Chapter 18
The Cardiovascular System: The Heart
673
18
2.
Transmission of the depolarization wave down the T tu-
bules (ultimately) causes the sarcoplasmic reticulum (SR)
to release Ca
2
1
into the sarcoplasm.
3.
Excitation-contraction coupling occurs as Ca
2
1
provides
the signal (via troponin binding) for cross bridge activa-
tion and couples the depolarization wave to the sliding of
the myofilaments.
±ese three steps are common to both skeletal and cardiac
muscle cells (see Figure 9.11), but the two muscle types differ in
how the SR is stimulated to release Ca
2
1
. Let’s take a look.
Some 10–20% of the Ca
2
1
needed for the calcium pulse that
triggers contraction enters the cardiac cells from the extracel-
lular space. Once inside, it stimulates the SR to release the other
80% of the Ca
2
1
needed. Ca
2
1
is barred from entering nonstim-
ulated cardiac fibers, but when Na
1
-dependent membrane de-
polarization occurs, the voltage change also opens channels that
allow Ca
2
1
to enter from the extracellular space. ±ese channels
are called
slow Ca
2
1
channels
because their opening is delayed
a bit. ±e local influxes of Ca
2
1
through these channels trigger
opening of nearby Ca
2
1
-sensitive channels in the SR tubules,
which liberates bursts of Ca
2
1
(“calcium sparks”) that dramati-
cally increase the intracellular Ca
2
1
concentration.
Although Na
1
channels have inactivated and repolarization
has begun by this point, the calcium surge across the sarco-
lemma prolongs the depolarization potential briefly, producing
a
plateau
in the action potential tracing (Figure 18.13
2
). At
the same time, few K
1
channels are open, which also prolongs
the plateau and prevents rapid repolarization. As long as Ca
2
1
is
entering, the cells continue to contract. Notice in Figure 18.13
Membrane potential (mV)
Tension (g)
Absolute
refractory
period
–80
–60
–40
–20
0
20
Tension
development
(contraction)
Plateau
Action
potential
0
150
300
Time (ms)
1
1
2
2
3
3
Depolarization
is due to Na
+
influx through fast
voltage-gated Na
+
channels. A positive feedback cycle
rapidly opens many Na
+
channels, reversing the
membrane potential. Channel inactivation ends this
phase.
Plateau phase
is due to Ca
2+
influx through slow
Ca
2+
channels. This keeps the cell depolarized because
few K
+
channels are open.
Repolarization
is due to Ca
2+
channels inactivating
and K
+
channels opening. This allows K
+
efflux, which
brings the membrane potential back to its resting
voltage.
Figure 18.13
The action potential of contractile cardiac muscle cells.
Relationship
between the action potential, period of contraction, and absolute refractory period in a single
ventricular cell.
that muscle tension develops during the plateau, and peaks just
a²er the plateau ends.
Notice also that the action potential and contractile phase lasts
much longer in cardiac muscle than in skeletal muscle. In skeletal
muscle, the action potential typically lasts 1–2 ms and the con-
traction (for a single stimulus) 15–100 ms. In cardiac muscle, the
action potential lasts 200 ms or more (because of the plateau), and
tension development persists for 200 ms or more, providing the
sustained contraction needed to eject blood from the heart.
A²er about 200 ms, the slope of the action potential tracing
falls rapidly (Figure 18.13
3
). ±is repolarization results from
inactivation of Ca
2
1
channels and opening of voltage-gated K
1
channels, which allows a rapid loss of potassium from the cell that
restores the resting membrane potential. During repolarization,
Ca
2
1
is pumped back into the SR and the extracellular space.
Energy Requirements
Cardiac muscle has more mitochondria than skeletal muscle
does, reflecting its greater dependence on oxygen for its energy
metabolism. ±e heart relies almost exclusively on aerobic res-
piration. As a result, cardiac muscle cannot operate effectively
for long without oxygen. ±is is in contrast to skeletal muscle,
which can contract for prolonged periods by carrying out anaer-
obic respiration, and then restore its reserves of oxygen and fuel
using excess postexercise oxygen consumption (EPOC).
Both types of muscle tissue use multiple fuel molecules,
including glucose and fatty acids. But cardiac muscle is much
more adaptable and readily switches metabolic pathways to use
whatever nutrients are available, including lactic acid generated
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