568
UNIT FOUR
The electrical changes that accompany ventricular mus-
cle f
ber repolarization slowly produce a
T wave
as the pen
defl
ects again, ending the ECG pattern (f g. 15.21
g
and
h
).
The record oF the atrial repolarization seems to be missing
From the pattern because the atrial f bers repolarize at the
same time that the ventricular f bers depolarize. Thus, the
QRS complex obscures the recording oF the atrial repolar-
ization. The graph in
f
gure 15.22
summarizes some oF the
changes that occur during a cardiac cycle with correspond-
ing ECG patterns and heart sounds.
In addition to the waves that comprise the classic electrocardiogram
are repeating subpatterns of other waves that occur at diF
erent time-
scales and in an irregular pattern. Although it may seem counterintui-
tive, this complex, varying backdrop to the cardiac cycle seems to be
necessary for health. It is disrupted in congestive heart failure.
Physicians use ECG patterns to assess the heart’s abil-
ity to conduct impulses. ±or example, the period between
the beginning oF a P wave and the beginning oF a QRS com-
plex called the
PQ interval
(or iF the initial portion oF the
QRS wave is upright, the
PR interval
) indicates the time For
the cardiac impulse to travel From the SA node through the
AV node. Ischemia or other problems aFFecting the f bers oF
the AV conduction pathways can increase this PQ interval.
Similarly, injury to the AV bundle can extend the QRS com-
plex, because it may take longer For an impulse to spread
throughout the ventricular walls
(f
g. 15.23)
.
PRACTICE
25
What is an electrocardiogram?
26
Which cardiac events do the P wave, QRS complex, and T wave
represent?
Regulation of the Cardiac Cycle
The volume oF blood pumped changes to accommodate cel-
lular requirements. ±or example, during strenuous exercise,
skeletal muscles require more blood, and heart rate increases
in response. The SA node normally controls heart rate, so
changes in this rate oFten involve Factors that aFFect the pace-
maker, such as the motor impulses carried on the parasym-
pathetic and sympathetic nerve f
bers (see
f gs.
11.38, 11.39,
15.24
, 15.37, and 15.38).
The parasympathetic f
bers that innervate the heart arise
From neurons in the medulla oblongata and make up parts oF
the
vagus nerves.
Most oF these f
bers branch to the SA and
AV nodes. When the nerve impulses reach nerve f
ber end-
ings, they secrete acetylcholine, which decreases SA and AV
nodal activity. As a result, heart rate decreases.
The vagus nerves continually carry impulses to the SA and
AV nodes, “braking” heart action. Consequently, parasympa-
thetic activity can change heart rate in either direction. An
Electrocardiogram
An
electrocardiogram
(e-lek
tro-kar
de-o-gram
) (ECG) is a
recording oF the electrical changes in the myocardium dur-
ing a cardiac cycle. (This pattern occurs as action potentials
stimulate cardiac muscle f
bers to contract, but it is not the
same as individual action potentials.) Because body fl uids
can conduct electrical currents, such changes can be detected
on the surFace oF the body.
To record an ECG, electrodes are placed on the skin and
connected by wires to an instrument that responds to weak
electrical changes by moving a pen or stylus on a moving
strip oF paper. Up-and-down movements oF the pen corre-
spond to electrical changes in the myocardium. The paper
moves past the pen at a known rate, so the distance between
pen defl ections indicates time elapsing between phases oF
the cardiac cycle.
A normal ECG pattern includes several deFlections, or
waves,
during each cardiac cycle, as
f gure 15.21
a
illustrates.
Between cycles, the muscle f bers remain polarized, with no
detectable electrical changes. The pen does not move and
marks along the baseline. When the SA node triggers a cardiac
impulse, the atrial f bers depolarize, producing an electrical
change. The pen moves, and at the end oF the electrical change,
returns to the base position. This f rst pen movement produces
a
P wave,
corresponding to depolarization oF the atrial f bers
that will lead to contraction oF the atria (f g. 15.21
b–d
).
When the cardiac impulse reaches the ventricular f
bers,
they rapidly depolarize. The ventricular walls are thicker
than those oF the atria, so the electrical change is greater,
and the pen defl ects more. When the electrical change ends,
the pen returns to the baseline. This leaves a mark called the
QRS complex,
which usually consists oF a
Q wave,
an
R wave,
and an
S wave.
The complex appears due to depolarization
oF the ventricular f bers just prior to the contraction oF the
ventricular walls (f
g. 15.21
e
and
f
).
Myocardial
muscle fibers
(a)
(b)
FIGURE 15.20
The muscle ±
bers within the ventricular walls form
whorled patterns. The ±
bers of groups (
a
) and (
b
) surround both
ventricles in these anterior views of the heart.
previous page 598 David Shier Hole's Human Anatomy and Physiology 2010 read online next page 600 David Shier Hole's Human Anatomy and Physiology 2010 read online Home Toggle text on/off