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Chapter 9
SECTION B SUMMARY
Structure of Smooth Muscle
I. Smooth muscle cells are spindle-shaped, lack striations, have
a single nucleus, and are capable of cell division. They contain
actin and myosin fi laments and contract by a sliding-fi lament
mechanism.
Smooth Muscle Contraction and Its Control
I. An increase in cytosolic calcium leads to the binding of
calcium by calmodulin. The calcium-calmodulin complex then
binds to myosin light-chain kinase, activating the enzyme,
which uses ATP to phosphorylate smooth muscle myosin. Only
phosphorylated myosin can bind to actin and undergo cross-
bridge cycling.
II. Smooth muscle myosin has a low rate of ATP splitting,
resulting in a much slower shortening velocity than in striated
muscle. However, the tension produced per unit cross-sectional
area is equivalent to that of skeletal muscle.
III. Two sources of the cytosolic calcium ions that initiate smooth
muscle contraction are the sarcoplasmic reticulum and
extracellular calcium. The opening of calcium channels in the
smooth muscle plasma membrane and sarcoplasmic reticulum,
mediated by a variety of factors, allows calcium ions to enter
the cytosol.
IV. The increase in cytosolic calcium resulting from most stimuli
does not activate all the cross-bridges. Therefore, smooth
muscle tension can be increased by agents that increase the
concentration of cytosolic calcium ions.
V. Table 9–5 summarizes the types of stimuli that can initiate
smooth muscle contraction by opening or closing calcium
channels in the plasma membrane or sarcoplasmic reticulum.
VI. Most, but not all, smooth muscle cells can generate action
potentials in their plasma membrane upon membrane
depolarization. The rising phase of the smooth muscle action
Table 9–6
Characteristics of Muscle Cells
Skeletal Muscle
Smooth Muscle
Cardiac Muscle
Characteristic
Single Unit
Multiunit
Thick and thin fi laments
Yes
Yes
Yes
Yes
Sarcomeres—banding pattern
Yes
No
No
Yes
Transverse tubules
Yes
No
No
Yes
Sarcoplasmic reticulum (SR)*
++++
+
+
++
Gap junctions between cells
No
Yes
Few
Yes
Source of activating calcium
SR
SR and
extracellular
SR and
extracellular
SR and extracellular
Site of calcium regulation
Troponin
Myosin
Myosin
Troponin
Speed of contraction
Fast-slow
Very slow
Very slow
Slow
Spontaneous production
of action potentials by
pacemakers
No
Yes
No
Yes in certain fi bers, but
most not spontaneously
active
Tone (low levels of maintained
tension in the absence of
external stimuli)
No
Yes
No
No
Effect of nerve stimulation
Excitation
Excitation or
inhibition
Excitation or
inhibition
Excitation or
inhibition
Physiological effects of
hormones on excitability and
contraction
No
Yes
Yes
Yes
Stretch of cell produces
contraction
No
Yes
No
No
*Number of plus signs (+) indicates the relative amount of sarcoplasmic reticulum present in a given muscle type.
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