CHAPTER 17The Autonomic Nervous System 269Noradrenergic 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 emergency-
induced 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 feed-
back regulation of blood pressure is effected. In addition, sym-
pathetic 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 fast-
ing 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 inhib-
itory input from several brain regions. Figure 17–6 shows the
source of some forebrain and brain stem descending inputs to
autonomic preganglionic neurons. For example, a major
source of excitatory drive to sympathetic preganglionic neu-
rons 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 analo-
gous 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 di-
gestive tract, all the way from the esophagus to the anus. It is
comprised of two well-organized neural plexuses. The myen-
teric 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 lu-
men 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 “mini-
brain” as it contains all the elements of a nervous system
including sensory neurons, interneurons, and motor neurons.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, va-
nillylmandelic acid. (Courtesy of DS Goldstein.)
NENENEDADOPADOPAC DOPACDHPG DHPGMHPGCOMT
COMTNMNVMAHVATYR[DOPAC] [DHPG] [TYR][NE][NMN][VMA][MHPG][HVA]THUptake–1Sympathetic
nerve endingSmooth
muscle cellMAOMAOMAOBloodstream