Ganong's Review of Medical Physiology, 23rd Edition

CHAPTER 33

Cardiovascular Regulatory Mechanisms 559

wall of the arch of the aorta. The receptors are located in the
adventitia of the vessels. The afferent nerve fibers from the
carotid sinus form a distinct branch of the glossopharyngeal
nerve, the
carotid sinus nerve.
The fibers from the aortic arch
form a branch of the vagus nerve, the
aortic depressor nerve.
The baroreceptors are stimulated by distention of the struc-
tures in which they are located, and so they discharge at an
increased rate when the pressure in these structures rises. Their
afferent fibers pass via the glossopharyngeal and vagus nerves
to the medulla. Most of them end in the nucleus of the tractus
solitarius (NTS), and the excitatory transmitter they secrete is
glutamate (Figure 33–2). Excitatory (glutamate) projections
extend from the NTS to the caudal ventrolateral medulla
(CVLM), where they stimulate
γ
-aminobutyrate (GABA)-
secreting inhibitory neurons that project to the RVLM. Excita-
tory projections also extend from the NTS to the vagal motor
neurons in the nucleus ambiguus and dorsal motor nucleus
(Figure 33–3). Thus, increased baroreceptor discharge
inhibits
the tonic discharge of sympathetic nerves and
excites
the vagal
innervation of the heart. These neural changes produce vasodi-
lation, venodilation, a drop in blood pressure, bradycardia, and
a decrease in cardiac output.

FIGURE 33–3
Basic pathways involved in the medullary
control of heart rate by the vagus nerves.
NTS neurons (dashed
lines) project to and inhibit cardiac preganglionic parasympathetic
neurons primarily in the nucleus ambiguus. Some are also located in
the dorsal motor nucleus of the vagus; however, this nucleus primarily
contains vagal motor neurons that project to the gastrointestinal tract.
AP, area postrema; Pyr, pyramid; XII, hypoglossal nucleus.

Dorsal motor

nucleus

AP

NTS

Nucleus

ambiguus Vagus nerve

Heart

Pyr

XII

TABLE 33–3

Factors affecting heart rate.

Heart rate accelerated by:

Decreased activity of arterial baroreceptors

Increased activity of atrial stretch receptors

Inspiration

Excitement

Anger

Most painful stimuli

Hypoxia

Exercise

Thyroid hormones

Fever

Heart rate slowed by:

Increased activity of arterial baroreceptors

Expiration

Fear

Grief

Stimulation of pain fibers in trigeminal nerve

Increased intracranial pressure

FIGURE 33–4

Baroreceptor areas in the carotid sinus and

aortic arch.

X, sites where receptors are located. The carotid and aor-

tic bodies, which contain chemoreceptors, are also shown.

XX

X

XX

X

XX

XX

XX

Carotid body External

carotid

artery

Carotid

sinus

Aortic

body

Internal

carotid artery

Common

carotid artery

X

XX XXX

XXX

X XX

Left

common

carotid artery

Left

subclavian

artery

Aortic

body

Innominate

artery

Aortic arch

(viewed from behind)