402 Chapter 12
- Which of these statements about cross bridges is false?
a. They are composed of myosin.
b. They bind to ATP after they detach from actin.
c. They contain an ATPase.
d. They split ATP before they attach to actin.- When a muscle is stimulated to contract, Ca^2 1 binds to
a. myosin. c. actin.
b. tropomyosin. d. troponin.- Which of these statements about muscle fatigue is false?
a. It may result when ATP is no longer available for the
cross-bridge cycle.
b. It may be caused by a loss of muscle cell Ca^2 1.
c. It may be caused by the accumulation of extracellular K^1.
d. It may be a result of lactic acid production.- Which of these types of muscle cells are not capable of
spontaneous depolarization?
a. Single-unit smooth muscle
b. Multiunit smooth muscle
c. Cardiac muscle
d. Skeletal muscle
e. Both b and d
f. Both a and c - Which of these muscle types is striated and contains gap
junctions?
a. Single-unit smooth muscle
b. Multiunit smooth muscle
c. Cardiac muscle
d. Skeletal muscle - In an isotonic muscle contraction,
a. the length of the muscle remains constant.
b. the muscle tension remains constant.
c. both muscle length and tension are changed.
d. movement of bones does not occur.- Which of the following is an example of an eccentric muscle
contraction?
a. Doing a “curl” with a dumbbell
b. Doing a breast stroke in a swimming pool
c. Extending the arms when bench-pressing a weight
d. Flexing the arms when bench-pressing to allow the
weight to return to the chest - Which of the following statements about the Ca^2 1 release
channels in the sarcoplasmic reticulum is false?
a. They are also called ryanodine receptors.
b. They are one-tenth the size of the voltage-gated Ca^2 1
channels.
c. They are opened by Ca^2 1 release channels in the
transverse tubules.
d. They permit Ca^2 1 to diffuse into the sarcoplasm from the
sarcoplasmic reticulum.
18. Which of the following statements is not characteristic of
smooth muscles?
a. Myosin phosphatase is required for contraction.
b. They are able to conduct graded depolarizations.
c. They can enter a latch state.
d. They can produce graded contractions in response to
graded depolarizations.
Test Your Understanding- Using the concept of motor units, explain how skeletal
muscles in vivo produce graded and sustained contractions. - Describe how an isometric contraction can be converted
into an isotonic contraction using the concepts of motor unit
recruitment and the series-elastic component of muscles. - Explain why the myosin heads don’t bind to the actin
when the muscle is at rest. Then, provide a step-by-
step explanation of how depolarization of the muscle
fiber plasma membrane by ACh leads to the binding of
the myosin heads to actin. (That is, explain excitation-
contraction coupling.) - Using the sliding filament theory of contraction, explain
why the contraction strength of a muscle is maximal at a
particular muscle length. - Explain why muscle tone is first decreased and then
increased when descending motor tracts are damaged. How
is muscle tone maintained? - Explain the role of ATP in muscle contraction and muscle
relaxation. - Why are all the muscle fibers of a given motor unit of the
same type? Why are smaller motor units and slow-twitch
muscle fibers used more frequently than larger motor units
and fast-twitch fibers? - What changes occur in muscle metabolism as the intensity
of exercise is increased? Describe the changes that occur as
a result of endurance training and explain how these changes
allow more strenuous exercise to be performed before the
onset of muscle fatigue. - Compare the mechanism of excitation-coupling in striated
muscle with that in smooth muscle. - Compare cardiac muscle, single-unit smooth muscle, and
multiunit smooth muscle with respect to the regulation of
their contraction.
Test Your Analytical Ability- Your friend eats huge helpings of pasta for two days prior to
a marathon, claiming such “carbo loading” is of benefit in
the race. Is he right? What are some other things he can do
to improve his performance? - Compare muscular dystrophy and amyotrophic lateral
sclerosis (ALS) in terms of their causes and their effects on
muscles. - Why is it important to have a large amount of stored high-
energy phosphates in the form of creatine phosphate for the
function of muscles during exercise? What might happen to
a muscle in your body if it ever ran out of ATP?