Chapter 9
Muscles and Muscle Tissue
277
9
Types of Muscle Tissue
Chapter 4 introduced the three types of muscle tissue—
skeletal
,
cardiac
, and
smooth
. Now we are ready to describe each type in
detail, but before we do, let’s introduce some terminology.
Skeletal and smooth muscle cells (but not cardiac muscle
cells) are elongated, and for this reason, they are called
mus-
cle fibers
.
Whenever you see the prefixes
myo
or
mys
(both are word roots
meaning “muscle”) or
sarco
(flesh), the reference is to muscle.
For example, the plasma membrane of muscle cells is called the
sarcolemma
(sar
0
ko-lem
9
ah), literally, “muscle” (sarco) “husk”
(lemma), and muscle cell cytoplasm is called
sarcoplasm
.
Okay, let’s get to it.
Skeletal Muscle
Skeletal muscle
tissue
is packaged into the
skeletal muscles
, or-
gans that attach to and cover the bony skeleton. Skeletal mus-
cle fibers are the longest muscle cells and have obvious stripes
called
striations
(see Figure 4.9a, p. 138). Although it is o±en
activated by reflexes, skeletal muscle is called
voluntary muscle
because it is the only type subject to conscious control.
When you think of skeletal muscle tissue, the key words to
keep in mind are
skeletal
,
striated
, and
voluntary
.
Skeletal muscle is responsible for overall body mobility. It
can contract rapidly, but it tires easily and must rest a±er short
periods of activity. Nevertheless, it can exert tremendous power,
a fact revealed by reports of people li±ing cars to save their loved
ones. Skeletal muscle is also remarkably adaptable. For example,
your forearm muscles can exert a force of a fraction of an ounce
to pick up a paper clip—or a force of about 6 pounds to pick up
this book!
Cardiac Muscle
Cardiac muscle
tissue
occurs only in the heart (the body’s
blood pump), where it constitutes the bulk of the heart walls.
Like skeletal muscle cells, cardiac muscle cells are striated (see
Figure 4.9b, p. 139), but cardiac muscle is not voluntary. Indeed,
it can and does contract without being stimulated by the nerv-
ous system. Most of us have no conscious control over how fast
our heart beats.
Key words to remember for cardiac muscle are
cardiac
,
stri-
ated
, and
involuntary
.
Cardiac muscle usually contracts at a fairly steady rate set
by the heart’s pacemaker, but neural controls allow the heart to
speed up for brief periods, as when you race across the tennis
court to make that overhead smash.
Smooth Muscle
Smooth muscle
tissue
is found in the walls of hollow visceral or-
gans, such as the stomach, urinary bladder, and respiratory passages.
Its role is to force fluids and other substances through internal body
channels. Like skeletal muscle, smooth muscle consists of elongated
cells, but smooth muscle has no striations (see Figure 4.9c, p. 139).
Like cardiac muscle, smooth muscle is not subject to voluntary con-
trol. Its contractions are slow and sustained.
We can describe smooth muscle tissue as
visceral
,
nonstri-
ated
, and
involuntary
.
Special Characteristics of Muscle Tissue
What enables muscle tissue to perform its duties? Four special
characteristics are key.
Excitability
, also termed
responsiveness
, is the ability to
receive and respond to a stimulus, that is, any change in the
environment either inside or outside the body. In the case of
muscle, the stimulus is usually a chemical—for example, a neu-
rotransmitter released by a nerve cell, or a local change in pH.
Te response (sometimes separated out as an additional charac-
teristic called conductivity) is to generate an electrical impulse
that travels along the plasma membrane of the muscle cell and
causes the cell to contract.
Contractility
is the ability to shorten forcibly when ade-
quately stimulated. Tis ability sets muscle apart from all other
tissue types.
Extensibility
is the ability to extend or stretch. Muscle cells
shorten when contracting, but they can stretch, even beyond
their resting length, when relaxed.
Elasticity
is the ability of a muscle cell to recoil and resume
its resting length a±er stretching.
Muscle Functions
Muscle performs at least four important functions for the body.
It produces movement, maintains posture, stabilizes joints, gen-
erates heat, and more.
Producing Movement
Just about all movements of the human body and its parts result
from muscle contraction. Skeletal muscles are responsible for
all locomotion (moving from place to place) and manipulation.
Tey enable you to respond quickly to changes in the external
environment—for example, jump out of the way of a car, direct
your eyes, and smile or frown.
Blood courses through your body because of the rhythmically
beating cardiac muscle of your heart and the smooth muscle
in the walls of your blood vessels, which helps maintain blood
pressure. Smooth muscle in organs of the digestive, urinary, and
reproductive tracts propels, or squeezes, substances (foodstuffs,
urine, semen) through the organs and along the tract.
Maintaining Posture and Body Position
We are rarely aware of the skeletal muscles that maintain body
posture. Yet these muscles function almost continuously, mak-
ing one tiny adjustment a±er another to counteract the never-
ending downward pull of gravity.
Stabilizing Joints
Even as muscles pull on bones to cause movement, they stabilize
and strengthen the joints of the skeleton (Chapter 8).
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