672
UNIT 4
Maintenance of the Body
18
the heart. Te bulk of heart muscle, however, is composed of
contractile muscle
fibers responsible for the heart’s pumping ac-
tivity. In these contractile muscle cells, the sequence of events
leading to contraction is similar to that in skeletal muscle fibers:
1.
Depolarization opens a few
fast voltage-gated Na
1
chan-
nels
in the sarcolemma, allowing extracellular Na
1
to
enter. Tis influx initiates a positive feedback cycle that
causes the rising phase of the action potential (and re-
versal of the membrane potential from
2
90 mV to nearly
1
30 mV; Figure 18.13
1
). Te period of Na
1
influx is
very brief, because the sodium channels quickly inactivate
and the Na
1
influx stops.
fibers, contractions last 15–100 ms with brief refractory
periods of 1–2 ms. Contrast this to cardiac muscle cells,
in which the absolute refractory period lasts over 200 ms,
nearly as long as the contraction
(Figure 18.13)
. Te long
cardiac refractory period normally prevents tetanic con-
tractions, which would stop the heart’s pumping action.
Having probed the major differences between cardiac and
skeletal muscle tissues, let’s now look at their similarities. As
with skeletal muscle, cardiac muscle contraction is triggered
by action potentials that sweep across cell membranes. About
1% of cardiac fibers are
autorhythmic
(“self-rhythmic”), having
the special ability to depolarize spontaneously and thus pace
Nucleus
Nucleus
Nucleus
Desmosomes
Gap junctions
Intercalated discs
I band
A band
Cardiac muscle cell
Cardiac muscle cell
Sarcolemma
Z disc
Mitochondrion
Mitochondrion
T tubule
Sarcoplasmic
reticulum
(a)
I band
Intercalated disc
(b)
Figure 18.12
Microscopic anatomy of cardiac muscle.
(a)
Photomicrograph of cardiac
muscle (600
3
). Notice that the cardiac muscle cells are short, branched, and striated. The dark-
staining areas are intercalated discs, or junctions, between adjacent cells.
(b)
Components of
intercalated discs and cardiac muscle fibers.
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