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
b. Action potentials propagate more slowly when the ﬁ
shortening, so extra time is required to activate the entire
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
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
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
b. ADP is rapidly converted back to ATP by creatine
c. Glucose is metabolized in glycolysis, producing large
quantities of ATP.
d. The mitochondria immediately begin oxidative
e. Fatty acids are rapidly converted to ATP by oxidative
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
d. high myoglobin content and intermediate glycolytic enzyme
e. small ﬁ ber diameter and fast onset of fatigue
8. Which is
regarding the structure of smooth muscle?
a. The thin ﬁ lament does not include the regulatory protein
b. The thick and thin ﬁ laments are not organized in sarcomeres.
c. Thick ﬁ laments are anchored to dense bodies instead of Z
d. The cells have a single nucleus.
e. Single-unit smooth muscles have gap junctions connecting
9. The role of myosin light-chain kinase in smooth muscle is to
a. bind to calcium ions to initiate excitation-contraction
b. phosphorylate cross-bridges, thus driving them to bind with
the thin ﬁ
c. split ATP to provide the energy for the power stroke of the
d. dephosphorylate myosin light chains of the cross-bridge,
thus relaxing the muscle.
e. pump calcium from the cytosol back into the sarcoplasmic
10. Single-unit smooth muscle differs from multiunit smooth
a. single-unit muscle contraction speed is slow, while multiunit
b. single-unit muscle has T-tubules, multiunit muscle
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
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
(c) A · M · ADP · P
(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
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.