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152
C
HAPTER
E
IGHT
Types of Muscle
muscle. They differ in having one nucleus per cell and
branching interconnections. The membranes between the
cells are specialized to allow electrical impulses to travel
rapidly through them, so that contractions can be better
coordinated. These membranes appear as dark lines be-
tween the cells
(see Table 8-1)
and are called intercalated
(in-TER-kah-la-ted) disks, because they are “inserted be-
tween” the cells. The electrical impulses that produce
contractions of cardiac muscle are generated within the
muscle itself but can be modified by nervous stimuli and
hormones.
There are three kinds of muscle tissue: smooth, cardiac,
and skeletal muscle, as introduced in Chapter 4. After a
brief description of all three types
(Table 8-1)
, this chap-
ter concentrates on skeletal muscle, which has been stud-
ied the most.
Smooth Muscle
Smooth muscle makes up the walls of the hollow body or-
gans as well as those of the blood vessels and respiratory
passageways. It moves involuntarily and produces the
wavelike motions of peristalsis that move substances
through a system. Smooth muscle can also regulate the
diameter of an opening, such as the central opening of
blood vessels, or produce contractions of hollow organs,
such as the uterus. Smooth muscle fibers (cells) are ta-
pered at each end and have a single, central nucleus. The
cells appear smooth under the microscope because they
do not contain the visible bands, or
striations
, that are
seen in the other types of muscle cells. Smooth muscle
may contract in response to a nerve impulse, hormonal
stimulation, stretching, and other stimuli. The muscle
contracts and relaxes slowly and can remain contracted
for a long time.
Skeletal Muscle
When viewed under the microscope, skeletal muscle cells
appear heavily striated. The arrangement of protein
threads within the cell that produces these striations is
described later. The cells are very long and cylindrical
and have multiple nuclei per cell. During development,
the nuclei of these cells divide repeatedly by mitosis
without division of the cell contents, resulting in a large,
multinucleated cell. Such cells can contract as a large unit
when stimulated. The nervous system stimulates skeletal
muscle to contract, and the tissue usually contracts and
relaxes rapidly. Because it is under conscious control,
skeletal muscle is described as voluntary.
Skeletal muscle is so named because most of these mus-
cles are attached to bones and produce movement at the
joints. There are a few exceptions. The muscles of the ab-
dominal wall, for example, are partly attached to other mus-
cles, and the muscles of facial expression are attached to the
Cardiac Muscle
Cardiac muscle, also involuntary, makes up the wall of
the heart and creates the pulsing action of that organ. The
cells of cardiac muscle are striated, like those of skeletal
Table 8•1
Comparison of the Different Types of Muscle
SMOOTH
CARDIAC
SKELETAL
Location
Wall of hollow organs,
vessels, respiratory
passageways
Tapered at each end,
branching networks,
nonstriated
Wall of heart
Attached to bones
Cell characteristics
Branching networks; special
membranes (intercalated
disks) between cells;
single nucleus; lightly
striated
Long and cylindrical; multinucleated;
heavily striated
Control
Action
Involuntary
Produces peristalsis;
contracts and relaxes
slowly; may sustain
contraction
Involuntary
Pumps blood out of heart;
self-excitatory but influ-
enced by nervous system
and hormones
Voluntary
Produces movement at joints; stimulated
by nervous system; contracts and
relaxes rapidly
T
HE
M
USCULAR
S
YSTEM
153
8
skin. Skeletal muscles constitute the
largest amount of the body’s muscle tis-
sue, making up about 40% of the total
body weight. This muscular system is
composed of more than 600 individual
skeletal muscles. Although each one is a
distinct structure, muscles usually act in
groups to execute body movements.
Table 8•2
Connective Tissue Layers in Skeletal
Muscle
NAME OF LAYER
LOCATION
Endomysium
Around each individual muscle fiber
Perimysium
Around fascicles (bundles) of muscle fibers
Epimysium
Around entire muscle; forms the innermost
layer of the deep fascia.
Checkpoint 8-1
What are the three types
of muscle?
The Muscular System
Structure of a Muscle
In forming whole muscles, individual muscle fibers are
arranged in bundles, or
fascicles
(FAS-ih-kls), held
together by fibrous connective tissue (
Fig. 8-1
,
Table 8-
2
). The deepest layer of this connective tissue, the
endomysium
(en-do-MIS-e-um) surrounds the individ-
ual fibers in the fascicles. Around each fascicle is a
connective tissue layer known as the
perimysium
(per-
ih-MIS-e-um). The entire muscle is then encased in a
tough connective tissue sheath, the
epimysium
(ep-ih-
MIS-e-um), which forms the innermost layer of the
deep
fascia
, the tough, fibrous sheath that encloses a muscle.
(Note that all these layers are named with prefixes that
describe their position added to the root
my/o,
meaning
“muscle.”) All of these supporting tissues merge to form
the
tendon
, the band of connective tissue that attaches a
muscle to a bone (see
Fig. 8-1
).
The three primary functions of skeletal muscles are:
Movement of the skeleton. Muscles are attached to bones
and contract to change the position of the bones at a joint.
Maintenance of posture. A steady partial contraction of
muscle, known as
muscle tone
, keeps the body in posi-
tion. Some of the muscles involved in maintaining pos-
ture are the large muscles of the thighs, back, neck, and
shoulders as well as the abdominal muscles.
F). Heat is a nat-
ural byproduct of muscle cell metabolism. When we are
cold, muscles can boost their heat output by the rapid
small contractions we know of as shivering.
C (98.6
Checkpoint 8-2
What are the three main functions of skeletal
muscle?
Epimysium
Muscle
fascicle
Muscle fiber
(cell)
Endomysium
Perimysium
Muscle fibers
Tendon
Bone
Blood
vessels
Endomysium
Body of muscle
Perimysium
Fascicle
A
B
Figure 8-1
Structure of a skeletal muscle. (A)
Structure of a muscle showing the tendon that attaches it to a bone.
(B)
Muscle tis-
sue seen under a microscope. Portions of several fascicles are shown with connective tissue coverings. (B, Reprinted with permission
from Gartner LP, Hiatt JL. Color Atlas of Histology. 3
rd
ed. Philadelphia: Lippincott Williams & Wilkins, 2000.)
ZOOMING IN
✦
What
is the innermost layer of connective tissue in a muscle? What layer of connective tissue surrounds a fascicle of muscle fibers?
Generation of heat. Muscles generate most of the heat
needed to keep the body at 37
 154
C
HAPTER
E
IGHT
Muscle Cells in Action
Nerve impulses coming from the brain and the spinal
cord stimulate skeletal muscle fibers (see Chapter 9). Be-
cause these impulses are traveling away from the central
nervous system (CNS), they are described as
motor
im-
pulses (as contrasted to sensory impulses traveling to-
ward the CNS), and the neurons (nerve cells) that carry
these impulses are described as motor neurons. As the
neuron contacts the muscle, its axon (fiber) branches to
supply from a few to hundreds of individual muscle cells,
or in some cases more than 1000
(Fig. 8-2)
.
A single neuron and all the muscle fibers it stimulates
comprise a
motor unit
. Small motor units are used in fine
coordination, as in movements of the eye. Larger motor
units are used for maintaining posture or for broad move-
ments, such as walking or swinging a tennis racquet.
cleft,
across which the neurotransmitter must travel.
Until its release, the neurotransmitter is stored in tiny
membranous sacs, called vesicles, in the endings of the
nerve fiber. Once released, the neurotransmitter crosses
the synaptic cleft and attaches to receptors, which are
proteins embedded in the muscle cell membrane. The
membrane forms multiple folds at this point that increase
surface area and hold a maximum number of receptors.
The receiving membrane of the muscle cell is known as
the
motor end plate
.
Muscle fibers, like nerve cells, show the property of
excitability;
that is, they are able to transmit electrical
current along the plasma membrane. When the muscle is
stimulated at the neuromuscular junction, an electrical
impulse is generated that spreads rapidly along the mus-
cle cell membrane. This spreading wave of electrical cur-
rent is called the
action potential
because it calls the mus-
cle cell into action. Chapter 9 provides more information
on synapses and the action potential.
The Neuromuscular Junction
The point at which a
nerve fiber contacts a muscle cell is called the
neuromus-
cular junction
(NMJ)
(Fig. 8-3)
. It is here that a chemical
classified as a
neurotransmitter
is released from the neu-
ron to stimulate the muscle fiber. The specific neurotrans-
mitter released here is
acetylcholine
(as-e-til-KO-lene),
abbreviated ACh, which is found elsewhere in the body as
well. A great deal is known about the events that occur at
this junction, and this information is important in under-
standing muscle action.
The neuromuscular junction is an example of a
synapse
(SIN-aps), a point of communication between
cells. Between the cells there is a tiny space, the
synaptic
Checkpoint 8-3
Muscles are activated by the nervous system.
What is the name of the special synapse where a nerve cell
makes contact with a muscle cell?
Checkpoint 8-4
What neurotransmitter is involved in the stimu-
lation of skeletal muscle cells?
Contraction
Another important property of muscle
tissue is
contractility
. This is the capacity of a muscle
fiber to undergo shortening and to change its shape, be-
coming thicker. Studies of muscle chemistry and obser-
vation of cells under the powerful electron microscope
have given a concept of how muscle cells work.
These studies reveal that each skeletal muscle fiber
contains many threads, or filaments, made of two kinds of
proteins, called
actin
(AK-tin) and
myosin
(MI-o-sin).
Filaments made of actin are thin and light; those made of
myosin are thick and dark. The filaments are present in
alternating bundles within the muscle cell
(Fig. 8-4)
. It is
the alternating bands of light actin and heavy myosin fil-
aments that give skeletal muscle its striated appearance.
They also give a view of what occurs when muscles con-
tract.
Note that the actin and myosin filaments overlap where
they meet, just as your fingers overlap when you fold your
hands together. A contracting subunit of skeletal muscle is
called a
sarcomere
(SAR-ko-mere). It consists of a band of
myosin filaments and the actin filaments on each side of
them
(see Fig. 8-4)
. In movement, the myosin filaments
“latch on” to the actin filaments in their overlapping region
by means of many paddlelike extensions called myosin
heads. In this way, the myosin heads form attachments be-
tween the actin and myosin filaments that are described as
cross-bridges. Using the energy of ATP for repeated move-
ments, the myosin heads, like the oars of a boat moving
water, pull all the actin strands closer together within each
sarcomere. As the overlapping filaments slide together, the
Skeletal muscle
fiber (cell)
Motor axon
Neuromuscular
junction
Axon branches
Figure 8-2
Nervous stimulation of skeletal muscle.
A
motor axon branches to stimulate multiple muscle fibers (cells).
The point of contact between the neuron and the muscle cell is
the neuromuscular junction. (Reprinted with permission from
Cormack DH. Essential Histology. 2
nd
ed. Philadelphia: Lippin-
cott Williams & Wilkins, 2001.)
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