294
Chapter 9
5. Why is the latent period longer during an isotonic twitch of a
skeletal muscle fi ber than it is during an isometric twitch?
a. Excitation-contraction coupling is slower during an
isotonic twitch.
b. Action potentials propagate more slowly when the fi
ber is
shortening, so extra time is required to activate the entire
ber.
c. In addition to the time for excitation-contraction
coupling, it takes extra time for enough cross-bridges to
attach to make the tension in the muscle fi ber greater than
the load.
d. Fatigue sets in much more quickly during isotonic
contractions, and when muscles are fatigued the cross-
bridges move much more slowly.
e. The latent period is longer because isotonic twitches only
occur in slow (Type I) muscle fi
bers.
6. What prevents a drop in muscle fi ber ATP concentration
during the fi rst few seconds of intense contraction?
a. Because cross-bridges are pre-energized, ATP is not
needed until several cross-bridge cycles have been
completed.
b. ADP is rapidly converted back to ATP by creatine
phosphate.
c. Glucose is metabolized in glycolysis, producing large
quantities of ATP.
d. The mitochondria immediately begin oxidative
phosphorylation.
e. Fatty acids are rapidly converted to ATP by oxidative
glycolysis.
7. Which correctly characterizes a “fast-oxidative” type of
skeletal muscle fi ber?
a. few mitochondria and high glycogen content
b. low myosin ATPase rate and few surrounding capillaries
c. low glycolytic enzyme activity and intermediate contraction
velocity
d. high myoglobin content and intermediate glycolytic enzyme
activity
e. small fi ber diameter and fast onset of fatigue
8. Which is
false
regarding the structure of smooth muscle?
a. The thin fi lament does not include the regulatory protein
troponin.
b. The thick and thin fi laments are not organized in sarcomeres.
c. Thick fi laments are anchored to dense bodies instead of Z
lines.
d. The cells have a single nucleus.
e. Single-unit smooth muscles have gap junctions connecting
individual cells.
9. The role of myosin light-chain kinase in smooth muscle is to
a. bind to calcium ions to initiate excitation-contraction
coupling.
b. phosphorylate cross-bridges, thus driving them to bind with
the thin fi
lament.
c. split ATP to provide the energy for the power stroke of the
cross-bridge cycle.
d. dephosphorylate myosin light chains of the cross-bridge,
thus relaxing the muscle.
e. pump calcium from the cytosol back into the sarcoplasmic
reticulum.
10. Single-unit smooth muscle differs from multiunit smooth
muscle because
a. single-unit muscle contraction speed is slow, while multiunit
is fast.
b. single-unit muscle has T-tubules, multiunit muscle
does not.
c. single-unit muscles are not innervated by autonomic nerves.
d. single-unit muscle contracts when stretched, whereas
multiunit muscle does not.
e. single-unit muscle does not produce action potentials
spontaneously, but multiunit muscle does.
11. Which of the following describes a similarity between cardiac
and smooth muscle cells?
a. An action potential always precedes contraction.
b. The majority of the calcium that activates contraction comes
from the extracellular fl
uid.
c. Action potentials are generated by pacemaker potentials.
d. An extensive system of T-tubules is present.
e. Calcium release and contraction strength are graded.
Chapter 9 Quantitative and Thought Questions
(Answers appear in Appendix A.)
1. Which of the following corresponds to the state of myosin (M)
under resting conditions and in rigor mortis? (a) M · ATP
(b) M · ADP · P
i
(c) A · M · ADP · P
i
(d) A · M
2. If the transverse tubules of a skeletal muscle are disconnected
from the plasma membrane, will action potentials trigger a
contraction? Give reasons.
3. When a small load is attached to a skeletal muscle that is then
tetanically stimulated, the muscle lifts the load in an isotonic
contraction over a certain distance, but then stops shortening
and enters a state of isometric contraction. With a heavier
load, the distance shortened before entering an isometric
contraction is shorter. Explain these shortening limits in terms
of the length-tension relation of muscle.
4. What conditions will produce the maximum tension in a
skeletal muscle fi ber?
5. A skeletal muscle can often maintain a moderate level of active
tension for long periods of time, even though many of its fi bers
become fatigued. Explain.
6. If the blood fl ow to a skeletal muscle were markedly decreased,
which types of motor units would most rapidly undergo a
severe reduction in their ability to produce ATP for muscle
contraction? Why?
7. As a result of an automobile accident, 50 percent of the muscle
fi bers in the biceps muscle of a patient were destroyed. Ten
months later, the biceps muscle was able to generate 80 percent
of its original force. Describe the changes that took place in the
damaged muscle that enabled it to recover.
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