Ganong's Review of Medical Physiology, 23rd Edition

NTS
Dorsal motor
nucleus
XII
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.
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 structures
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. Excitatory
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 vasodilation,
venodilation, a drop in blood pressure, bradycardia, and
a decrease in cardiac output.
AP
Pyr
Nucleus
ambiguus Vagus nerve
Heart
CHAPTER 33 Cardiovascular Regulatory Mechanisms 559
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
Internal
carotid artery
Left
common
carotid artery
Left
subclavian
artery
Aortic
body
Aortic arch
(viewed from behind)
Carotid body External
carotid
artery
X X X X
X X X X X
X X X
Common
carotid artery
X
X
X X X X
X X X
X X X
Carotid
sinus
Aortic
body
Innominate
artery
FIGURE 33–4 Baroreceptor areas in the carotid sinus and
aortic arch. X, sites where receptors are located. The carotid and aortic
bodies, which contain chemoreceptors, are also shown.