Higher Centers
■ FIGURE 6.2 Schematic representation of autonom ic sym pathetic and vagal interconnections within
the central nervous system. Receptor afferent nerve fibers (e.g., from baroreceptors) enter the medulla
at the nucleus tractus solitarius
which projects inhibitory interneurons to the sym pathetic neurons
in the rostral ventrolateral medulla
and excitatory fibers to the vagal neurons in the dorsal vagal
and nucleus ambiguus
The medulla receives input from the hypothalamus and higher
brain centers. Sym pathetic activation (+) of blood vessels and the heart causes sm ooth muscle contrac-
tion (vasoconstriction), increased heart rate (positive chronotropy), increased conduction velocity within
the heart (positive drom otropy), and increased contractility (positive inotropy). Vagal activation of the
heart decreases ( -) chronotropy, drom otropy, and inotropy.
node, whereas the left vagus primarily inner-
vates the AV node. This can be demonstrated
experimentally by electrically stimulating the
right vagus nerve, which causes bradycardia
(or SA nodal arrest) with little change in AV
nodal conduction, as evidenced by a rela-
tively small increase in the P-R interval of the
electrocardiogram. Left vagal stimulation, in
contrast, usually results in a pronounced AV
nodal block (see Chapter 2), with relatively
little decrease in heart rate. However, these
responses to vagal stimulation can be mark-
edly different between individuals because of
crossover of the left and right vagal efferent
Some efferent parasympathetic fibers inner-
vate blood vessels in specific organs in which
they directly or indirectly cause vasodilation.
activation in some tissues (e.g., genitalia erec-
tile tissue) is achieved through the release
of acetylcholine (Ach), which binds to mus-
carinic receptors on the vascular endothelium
to cause vasodilation through the subsequent
formation of nitric oxide (see Chapter 3).
Parasympathetic stimulation causes indirect
vasodilation in some organs (e.g., gastrointes-
tinal circulation) by stimulating nonvascular
tissue to produce vasodilator substances such
as bradykinin, which then binds to vascular
receptors to cause vasodilation. Note that
existing parasympathetic nerves primarily serve
to regulate blood flow within specific organs and
do not play a significant role in the regulation of
systemic vascular resistance and arterial blood
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