Muscle
293
potential is due to the infl ux of calcium ions into the cell
through voltage-gated calcium channels.
VII. Some smooth muscles generate action potentials
spontaneously, in the absence of any external input, because
of pacemaker potentials in the plasma membrane that
repeatedly depolarize the membrane to threshold. Slow
waves are a pattern of spontaneous, periodic depolarization
of the membrane potential seen in some smooth muscle
pacemaker cells.
VIII. Smooth muscle cells do not have a specialized end-plate
region. A number of smooth muscle cells may be infl uenced
by neurotransmitters released from the varicosities on a
single nerve ending, and a single smooth muscle cell may be
infl uenced by neurotransmitters from more than one neuron.
Neurotransmitters may have either excitatory or inhibitory
effects on smooth muscle contraction.
IX. Smooth muscles can be classifi ed broadly as single-unit or
multiunit smooth muscle.
Cardiac Muscle
I. Cardiac muscle combines features of skeletal and smooth
muscles. Like skeletal muscle, it is striated, is composed of
myofi brils with repeating sarcomeres, has troponin in its thin
fi laments, has T-tubules that conduct action potentials, and
has sarcoplasmic reticulum lateral sacs that store calcium.
Like smooth muscle, cardiac muscle cells are small and
single-nucleated, arranged in layers around hollow cavities,
and connected by gap junctions.
II. Cardiac muscle excitation-contraction coupling involves
entry of a small amount of calcium through L-type calcium
channels, which triggers opening of ryanodine receptors that
release a larger amount of calcium from the sarcoplasmic
reticulum. Calcium activates the thin fi lament and cross-
bridge cycling as in skeletal muscle.
III. Cardiac contractions and action potentials are prolonged,
tetany does not occur, and both the strength and
frequency of contraction are modulated by autonomic
neurotransmitters and hormones.
SECTION B REVIEW QUESTIONS
1. How does the organization of thick and thin fi laments in
smooth muscle fi bers differ from that in striated muscle fi
bers?
2. Compare the mechanisms by which a rise in cytosolic calcium
concentration initiates contractile activity in skeletal, smooth,
and cardiac muscle cells.
3. What are the two sources of calcium that lead to the increase in
cytosolic calcium that triggers contraction in smooth muscle?
4. What types of stimuli can trigger a rise in cytosolic calcium in
smooth muscle cells?
5. What effect does a pacemaker potential have on a smooth
muscle cell?
6. In what ways does the neural control of smooth muscle activity
differ from that of skeletal muscle?
7. Describe how a stimulus may lead to the contraction of a
smooth muscle cell without a change in the plasma membrane
potential.
8. Describe the differences between single-unit and multiunit
smooth muscles.
9. Compare and contrast the physiology of cardiac muscle with
that of skeletal and smooth muscles.
10. Explain why cardiac muscle cannot undergo tetanic
contractions.
dense body
284
intercalated disk
290
latch state
286
L-type calcium channel
290
multiunit smooth muscle
289
myosin light-chain kinase
285
myosin light-chain
phosphatase
286
pacemaker potential
287
single-unit smooth muscle
289
slow waves
287
smooth muscle tone
287
varicosity
288
IV. Table 9–6 summarizes and compares the features of skeletal,
smooth, and cardiac muscles.
SECTION B KEY TERMS
Chapter 9 Test Questions
(Answers appear in Appendix A.)
1. Which is a false statement about skeletal muscle structure?
a. A myofi bril is composed of multiple muscle fi bers.
b. Most skeletal muscles attach to bones by connective-tissue
tendons.
c. Each end of a thick fi lament is surrounded by six thin
fi l a m e n t s .
d. A cross-bridge is a portion of the myosin molecule.
e. Thin fi laments contain actin, tropomyosin, and troponin.
2. Which is correct regarding a skeletal muscle sarcomere?
a. M lines are found in the center of the I band.
b. The I band is the space between one Z line and the next.
c. The H zone is the region where thick and thin fi laments
overlap.
d. Z lines are found in the center of the A band.
e. The width of the A band is equal to the length of a thick
fi l a m e n t .
3. When a skeletal muscle fi ber undergoes a concentric isotonic
contraction:
a. M lines remain the same distance apart.
b. Z lines move closer to the ends of the A bands.
c. A bands become shorter.
d. I bands become wider.
e. M lines move closer to the end of the A band.
4. During excitation-contraction coupling in a skeletal muscle fi ber
a. the Ca
2+
-ATPase pumps calcium into the T-tubule.
b. action potentials propagate along the membrane of the
sarcoplasmic reticulum.
c. calcium fl oods the cytosol through the dihydropyridine
(DHP) receptors.
d. DHP receptors trigger the opening of lateral sac ryanodine
receptor calcium channels.
e. acetylcholine opens the DHP receptor channel.
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