preferentially innervates the sinoatrial (SA)
node, whereas the left vagus nerve inner-
vates the AV node; however, significant over-
lap can occur in the anatomical distribution.
Atrial muscle is also innervated by vagal
efferents; the ventricular myocardium is only
sparsely innervated by vagal efferents. Sym-
pathetic efferent nerves are present through-
out the atria (especially in the SA node) and
ventricles, and in the conduction system of
the heart.
Vagal activation of the heart decreases heart
rate (negative chronotropy), decreases con-
duction velocity (negative dromotropy), and
decreases contractility (negative inotropy) of
the heart. Vagal-mediated inotropic influences
are moderate in the atria and relatively weak
in the ventricles. Activation of the sympa-
thetic nerves to the heart increases heart rate,
conduction velocity, and inotropy.
thetic influences are pronounced in both the
atria and ventricles.
As Chapter 6 describes in more detail, the
heart also contains vagal and sympathetic
afferent nerve fibers that relay information
from stretch and pain receptors. The stretch
receptors are involved in feedback regula-
tion of blood volume and arterial pressure,
whereas the pain receptors produce chest pain
when activated during myocardial ischemia.
Cardiac Cycle Diagram
To understand how cardiac function is regu-
lated, one must know the sequence of mechan-
ical events during a complete cardiac cycle
and how these mechanical events relate to
the electrical activity of the heart. The cardiac
cycle diagram in Figure 4.2 (sometimes called
the Wiggers diagram) depicts changes in the
left side of the heart (left ventricular pressure
and volume, left atrial pressure, and aortic
pressure) as a function of time. Although not
shown in this figure, pressure and volume
changes in the right side of the heart (right
atrium and ventricle and pulmonary artery)
are qualitatively similar to those in the left
side. Furthermore, the timing of mechanical
events in the right side of the heart is very
similar to that of the left side. The main differ-
ence is that the pressures in the right side of
the heart are much lower than those found in
the left side. For example, the right ventricu-
lar pressure typically changes from about 0 to
4 mm Hg during filling to a maximum of 25 to
30 mm Hg during contraction.
A catheter can be placed in the ascending
aorta and left ventricle to obtain the pressure
and volume information shown in the cardiac
cycle diagram and to measure simultaneous
changes in aortic and intraventricular pres-
sure as the heart beats. This catheter can also
be used to inject a radiopaque contrast agent
into the left ventricular chamber. This per-
mits fluoroscopic imaging (contrast ventricu-
lography) of the ventricular chamber, from
which estimates of ventricular volume can be
obtained; however, real-time echocardiogra-
phy and nuclear imaging of the heart are more
commonly used to obtain clinical assessment
of volume and function.
In the following discussion, a complete
cardiac cycle is defined as the cardiac events
initiated by the P wave in the electrocardio-
gram (ECG) and continuing until the next
P wave. The cardiac cycle is divided into two
general categories: systole and diastole. Sys-
tole refers to events associated with ventricu-
lar contraction and ejection. Diastole refers to
the rest of the cardiac cycle, including ventric-
ular relaxation and filling. The cardiac cycle
is further divided into seven phases, begin-
ning when the P wave appears. These phases
are atrial systole, isovolumetric contraction,
rapid ejection, reduced ejection, isovolumet-
ric relaxation, rapid filling, and reduced fill-
ing. The events associated with each of these
phases are described below.
Phase 1. Atrial Systole: AV Valves
Open; Aortic and Pulmonic
Valves Closed
The P wave of the ECG represents electrical
depolarization of the atria, which initiates
contraction of the atrial musculature. As the
atria contract, the pressures within the atrial
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