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
axon and the sarcolemma of the skeletal muscle fiber meet. The ends of the
nerve fibers are called synaptic knobs. Each synaptic knob overlies a region
of the skeletal muscle fiber membrane called the motor end plate.
10-1
Chemical communication between the synaptic knob of the neuron and the
motor end plate of the skeletal muscle fiber occurs at the neuromuscular
junction.
7.
Calcium ions bind to subunits of troponin causing a change in the
conformation of the troponin molecule and move 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.
8.
During a muscle contraction, the heads of the myosin molecules bind 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.
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.
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 extensive, thus, they receive more nutrients and oxygen. Slow
fibers are also called red fibers because they contain the red oxygen binding
10-2
pigment myoglobin. Additionally, they have a relatively large number of
mitochondria, permitting these fibers to produce a greater amount of ATP than
fast-fiber muscles 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 have a large diameter, and they
contain large glycogen reserves, densely packed 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 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, convergent muscles, parallel muscles, and pennate muscles.
12.
In hypertrophy, each muscle fiber develops more myofibrils, and each myofibril
contains a larger number of myofilaments. This results in increased fiber
diameter.
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
10-3
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 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
10-4
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 structu re
is that of a thin, flattened sheet.
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.
Neither the thick nor thin filaments change length.
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 down the transverse tubules, 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
10-5
on G-actin molecules. Myosin crossbridges form between myosin and actin. The
myosin head pivots toward the center of the sarcomere pulling the actin. 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 thus,
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
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 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 near-maximal 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
10-6
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 towards body surfaces.
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.
10-7