Ganong's Review of Medical Physiology, 23rd Edition

Noradrenergic discharge also leads to elevated plasma glucose
and free fatty acid levels (supplying more energy). On the basis
of effects like these, Walter Cannon called the emergencyinduced
discharge of the noradrenergic nervous system the
“preparation for flight or fight.”
The emphasis on mass discharge in stressful situations should
not obscure the fact that the sympathetic fibers also subserve
other functions. For example, tonic sympathetic discharge to
the arterioles maintains arterial pressure, and variations in this
tonic discharge are the mechanism by which carotid sinus feedback
regulation of blood pressure is effected. In addition, sympathetic
discharge is decreased in fasting animals and increased
when fasted animals are refed. These changes may explain the
decrease in blood pressure and metabolic rate produced by fasting
and the opposite changes produced by feeding.
DESCENDING INPUT
TO AUTONOMIC
PREGANGLIONIC NEURONS
As is the case for α-motor neurons, the activity of autonomic
nerves is dependent on both reflexes (eg, baroreceptor and
chemoreceptor reflexes) and descending excitatory and inhibitory
input from several brain regions. Figure 17–6 shows the
source of some forebrain and brain stem descending inputs to
NE
NE
CHAPTER 17 The Autonomic Nervous System 269
DHPG DHPG
DOPAC
DA
COMT
DOPAC
COMT
[DOPAC]
MAO
[DHPG]
Sympathetic
nerve ending
DOPA
TYR
TH
Uptake–1
FIGURE 17–5 Catecholamine metabolism in the sympathetic nervous system. COMT, catechol-O-methyltransferase; DA, dopamine;
DHPG, dihydroxyphenylglycol; DOPA, dihydroxyphenylalanine; DOPAC, dihydroxyphenylacetic acid; HVA, homovanillic acid; MHPG, 3-methoxy-
4-hydroxyphenylglycol; MOA, monoamine oxidase; NE, norepinephrine; NMN, normetanephrine; TH, tyrosine hydroxylase; TYR, tyrosine; VMA, vanillylmandelic
acid. (Courtesy of DS Goldstein.)
NE
NMN
MHPG
VMA
HVA
[TYR] [NE] [NMN] [VMA] [MHPG] [HVA]
Bloodstream
MAO
Smooth
muscle cell
MAO
autonomic preganglionic neurons. For example, a major
source of excitatory drive to sympathetic preganglionic neurons
comes from the rostral ventrolateral medulla. Although
not shown, medullary raphé neurons project to the spinal cord
to inhibit or excite sympathetic activity. In addition to these
direct pathways to preganglionic neurons, there are many
brain stem nuclei that feed into these pathways. This is analogous
to the control of somatomotor function by areas such as
the basal ganglia.
ENTERIC NERVOUS SYSTEM
The enteric nervous system, which can be considered as the
third division of the ANS, is located within the wall of the digestive
tract, all the way from the esophagus to the anus. It is
comprised of two well-organized neural plexuses. The myenteric
plexus is located between longitudinal and circular layers
of muscle; it is involved in control of digestive tract motility.
The submucosal plexus is located between the circular muscle
and the luminal mucosa; it senses the environment of the lumen
and regulates gastrointestinal blood flow and epithelial
cell function.
The enteric nervous system contains as many neurons as
the entire spinal cord. It is sometimes referred to as a “minibrain”
as it contains all the elements of a nervous system
including sensory neurons, interneurons, and motor neurons.