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Human Anatomy, 2nd Edition
McKinley/O’Loughlin
CHAPTER 10
Answers to “What Did You Learn?”
1.
The properties of muscle tissue are excitability, contractility, elasticity, and
extensibility.
2.
The three connective tissue layers are the outer epimysium, the central
perimysium, and the inner endomysium. The epimysium is a layer of dense
irregular connective tissue that surrounds the whole skeletal muscle. The
perimysium is a connective tissue layer that surrounds each fascicle. The
endomysium is the innermost connective tissue layer, and is a thin sheath of
delicate loose connective tissue that surrounds each muscle fiber.
3.
The origin is the stationary or less movable attachment of a muscle, whereas
the insertion is the more movable attachment. In the limbs, the origin
typically lies proximal to the insertion. Usually, the insertion is pulled toward
the origin.
4.
Myofibrils are small, cylindrical structures located within the sarcoplasm of a
skeletal muscle. Myofibrils consist of bundles of thick and thin protein
filaments, generally called myofilaments.
5.
Thick filaments are composed of myosin only. The proteins in thin filaments are
actin, tropomyosin, and troponin.
6.
A neuromuscular junction is the site where the membrane of a motor neuron
fiber and the sarcolemma of the skeletal muscle fiber meet. The ends of the
nerve fibers are called synaptic terminals. Each synaptic terminal overlies a
region of the skeletal muscle fiber membrane called the motor end plate.
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Human Anatomy, 2nd Edition
McKinley/O’Loughlin
Chemical communication between the synaptic terminal of the neuron and
the motor end plate of the skeletal muscle fiber occurs at the neuromuscular
junction.
7.
When the muscle fiber is at rest, troponin holds tropomyosin in place within
the thin filaments to cover the active sites for myosin binding to G-actin.
When a muscle impulse passes along the T-tubule membrane, calcium ions
are released from the terminal cisternae of the sarcoplasmic reticulum, and
some of the calcium binds to a subunit of troponin. This causes a change in
the conformation of the troponin molecule and a resultant movement of the
tropomyosin off of the active sites for myosin on the G-actin molecule.
Consequently, cross-bridges form between the thick and thin filaments, and a
contraction begins. Thin filaments, allowing cross bridges to form between
the thick and thin filaments, and a contraction begins
8.
When a muscle fiber is at rest, tropomyosin molecules prevent the
interactions between thin and thick filaments. Tropomyosin is a doublehelical protein that covers the active sites for myosin on the individual Gactin molecules, thus preventing the actin-myosin interaction necessary for
contraction of a muscle. As a result a muscle impulse, the heads of the
myosin molecules are permitted to bind to active to active sites on G-actin
molecules and then the heads swing toward the center of the A band, pulling
the attached thin filaments toward the center of the A band and shortening the
muscle fiber.
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Human Anatomy, 2nd Edition
McKinley/O’Loughlin
9.
During an isometric contraction, the length of the muscle does not change because
the tension produced by this contracting muscle never exceeds the resistance, so
the muscle does not shorten. In an isotonic contraction, the tension produced
exceeds the resistance, and the muscle fibers shorten, resulting in movement. In
the arm muscles, an isometric contraction occurs when a person tries to lift a
weight that is too heavy, while an isotonic allows a person to lift a book.
10.
In hypertrophy, each muscle fiber develops more myofibrils, and each myofibril
contains a larger number of myofilaments. This results in increased fiber size.
11.
The three types of skeletal muscle fibers are slow (Type I, slow oxidative),
intermediate (Type IIa, fast aerobic), and fast (Type IIb, fast anaerobic). Slow
fibers are usually about half the diameter of fast fibers, contract more slowly, and
are specialized to continue contracting for extended periods of time. Their
vascular supply is more extensive than the network of capillaries around fast
muscle fibers; thus, they receive more nutrients and oxygen. Slow fibers are also
called red fibers because they contain the red pigment myoglobin. Additionally,
they have a relatively large number of mitochondria. This permits slow muscle
fibers to produce a greater amount of ATP than fast-fiber muscles while
contractions are under way, making the cell less dependent on anaerobic
metabolism. Intermediate fibers exhibit properties that are somewhere between
those of fast fibers and slow fibers. The intermediate fibers contract faster than
the slow fibers and slower than the fast fibers. Histologically, they resemble fast
fibers, but they have a greater resistance to fatigue. Fast fibers (white fibers) have
a large diameter, and they contain large glycogen reserves, densely packed
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Human Anatomy, 2nd Edition
McKinley/O’Loughlin
myofibrils, and relatively few mitochondria. They are also called white fibers
because they lack myoglobin. Fast-fiber muscles produce powerful contractions
because they contain a large number of sarcomeres. These contractions use vast
quantities of ATP; therefore, their prolonged activity causes the fast fibers to
rapidly fatigue.
12.
The four main patterns of skeletal muscle fiber organization are circular
muscles, convergent muscles, parallel muscles, and pennate muscles.
13.
An agonist, also called a prime mover, is a muscle that contracts to produce a
particular movement, such as extension of the forearm. The triceps brachii is an
agonist of the forearm causing extension. A muscle that assists the prime mover in
performing its action is a synergist. The contraction of a synergist usually either
contributes to tension exerted close to the insertion of the muscle or stabilizes the point
of origin. Usually, synergists are most useful at the start of a movement when the
agonist is stretched and cannot exert much power. Synergists may also assist an
agonist by preventing movement at a joint and thereby stabilizing the origin of the
agonist. These synergistic muscles are called fixators.
14.
Muscles can be named according to (1) orientation of muscle fibers, (2)
muscle attachments, (3) specific body regions, (4) muscle shape, (5) muscle
size, (6) muscle heads/tendons of origin, (7) muscle function/movement, and
(8) muscle position at body surface.
15.
The gluteus maximus gets its name from (1) the gluteal region of the body
(buttocks) = gluteus and from (2) the shape and size of the muscle = maximus
(largest).
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Human Anatomy, 2nd Edition
McKinley/O’Loughlin
16.
Smooth muscle is composed of short muscle fibers that have a fusiform shape
and a single, centrally located nucleus. Although smooth muscle fibers have
both thick and thin filaments, they are not precisely aligned, so no striations
or sarcomeres are visible. Dense bodies are small concentrations of protein
scattered throughout the sarcoplasm of the smooth muscle fiber and on the
inner face of the sarcolemma. Thin filaments are attached to dense bodies by
elements of the cytoskeleton.
Answers to “Content Review”
1.
The three connective tissue layers are the inner endomysium, the central
perimysium, and the outer epimysium. The endomysium is the innermost
layer and it surrounds each individual muscle fiber. This thin sheath of loose
connective tissue and reticular fibers binds together the neighboring fibers
and supports the capillaries that supply these fibers. The perimysium
surrounds each fascicle. It is a layer of dense irregular connective tissue that
supports and contains neurovascular bundles that branch to supply each
individual fascicle. The epimysium is a layer of dense irregular connective
tissue that surrounds the whole skeletal muscle. Often the epimysium
integrates or blends with the deep fascia between adjacent muscles.
2.
At the ends of a muscle, the connective tissue layers merge to form a fibrous
tendon. A tendon attaches a muscle to bone, skin or another muscle.
Tendons usually have a thick cord or cable-like structure. An aponeurosis
also attaches a muscle to bone, skin or another muscle. However, its structure
is that of a thin, flattened sheet.
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Human Anatomy, 2nd Edition
McKinley/O’Loughlin
3.
The H zone is the lighter central region of the A band. The H zone is lighter in
color because there are only thick filaments here (thin filaments are not present in
the H zone when the muscle fiber is at rest). In the center of the H zone there is a
structure called the M line. It is composed of proteins that serve as an attachment
site for the thick filaments.
4.
The thick filaments do not change in shape. Myosin proteins in the thick
filaments do not shorten. The I band becomes narrower as the thin filaments slide
past the thick filaments. The sarcomere shortens as the Z-discs move toward
each other.
5.
A muscle impulse travels in the membrane of a transverse tubule, causing calcium
ions to be released from the terminal cisternae. When calcium ions enter the
sarcoplasm, some bind to a subunit of troponin. This causes a change in the
conformation of the troponin, resulting in the movement of tropomyosin
molecules off active sites on G-actin molecules. Myosin crossbridges form
between myosin and actin resulting in the pivoting of the myosin head towards
the center of the sarcomere. When ATP then binds to the myosin heads, they
detach from the actin and resume their original orientation toward the edge of the
thick filament.
6.
A motor unit is composed of a single motor neuron and all of the muscle
fibers it controls. The movements of an eye require very precise control;
therefore each motor unit must be small. In contrast, there is much less
precision of movement needed in the postural muscles of the leg, so one
motor neuron will innervate many more muscle fibers
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Human Anatomy, 2nd Edition
McKinley/O’Loughlin
7.
Atrophy is a reduction in muscle size, tone, and power. If a skeletal muscle
experiences markedly reduced stimulation, it loses both muscle mass and tone.
The muscle becomes flaccid and its fibers decrease in size and become weaker.
Causes of muscle atrophy include any situation or condition that causes a muscle
to experience little or no use, such as wearing a cast or paralysis.
Hypertrophy,
in contrast, is an increase in muscle cell size. The repetitive, exhaustive
stimulation of muscle fibers results in an increased number of mitochondria,
larger glycogen reserves and an increased ability to produce ATP. Ultimately,
each muscle cell develops more myofibrils, and each myofibril contains a larger
number of myofilaments. Repeated stimulation of muscles to produce nearmaximal tension, such as experienced during weight lifting, causes muscle
hypertrophy.
8.
Slow fibers contract more slowly than fast fibers, often taking two or three times
as long to contract after stimulation. These fibers are specialized to continue
contracting for extended periods of time. Therefore, an athlete who excels at
sprinting would benefit from having more fast fibers than slow because sprinting
is not a long-term aerobic activity. A successful sprinter would have more fast
fibers due to the powerful contractions they produce because of their large
number of sarcomeres.
9.
Longus means “long”. Extensor indicates that the muscle causes an extension at a
joint. Triceps muscles have three muscular head origins. Rectus means “straight”.
Superficialis muscles are found at body surfaces.
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Human Anatomy, 2nd Edition
McKinley/O’Loughlin
10.
Intercalated discs are complex junctions where cardiac muscle fibers joint
together at their ends. Intercalated discs appear as thick dark lines between
cells in histological sections. At these junctions, the sarcolemmas of adjacent
cardiac muscle fibers interlock through desmosomes and gap junctions.
Desmosomes hold the adjacent membranes together and the gap junctions
facilitate ion passage between cells. This free movement of ions between
cells allows each cardiocyte to transmit an electrical impulse and directly
stimulate its neighbors.
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