Download Chapter 10 Answer Keys

CHAPTER 10
Answers to “What Did You Learn?”
1.
The properties of muscle tissue are excitability, contractility, elasticity, and
extensibility.
2.
Three concentric layers of connective tissue encircle each muscle fiber, groups of
muscle fibers, and the entire muscle. These layers are the endomysium, the
perimysium, and the epimysium. The endomysium is the innermost connective tissue
layer, and is a delicate, areolar connective tissue layer with reticular fibers to bind
neighboring muscle fibers together. The perimysium is a dense irregular connective
tissue layer that surrounds bundles of muscle fibers (fascicles). The epimysium is a
layer of dense irregular connective tissue that surrounds the whole skeletal muscle.
3.
The origin is the less mobile attachment of a muscle, whereas the insertion is the more
mobile attachment. Usually, the insertion is pulled toward the origin. In the limbs, the
origin typically lies proximal to the insertion. Sometimes neither origin nor insertion
can be determined easily by movement or position.
4.
Myofibrils are long, cylindrical structures located within the sarcoplasm of a skeletal
muscle fiber. Each myofibril extends the length of the entire muscle fiber, and it
consists of bundles of thick and thin protein filaments called myofilaments.
5.
Thick filaments are composed of bundles of the protein myosin only. The thin
filaments are primarily composed of two strands of the protein actin. Two regulatory
proteins, tropomyosin and troponin, are part of the thin filaments.
6.
A neuromuscular junction is the site where the membranes of a nerve fiber (motor
neuron axon) and muscle fiber (the sarcolemma of the skeletal muscle fiber) meet.
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The expanded tips of the nerve fibers are called synaptic knobs. Each synaptic knob
covers a region of the skeletal muscle fiber membrane called the motor end plate.
Chemical communication between the synaptic knob of the motor neuron and the
motor end plate of the skeletal muscle fiber occurs at the neuromuscular junction.
7.
Calcium ions bind to subunits of the regulatory protein, troponin, causing a change in
the conformation of the troponin molecule. This shape-change moves the
tropomyosin molecule causing exposure 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.
8.
During a muscle contraction, the heads of the myosin molecules bind to active sites on
the thin filament G-actin molecules. Following this binding, the myosin heads swing
toward the center of the A band. This pulls the thin filaments to where the heads are
attached, toward the center of the A band, resulting in the shortening of the muscle
fiber.
9.
The length of the muscle does not change during an isometric contraction because the
tension produced by this contracting muscle never exceeds the resistance (load), thus
the muscle does not shorten. In an isotonic contraction, the tension produced exceeds
the resistance, so the muscle fibers shorten, resulting in movement. An isometric
contraction occurs in arm muscles when a person tries to lift a weight that is too heavy,
while an isotonic allows a person to lift a book.
10.
The three types of skeletal muscle fibers are slow oxidative (type I), fast oxidative
(type IIa), and fast glycolytic (type IIb). Slow oxidative fibers are usually about half
the diameter of fast glycolytic fibers, contract more slowly, and are specialized to
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continue contracting for extended periods of time. Their vascular supply is extensive,
thus, they receive more nutrients and oxygen. Slow oxidative fibers are also called red
fibers because they contain myoglobin, the red oxygen binding pigment. They have a
large number of mitochondria and can produce a greater amount of ATP than fastfiber muscles. These fibers have high aerobic capacity. Fast oxidative fibers exhibit
properties that range between those of slow oxidative fibers and fast glycolytic fibers.
The fast oxidative fibers contract faster than the slow oxidative fibers and slower than
the fast glycolytic fibers. Histologically, fast oxidative fibers resemble fast glycolytic
fibers, but they have a greater resistance to fatigue. Fast glycolytic fibers have a larger
diameter than fast oxidative fibers and they contain large glycogen reserves, densely
packed myofibrils, and relatively few mitochondria. They are also called white fibers
because they lack myoglobin. Fast glycolytic fiber muscles produce powerful
contractions because they contain a large number of sarcomeres. These contractions
use vast quantities of ATP, but they use anaerobic mechanisms to make their ATP, so
their prolonged activity causes the fast fibers to rapidly fatigue.
11.
The four main patterns of skeletal muscle fiber organization are circular muscles,
parallel muscles, convergent muscles, and pennate muscles.
12.
Each fast muscle fiber increases its number of myofibrils during hypertrophy and each
myofibril contains a larger number of myofilaments. Additionally, there is an increase
in mitochondria number and larger glycogen reserves. This results in increased fiber
diameter.
13.
An agonist is called a prime mover. It is a muscle that contracts to produce a
particular movement, such as extension of the forearm. The triceps brachii is an
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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. Synergists are useful at the start of a movement when the agonist is
stretched and cannot exert much power. Also, they may assist an agonist by
preventing movement at a joint and thereby, stabilizing the origin of the agonist.
14.
Muscles can be named according to (1) muscle action, (2) specific body regions, (3)
muscle attachments, (4) orientation of muscle fibers, (5) muscle shape and size, and
(6) muscle heads/tendons of origin.
15.
The gluteus maximus gets its name from (1) the gluteal region of the body gluteus =
(buttocks) and from (2) the size of the muscle = maximus (largest).
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
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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.
The connective tissue layers merge at the end of a muscle to form a fibrous tendon that
attaches a muscle to bone, skin, or another muscle. Tendons usually have a thick cord
or cablelike structure. An aponeurosis is a thin, flattened tendon sheet that also
attaches a muscle to bone, skin, or another muscle.
3.
Skeletal muscle has alternating dark and light striations as a result of size and density
differences between the thick and thin protein filaments that compose the muscle fiber.
The A bands are dark bands that contain the entire thick filament. The H zone is the
lighter central region of the A band. It appears lighter because there are only thick
filaments here (thin filaments have only overlapped the peripheral regions of the thick
filaments) when the muscle fiber is at rest. In the center of the H zone, there is a
structure called the M line. It is a transverse protein meshwork in the center of the H
zone in relaxed fibers. The M line is an attachment site for thick filaments to keep
them aligned during contraction and relaxation.
4.
Neither the thick nor thin filaments change length. They merely slide past each other
during contraction. The I band is the light band composed of thin filaments only. It
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 down the transverse tubules, causing calcium ions to be
released from the terminal cisternae. When calcium ions enter the sarcoplasm of the
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cell, some bind to a subunit of troponin. This causes a change in the conformation
(shape) of the troponin, resulting in the movement of tropomyosin molecules off
active sites on G-actin molecules of the thin filament. Myosin crossbridges form
between myosin and actin. Then the myosin head pivots pulling the actin thin
filament toward the center of the sarcomere. Thereafter, ATP then binds to the
pivoted myosin heads, which then detach from the actin. Now the ATP is broken
down to ADP and Pi and the myosin heads resume their original orientation projecting
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. An inverse relationship exists between motor unit size and degree of control
needed. The movements of an eye require very precise control thus, each motor unit
must be small. In contrast, much less precision of movement is needed in the postural
muscles of the leg, so one motor neuron will innervate many more muscle fibers.
7.
Atrophy is a reduction in muscle size, tone, and power. Both muscle mass and tone
are lost if a skeletal muscle experiences markedly reduced stimulation. The muscle
becomes flaccid when its fibers decrease in size and become weaker. Causes of
muscle atrophy include any situation or condition that causes decrease or lack of
muscle use, such as the wearing of a cast or paralysis. Hypertrophy is an increase in
muscle cell size. The repetitive, exhaustive stimulation of muscle fibers results in an
increased number of mitochondria, accumulation of larger glycogen reserves, and an
increased ability to produce ATP. Each muscle cell develops more myofibrils, which
contain a larger number of myofilaments. Repeated stimulation of muscles to produce
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near-maximal tension, such as experienced during weight lifting, causes muscle
hypertrophy.
8.
Slow oxidative 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 without fatigue. Therefore, an athlete who
runs short sprints would benefit from having more fast glycolytic fibers than slow
because sprinting is not a long-term aerobic activity. A successful sprinter would have
more fast fibers due to the quick powerful contractions they produce.
9.
Longus means “long.” Extensor indicates that the muscle causes an “extension” at a joint.
Triceps muscles have “three muscular heads.” Rectus means “straight.” Superficialis
muscles are found towards body “surfaces.”
10.
Cardiac muscle cells are individual cells arranged in thick bundles within the heart
wall. These cells are autorhythmic, meaning that individual cells can generate a
muscle impulse without nervous stimulation. They are short, thick, and striated. Each
cell has Y-shaped branches and is connected to adjacent cells by special structures
called intercalated discs. Intercalated discs are complex junctions where cardiac
muscle fibers join together at their ends. They 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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