301
CHAPTER NINE
Muscular System
f
ber are less developed and store less calcium than those
oF a skeletal muscle f ber. On the other hand, the transverse
tubules oF cardiac muscle f
bers are larger than those in skel-
etal muscle, and they release many calcium ions into the sar-
coplasm in response to a single muscle impulse.
The calcium ions in transverse tubules come From the
fl uid outside the muscle f ber. In this way, extracellular cal-
cium partially controls the strength oF cardiac muscle con-
traction and enables cardiac muscle f
bers to contract longer
than skeletal muscle f
bers can.
Drugs called calcium channel blockers are used to treat irregular heart
rhythms. They do this by blocking ion channels that admit extracellu-
lar calcium into cardiac muscle cells.
The opposing ends oF cardiac muscle cells are connected
by cross-bands called
intercalated discs.
These bands are
complex membrane junctions. Not only do they help join
cells and transmit the Force oF contraction From cell to cell,
but the intercellular junctions oF the Fused membranes oF
intercalated discs allow ions to diFFuse between the cells.
This allows muscle impulses to travel rapidly From cell to
cell (see
f
gs.
5.30 and
9.19
).
When one portion oF the cardiac muscle network is stimu-
lated, the impulse passes to other f bers oF the network, and
the whole structure contracts as a unit (a
syncytium
); that
is, the network responds to stimulation in an all-or-none
manner. Cardiac muscle is also selF-exciting and rhythmic.
Consequently, a pattern oF contraction and relaxation repeats,
generating the rhythmic contraction oF the heart. Also, the
reFractory period oF cardiac muscle is longer than in skeletal
muscle and lasts until the contraction ends. Thus, sustained or
tetanic contractions do not occur in the heart muscle.
Table 9.2
summarizes characteristics oF the three types oF muscles.
PRACTICE
23
How is cardiac muscle similar to skeletal muscle?
24
How does cardiac muscle dif
er From skeletal muscle?
25
What is the Function oF intercalated discs?
26
What characteristic oF cardiac muscle causes the heart to contract
as a unit?
9.7
SKELETAL MUSCLE ACTIONS
Skeletal muscles generate a great variety oF body move-
ments. The action oF each muscle mostly depends upon the
type oF joint it is associated with and the way the muscle is
attached on either side oF that joint.
Body Movement
Whenever limbs or other body parts move, bones and muscles
interact as simple mechanical devices called
levers
(lev
erz).
reactions oF actin and myosin; both are triggered by mem-
brane impulses and release oF calcium ions; and both use
energy From ATP molecules. However, smooth and skel-
etal muscle action also diFFers. ±or example, smooth muscle
f bers lack troponin, the protein that binds to calcium ions in
skeletal muscle. Instead, smooth muscle uses a protein called
calmodulin,
which binds to calcium ions released when its
Fibers are stimulated, activating contraction. In addition,
much oF the calcium necessary For smooth muscle contrac-
tion diFFuses into the cell From the extracellular fl
uid.
Acetylcholine, the neurotransmitter in skeletal muscle, as
well as
norepinephrine,
aFFect smooth muscle. Each oF these
neurotransmitters stimulates contractions in some smooth
muscles and inhibits contractions in others. The discussion oF
the autonomic nervous system in chapter 11 (p. 424) describes
these actions in greater detail.
Hormones aFFect smooth muscles by stimulating or
inhibiting contraction in some cases and altering the degree
oF response to neurotransmitters in others. ±or example, dur-
ing the later stages oF childbirth, the hormone oxytocin stim-
ulates smooth muscles in the wall oF the uterus to contract
(see chapter 23, pp. 899–901).
Stretching oF smooth muscle f bers can also trigger contrac-
tions. This response is particularly important to the Function oF
visceral smooth muscle in the walls oF certain hollow organs,
such as the urinary bladder and the intestines. ±or example,
when partially digested Food stretches the wall oF the intestine,
contractions move the contents Further along the intestine.
Smooth muscle is slower to contract and relax than skel-
etal muscle, yet smooth muscle can ForceFully contract longer
with the same amount oF ATP. Unlike skeletal muscle, smooth
muscle f bers can change length without changing tautness;
because oF this, smooth muscles in the stomach and intesti-
nal walls can stretch as these organs f ll, holding the pressure
inside the organs constant.
PRACTICE
19
Describe the two major types oF smooth muscle.
20
What special characteristics oF visceral smooth muscle make
peristalsis possible?
21
How is smooth muscle contraction similar to skeletal muscle
contraction?
22
How do the contraction mechanisms oF smooth and skeletal
muscles dif
er?
9.6
CARDIAC MUSCLE
Cardiac muscle appears only in the heart. It is composed oF
striated cells joined end to end, Forming f
bers interconnected
in branching, three-dimensional networks. Each cell con-
tains a single nucleus and many f laments oF actin and myo-
sin similar to those in skeletal muscle. A cardiac muscle cell
also has a well-developed sarcoplasmic reticulum, a system
oF transverse tubules, and many mitochondria. However, the
cisternae oF the sarcoplasmic reticulum oF a cardiac muscle
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