.
Photomicrograph:
Skeletal muscle (approx. 440
m
).
Notice the obvious banding pattern and the
fact that these large cells are multinucleate.
Nuclei
Striations
Part of
muscle
fiber (cell)
Figure 4.9
Muscle tissues.
(a)
Skeletal muscle tissue. (For a related image, see
A Brief Atlas
of the Human Body
, Plate 28.)
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