278
UNIT 2
Covering, Support, and Movement of the Body
9
Generating Heat
Muscles generate heat as they contract. Tis heat is vitally im-
portant in maintaining normal body temperature. Because
skeletal muscle accounts for at least 40% of body mass, it is the
muscle type most responsible for generating heat.
Additional Functions of Muscle
What else do muscles do? Skeletal muscles protect the more frag-
ile internal organs (the viscera) by enclosing them. Smooth mus-
cle forms valves to regulate the passage of substances through
internal body openings, dilates and constricts the pupils of your
eyes, and forms the arrector pili muscles attached to hair follicles.
■ ■ ■
In this chapter, we examine the structure and function of
skeletal muscle. Ten we consider smooth muscle more briefly,
largely by comparing it with skeletal muscle. We describe car-
diac muscle in detail in Chapter 18, but for easy comparison,
±able 9.3 on p. 310 summarizes the characteristics of all three
muscle types.
Check Your Understanding
1.
When describing muscle, what does “striated” mean?
2.
Harry is pondering an exam question that asks, “Which
muscle type has elongated cells and is found in the walls of
the urinary bladder?” How should he respond?
For answers, see Appendix H.
Skeletal Muscle
Describe the gross structure of a skeletal muscle.
Describe the microscopic structure and functional roles of
the myofibrils, sarcoplasmic reticulum, and T tubules of
skeletal muscle fibers.
Describe the sliding filament model of muscle contraction.
For easy reference,
Table 9.1
on p. 283 summarizes the levels
of skeletal muscle organization, gross to microscopic, that we
describe in the following sections.
Gross Anatomy of a Skeletal Muscle
Each
skeletal muscle
is a discrete organ, made up of several
kinds of tissues. Skeletal muscle fibers predominate, but blood
vessels, nerve fibers, and substantial amounts of connective tis-
sue are also present. We can easily examine a skeletal muscle’s
shape and its attachments in the body without a microscope.
Nerve and Blood Supply
In general, one nerve, one artery, and one or more veins serve
each muscle. Tese structures all enter or exit near the central
part of the muscle and branch profusely through its connec-
tive tissue sheaths (described below). Unlike cells of cardiac
and smooth muscle tissues, which can contract without nerve
stimulation, every skeletal muscle fiber is supplied with a nerve
ending that controls its activity.
Skeletal muscle has a rich blood supply. Tis is understand-
able because contracting muscle fibers use huge amounts of
energy and require almost continuous delivery of oxygen and
nutrients via the arteries. Muscle cells also give off large amounts
of metabolic wastes that must be removed through veins if con-
traction is to remain efficient. Muscle capillaries, the smallest of
the body’s blood vessels, are long and winding and have numer-
ous cross-links, features that accommodate changes in muscle
length. Tey straighten when the muscle stretches and contort
when the muscle contracts.
Connective Tissue Sheaths
In an intact muscle, several different connective tissue sheaths
wrap individual muscle fibers. ±ogether these connective tissue
sheaths support each cell and reinforce and hold together the
muscle as a whole, preventing the bulging muscles from burst-
ing during exceptionally strong contractions.
Let’s consider these connective tissue sheaths from external
to internal (see
Figure 9.1
and the top three rows of ±able 9.1).
Epimysium.
Te
epimysium
(ep
0
ĭ-mis
9
e-um; meaning “out-
side the muscle”) is an “overcoat” of dense irregular connec-
tive tissue that surrounds the whole muscle. Sometimes it
blends with the deep fascia that lies between neighboring
muscles or the superficial fascia deep to the skin.
Perimysium and fascicles.
Within each skeletal muscle, the
muscle fibers are grouped into
fascicles
(fas
9
ĭ-klz; “bundles”)
that resemble bundles of sticks. Surrounding each fascicle is
a layer of fibrous connective tissue called
perimysium
(per
0
ĭ-
mis
9
e-um; meaning “around the muscle [fascicles]”).
Endomysium.
Te
endomysium
(en
0
do-mis
9
e-um; meaning
“within the muscle”) is a wispy sheath of connective tissue
that surrounds each individual muscle fiber. It consists of
fine areolar connective tissue.
As shown in Figure 9.1, all of these connective tissue sheaths
are continuous with one another as well as with the tendons that
join muscles to bones. When muscle fibers contract, they pull
on these sheaths, which transmit the pulling force to the bone
to be moved. Te sheaths contribute somewhat to the natural
elasticity of muscle tissue, and also provide entry and exit routes
for the blood vessels and nerve fibers that serve the muscle.
Attachments
Recall from Chapter 8 that most skeletal muscles span joints
and attach to bones (or other structures) in at least two places.
When a muscle contracts, the movable bone, the muscle’s
inser-
tion
, moves toward the immovable or less movable bone, the
muscle’s
origin
. In the muscles of the limbs, the origin typically
lies proximal to the insertion.
Muscle attachments, whether origin or insertion, may be di-
rect or indirect.
In
direct
, or
fleshy
,
attachments
, the epimysium of the mus-
cle is fused to the periosteum of a bone or perichondrium of
a cartilage.
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