Chapter 14
The Autonomic Nervous System
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14
postsynaptic membranes in the brain help to relieve depression.
Hundreds of over-the-counter drugs used to treat colds, coughs,
allergies, and nasal congestion contain
sympathomimetics
sympathetic-mimicking drugs such as phenylephrine that stim-
ulate
a
-adrenergic receptors. Tis constricts the blood vessels in
the nasal mucosa, inhibiting secretions.
Much pharmaceutical research is directed toward finding
medications that affect only one subclass of receptor without
upsetting the whole adrenergic or cholinergic system. An im-
portant breakthrough was finding drugs that mainly activate
β
2
receptors. People with asthma use such β
2
activators to dilate
their lung bronchioles without activating β
1
receptors, which
would increase their heart rate.
Table 14.3
summarizes selected
drug classes that influence ANS activity.
Interactions of the Autonomic Divisions
As we mentioned earlier, most visceral organs receive
dual in-
nervation
from both the parasympathetic and sympathetic divi-
sions. Normally, both ANS divisions are partially active. Action
potentials continually fire down both sympathetic and parasym-
pathetic axons, producing a dynamic antagonism that precisely
controls visceral activity. However, one division or the other usu-
ally predominates in given circumstances, and in a few cases,
the two divisions actually cooperate with each other.
Table 14.4
contains an organ-by-organ summary of how their effects differ.
respond to NE (or to epinephrine) have one or more of these
receptor subtypes.
NE or epinephrine can be either excitatory or inhibitory
depending on which subclass of receptor predominates in the
target organ. For example, NE binding to the β
1
receptors of car-
diac muscle prods the heart into more vigorous activity, whereas
epinephrine binding to β
2
receptors in bronchiole smooth mus-
cle causes it to relax, dilating the bronchiole.
The Effects of Drugs
Knowing the locations of the cholinergic and adrenergic re-
ceptor subtypes allows clinicians to prescribe specific drugs to
stimulate or inhibit selected target organs.
For example,
atropine
is an anticholinergic drug that blocks
muscarinic ACh receptors. It is routinely administered before
surgery to prevent salivation and dry up respiratory system se-
cretions. Ophthalmologists also administer atropine to dilate
the pupils for eye examination.
Te anticholinesterase drug
neostigmine
inhibits the enzyme
acetylcholinesterase that breaks down ACh. Tis allows ACh to
accumulate in synapses. Neostigmine is used to treat myasthe-
nia gravis, a condition in which inadequate ACh stimulation
impairs skeletal muscle activity.
As we described in Chapter 11, NE is one of our “feel good”
neurotransmitters, and drugs that prolong the activity of NE on
Table 14.3
Selected Drug Classes That Influence the Autonomic Nervous System
DRUG CLASS
RECEPTOR BOUND
EFFECTS
EXAMPLE
CLINICAL APPLICATION
Nicotinic agents (little
therapeutic value, but
important for public
health because tobacco
contains nicotine)
Nicotinic ACh receptors
on all postganglionic
neurons and in CNS
Typically stimulates
sympathetic effects;
blood pressure rises
Nicotine
Smoking cessation products
Parasympathomimetic
agents (muscarinic
agents)
Muscarinic ACh
receptors
Enhance
parasympathetic activity
by mimicking effects of
ACh
Pilocarpine
Glaucoma (opens aqueous
humor drainage pores)
 
 
 
Bethanechol
Difficulty urinating (increases
bladder contraction)
Acetylcholinesterase
inhibitors
None; bind to the
enzyme (AChE) that
degrades ACh
Indirect effect at all ACh
receptors; prolong the
effect of ACh
Neostigmine
Myasthenia gravis (increases
availability of ACh)
 
 
 
Sarin
Used as chemical warfare
agent (similar to widely used
insecticides)
Sympathomimetic
agents
Adrenergic receptors
Enhance sympathetic
activity by binding to
adrenergic receptors or
increasing NE release
Albuterol (Ventolin)
Asthma (dilates bronchioles by
binding to
b
2
receptors)
 
 
 
Phenylephrine
Colds (nasal decongestant, binds
to
a
1
receptors)
Sympatholytic agents
Adrenergic receptors
Decrease sympathetic
activity by blocking
adrenergic receptors or
inhibiting NE release
Propranolol
Hypertension (member of
a class of drugs called
beta-
blockers
that block
b
receptors,
decreasing blood pressure)
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