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[ Pobierz całość w formacie PDF ]Muscle Tissue
10
Did you know...?
If all the body muscles were pulling in one
direction, they would develop 25 tons of force.
Learning Outcomes
After completing this chapter, you should be able to do the following:
10-1
Specify the functions of skeletal muscle tissue.
10-2
Describe the organization of muscle at the tissue level.
10-3
Explain the characteristics of skeletal muscle fibers, and identify
the structural components of a sarcomere.
10-4
Identify the components of the neuromuscular junction, and
summarize the events involved in the neural control of skeletal
muscles.
10-5
Describe the mechanism responsible for tension production in a
muscle fiber, and compare the different types of muscle
contraction.
10-6
Describe the mechanisms by which muscle fibers obtain the
energy to power contractions.
10-7
Relate the types of muscle fibers to muscle performance, and
distinguish between aerobic and anaerobic endurance.
10-8
Identify the structural and functional differences between
skeletal muscle fibers and cardiac muscle cells.
10-9
Identify the structural and functional differences between
skeletal muscle fibers and smooth muscle cells, and discuss the
roles of smooth muscle tissue in systems throughout the body.
Clinical Notes
Tetanus p. 304
Rigor Mortis p. 311
Delayed-Onset Muscle Soreness p. 326
294
Unit 2
Support and Movement
An Introduction to Muscle Tissue
5.
Maintain Body Temperature.
Muscle contractions require
energy; whenever energy is used in the body, some of it is
converted to heat. The heat released by working muscles
keeps body temperature in the range required for normal
functioning.
6.
Store Nutrient Reserves.
When the diet contains inade-
quate proteins or calories, the contractile proteins in
skeletal muscles are broken down, and the amino acids
released into the circulation. Some of these amino acids
can be used by the liver to synthesize glucose; others can
be broken down to provide energy.
This chapter discusses one of the four primary tissue types,
with particular attention to skeletal muscle tissue. It also ex-
amines the histological and physiological characteristics of
skeletal muscle cells, and relates those features to the func-
tional properties of the entire tissue.
10-1
Skeletal muscle performs six
major functions
We will begin our discussion with the functional anatomy
of a typical skeletal muscle, with particular emphasis on the mi-
croscopic structural features that make contractions possible.
Muscle tissue, one of the four primary types of tissue, consists
chiefly of muscle cells that are highly specialized for contraction.
Three types of muscle tissue exist: (1)
skeletal muscle
, (2)
cardiac
muscle
, and (3)
smooth muscle
.
l
p. 138
Without these muscle
tissues, nothing in the body would move, and no body movement
could occur. Skeletal muscle tissue moves the body by pulling on
bones of the skeleton, making it possible for us to walk, dance,
bite an apple, or play the ukulele. Cardiac muscle tissue pushes
blood through the circulatory system. Smooth muscle tissue
pushes fluids and solids along the digestive tract and regulates the
diameters of small arteries, among other functions.
This chapter primarily describes the structure and func-
tion of skeletal muscle tissue, in preparation for our discus-
sion of the muscular system (Chapter 11). This chapter also
provides an overview of the differences among skeletal, car-
diac, and smooth muscle tissues.
Skeletal muscles
are organs composed primarily of
skeletal muscle tissue, but they also contain connective tis-
sues, nerves, and blood vessels. Each cell in skeletal muscle
tissue is a single muscle
fiber
. Skeletal muscles are directly or
indirectly attached to the bones of the skeleton. Our skeletal
muscles perform the following six functions:
CHECKPOINT
1. Identify the three types of muscle tissue.
2. Identify the six major functions of skeletal muscle.
See the blue Answers tab at the end of the book.
10-2
A skeletal muscle contains
muscle tissue, connective tissues,
blood vessels, and nerves
Figure 10–1
illustrates the organization of a representative
skeletal muscle. In the next section we will examine skeletal
muscle tissue in detail; here we consider how connective tis-
sues are organized in skeletal muscle, and how skeletal mus-
cles are supplied with blood vessels and nerves.
1.
Produce Skeletal Movement.
Skeletal muscle contractions
pull on tendons and move the bones of the skeleton. The
effects range from simple motions such as extending the
arm or breathing, to the highly coordinated movements
of swimming, skiing, or typing.
2.
Maintain Posture and Body Position.
Tension in our skeletal
muscles maintains body posture—for example, holding
your head still when you read a book or balancing your body
weight above your feet when you walk. Without constant
muscular activity, we could neither sit upright nor stand.
3.
Support Soft Tissues.
The abdominal wall and the floor of
the pelvic cavity consist of layers of skeletal muscle.
These muscles support the weight of visceral organs and
shield internal tissues from injury.
4.
Guard Entrances and Exits.
The openings of the digestive
and urinary tracts are encircled by skeletal muscles.
These muscles provide voluntary control over swallow-
ing, defecation, and urination.
Organization of Connective Tissues
Three layers of connective tissue are part of each muscle: (1) an
epimysium, (2) a perimysium, and (3) an endomysium. These
layers and the relationships among them are diagrammed in
Figure 10–1
.
The entire muscle is surrounded by the
epimysium
(ep-
i-MIZ-e-um;
epi
-, on
mys
, muscle), a dense layer of colla-
gen fibers. The epimysium separates the muscle from
surrounding tissues and organs. It is connected to the deep
fascia, a dense connective tissue layer.
The connective tissue fibers of the
perimysium
(per-i-
MIZ-e-um;
peri
-, around) divide the skeletal muscle into a se-
ries of compartments, each containing a bundle of muscle
fibers called a
fascicle
(FAS-i-kl;
fasciculus
, a bundle). In addi-
tion to possessing collagen and elastic fibers, the perimysium
contains blood vessels and nerves that maintain blood flow and
innervate the muscle fibers within the fascicles. Each fascicle
receives branches of these blood vessels and nerves.
295
Chapter 10
Muscle Tissue
Nerve
Epimysium
Muscle fascicle
Muscle fibers
Endomysium
Blood vessels
Perimysium
SKELETAL MUSCLE
(organ)
Perimysium
Muscle fiber
Endomysium
Epimysium
Blood vessels
and nerves
MUSCLE FASCICLE
(bundle of cells)
Capillary
Mitochondria
Endomysium
Sarcolemma
Endomysium
Myosatellite
cell
Tendon
Myofibril
Axon
Perimysium
Nucleus
Sarcoplasm
MUSCLE FIBER
(cell)
Figure 10–1
The Organization of Skeletal Muscles.
A skeletal muscle consists of fascicles (bundles of muscle fibers) enclosed by the
epimysium. The bundles are separated by connective tissue fibers of the perimysium, and within each bundle the muscle fibers are surrounded
by the endomysium. Each muscle fiber has many superficial nuclei, as well as mitochondria and other organelles (
See
Figure 10–3
).
Within a fascicle, the delicate connective tissue of the
endomysium
(en-d¯-MIZ-e-um;
endo
-, inside) surrounds the
individual skeletal muscle cells, or
muscle fibers
, and loosely
interconnects adjacent muscle fibers. This flexible, elastic
connective tissue layer contains (1) capillary networks that
supply blood to the muscle fibers; (2)
myosatellite cells
, em-
bryonic stem cells that function in the repair of damaged
muscle tissue; and (3) nerve fibers that control the muscle.
All these structures are in direct contact with the individual
muscle fibers.
l
p. 138
The collagen fibers of the perimysium and endomysium
are interwoven and blend into one another. At each end of the
muscle, the collagen fibers of the epimysium, perimysium,
and endomysium come together to form either a bundle
known as a
tendon
, or a broad sheet called an
aponeurosis
(ap-¯-noo-RO-sis). Tendons and aponeuroses usually attach
skeletal muscles to bones. Where they contact the bone, the
collagen fibers extend into the bone matrix, providing a firm
attachment. As a result, any contraction of the muscle will ex-
ert a pull on the attached bone (or bones).
Blood Vessels and Nerves
The connective tissues of the endomysium and perimysium
contain the blood vessels and nerves that supply the muscle
fibers. Muscle contraction requires tremendous quantities of
energy. An extensive vascular network delivers the necessary
oxygen and nutrients and carries away the metabolic wastes
generated by active skeletal muscles. The blood vessels and
the nerve supply generally enter the muscle together and fol-
low the same branching course through the perimysium.
Within the endomysium, arterioles supply blood to a capil-
lary network that services the individual muscle fiber.
Skeletal muscles contract only under stimulation from
the central nervous system. Axons, or
nerve fibers
, penetrate
the epimysium, branch through the perimysium, and enter
the endomysium to innervate individual muscle fibers.
Skeletal muscles are often called voluntary muscles, because
we have voluntary control over their contractions. Many
skeletal muscles may also be controlled at a subconscious
level. For example, skeletal muscles involved with breathing,
296
Unit 2
Support and Movement
such as the
diaphragm
, usually work outside our conscious
awareness.
Next, we will examine the microscopic structure of a typ-
ical skeletal muscle fiber and relate that microstructure to the
physiology of the contraction process.
Skeletal muscle fibers are enormous. A muscle fiber from a
thigh muscle could have a diameter of 100 mm and a length
equal to the distance between the tendons at either end (up to
30 cm, or 12 in.). A second obvious difference is that skeletal
muscle fibers are
multinucleate
: Each contains hundreds of nu-
clei just internal to the plasma membrane. The genes in these
nuclei control the production of enzymes and structural pro-
teins required for normal muscle contraction, and the more
copies of these genes, the faster these proteins can be produced.
The distinctive features of size and multiple nuclei are re-
lated. During development, groups of embryonic cells called
myoblasts
(
myo
-, muscle
CHECKPOINT
3. Describe the connective tissue layers associated with
skeletal muscle tissue.
4. How would severing the tendon attached to a muscle
affect the muscle’s ability to move a body part?
See the blue Answers tab at the end of the book.
blastos
, formative cell or germ)
fuse, forming individual multinucleate skeletal muscle fibers
(
Figure 10–2
). Each nucleus in a skeletal muscle fiber reflects
the contribution of a single myoblast. Some myoblasts, how-
ever, do not fuse with developing muscle fibers. These un-
fused cells remain in adult skeletal muscle tissue as the
myosatellite cells seen in
Figures 10–1
and
10–2a
. After an in-
jury, myosatellite cells may enlarge, divide, and fuse with
damaged muscle fibers, thereby assisting in the repair of the
tissue.
10-3
Skeletal muscle fibers have
distinctive features
Skeletal muscle fibers are quite different from the “typical” cells
we described in Chapter 3. One obvious difference is size:
Muscle fibers develop
through the fusion of
mesodermal cells
called
myoblasts
Myoblasts
(a)
LM
612
Nuclei
Sarcolemma
Myofibrils
Myosatellite cell
Nuclei
Mitochondria
Immature
muscle fiber
(b)
Myosatellite cell
Mature muscle fiber
Figure 10–2
The Formation of a Multinucleate Skeletal Muscle Fiber.
(a)
The formation of a muscle fiber by the fusion of myoblasts.
(b)
A diagrammatic view and a micrograph of one muscle fiber.
297
Chapter 10
Muscle Tissue
The Sarcolemma and Transverse Tubules
The
sarcolemma
(sar-k¯-LEM-uh;
sarkos
, flesh
eral properties as the sarcolemma, so electrical impulses con-
ducted by the sarcolemma travel along the T tubules into the
cell interior. These impulses, or
action potentials
, are the trig-
gers for muscle fiber contraction.
lemma
,
husk), or plasma membrane of a muscle fiber, surrounds the
sarcoplasm
(SAR-k¯-plazm), or cytoplasm of the muscle
fiber (
Figure 10–3
). Like other plasma membranes, the sar-
colemma has a characteristic transmembrane potential due to
the unequal distribution of positive and negative charges
across the membrane.
l
p. 99
In a skeletal muscle fiber, a
sudden change in the transmembrane potential is the first
step that leads to a contraction.
Even though a skeletal muscle fiber is very large, all re-
gions of the cell must contract simultaneously. Thus, the sig-
nal to contract must be distributed quickly throughout the
interior of the cell. This signal is conducted through the
transverse tubules.
Transverse tubules
, or
T tubules
, are
narrow tubes that are continuous with the sarcolemma and
extend into the sarcoplasm at right angles to the cell surface
(
Figure 10–3
). Filled with extracellular fluid, T tubules form
passageways through the muscle fiber, like a network of tun-
nels through a mountain. The T tubules have the same gen-
Myofibrils
Inside the muscle fiber, branches of the transverse tubules en-
circle cylindrical structures called
myofibrils
(
Figure 10–3
).
A myofibril is 1–2 mm in diameter and as long as the entire
cell. Each skeletal muscle fiber contains hundreds to thou-
sands of myofibrils.
Myofibrils consist of bundles of protein filaments called
myofilaments
. Two types of myofilaments were introduced in
Chapter 3):
Thin filaments
are composed primarily of actin,
whereas
thick filaments
are composed primarily of myosin.
l
pp. 73, 74
In addition, myofibrils contain
titin
, elastic
myofilaments associated with the thick filaments. (We will
consider the role of titin later in the chapter.)
Myofibrils, which can actively shorten, are responsible for
skeletal muscle fiber contraction. At each end of the skeletal
Myofibril
Sarcolemma
Nuclei
Sarcoplasm
MUSCLE FIBER
Mitochondria
Terminal cisterna
Sarcolemma
Sarcolemma
Sarcoplasm
Myofibril
Myofibrils
Thin filament
Thick filament
Triad
Sarcoplasmic
reticulum
T tubules
Figure 10–3
The Structure of a Skeletal Muscle Fiber.
The internal organization of a muscle fiber.
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