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Chapter 9
The Autonomic
Nervous System
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Chapter 9 Outline
Introduction
ANS
Neurons
Divisions of ANS
ANS Neurotransmitters
ANS Innervation of Organs
Higher Control of ANS
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Introduction
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Overview
Autonomic
nervous system (ANS) manages our
physiology
By regulating organs and organ systems, and their
smooth muscles and glands
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ANS Control of Smooth Muscle
Smooth
muscle maintains a resting tone in absence of
nerve stimulation
Smooth becomes more sensitive when ANS input is
cut (=denervation hypersensitivity)
Many types of smooth are spontaneously active and
contract rhythmically without ANS input
ANS input simply increases or decreases intrinsic
activity
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ANS Neurons
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Autonomic Neurons
 ANS
has 2 neurons in
its efferent pathway
 1st neuron
(=preganglionic
neuron) has cell body
in brain or spinal cord
 Synapses with 2nd
neuron
(=postganglionic
neuron) in an
autonomic ganglion
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Autonomic Neurons
 Postganglionic
axon
extends from
autonomic ganglion to
target tissue
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Divisions of the ANS
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Divisions of the ANS
ANS
has sympathetic and parasympathetic divisions
Which usually have antagonistic effects
These coordinate physiology with what’s going on in
person's life
Sympathetic mediates "fight, flight, and stress"
reactions
Parasympathetic mediates "rest and digest"
reactions
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Sympathetic Division
 Is
also called thoracolumbar division because its preganglionics
exit spinal cord from T1 to L2
 Most then synapse on postganglionics in the paravertebral
ganglia
 Which form chain of interconnected ganglia paralleling
spinal cord
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Sympathetic Division continued
Is
characterized by divergence and convergence
which cause Symp to mostly act as a unit (=mass
activation)
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Sympathetic Division continued
Divergence:
preganglionics branch to synapse with a
number of postganglionic neurons
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Sympathetic Division continued
Convergence:
postganglionics receive synaptic input
from a large number of preganglionics
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Sympathetic Division continued
 Some
postganglionics
do not synapse in
paravertebral
ganglion but go to
outlying collateral
ganglion
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Sympathoadrenal System
The
adrenal medulla, located in adrenal gland on top
of kidney, appears to be a modified collateral ganglion
Its secretory cells appear to be modified
postganglionics
That release 85% epinephrine (Epi) and 15%
norepinephrine (Norepi) into blood in response to
preganglionic stimulation
Adrenal is stimulated during mass activation
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Sympathoadrenal System continued
Epi
is made by methylating Norepi
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Parasympathetic Division
Is
also called the craniosacral division because its long
preganglionics originate in midbrain, medulla, pons,
and S2 - S4
These synapse on postganglionics in terminal
ganglia located next to or within a target organ
Postganglionics have short axons that innervate
target
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Parasympathetic Division continued
 The
long vagus nerve
carries most Parasymp
fibers
 Innervates heart,
lungs, esophagus,
stomach, pancreas,
liver, small intestine,
and upper half of the
large intestine
 Preganglionic fibers from
S2-4 innervate lower half
of large intestine, rectum,
urinary and reproductive
systems
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ANS Overview
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ANS Neurotransmitters
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ANS Neurotransmitters
 Both
Symp and Parasymp
preganglionics release
ACh
 Parasymp postganglionics
also release ACh
 Called cholinergic
synapses
 Most Symp
postganglionics release
Norepi (noradenaline)
 Called adrenergic
synapses
 A small number release
ACh
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ANS Neurotransmitters continued
Postganglionics
have unusual
synapses called
varicosities
Which
release NTs
along a
length of
axon
= synapses
en passant
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Adrenergic Stimulation
 Causes
both excitation and inhibition depending on tissue
 Because of different subtypes of receptors for same NT
 2 major subtypes are  and  adrenergic receptors
 Each has own subtypes: 1, 2 and 1, 2
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Adrenergic Stimulation continued
 Many
useful drugs
have been developed
to affect ANS
receptors
 Drugs that
promote actions of
a NT are agonists
 Drugs that inhibit
actions of a NT
are antagonists
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Cholinergic Stimulation
ACh
is used at all motor neuron synapses on skeletal
muscle, all preganglionics, and Parasymp
postganglionics
Cholinergic receptors have 2 subtypes:
Nicotinic which is stimulated by nicotine; blocked by
curare
And muscarinic which is stimulated by muscarine
(from poisonous mushrooms); blocked by atropine
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Other ANS NTs
Some
postganglionics do not use Norepi or ACh
Called nonadrenergic, noncholinergic fibers
Appear to use ATP, VIP, or NO as NTs
NO produces smooth muscle relaxation in many
tissues
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ANS Innervation of Organs
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Organs With Dual Innervation
Most
visceral organs receive dual innervation
(supplied by both Symp and Parasymp)
The 2 branches are usually antagonistic, e.g. in
controlling heart rate
But can be complementary (cause similar
effects), e.g. in controlling salivation
Or cooperative (produce different effects that
work together to cause desired effect) such as
with micturition
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Organs Without Dual Innervation
Regulation
is achieved by increasing or decreasing
firing rate
E.g. adrenal medulla, arrector pili muscle, sweat
glands, and most blood vessels receive only
sympathetic innervation
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Higher Control of ANS
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Control of the ANS by Higher Brain Centers
The
medulla oblongata most directly controls activity of
ANS
It has centers for control of cardiovascular,
pulmonary, urinary, reproductive, and digestive
systems
Hypothalamus has centers for control of body
temperature, hunger, and thirst; and can regulate
medulla
Limbic system is responsible for visceral responses
that reflect emotional states
Cerebral cortex and cerebellum also influence ANS
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