copyright © john wiley & sons, inc. all rights reserved. chapter 15 the autonomic nervous...
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Copyright © John Wiley & Sons, Inc. All rights reserved.
Chapter 15
The Autonomic
Nervous SystemLecture slides prepared by Curtis DeFriez, Weber State University
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Introduction to the ANS In this chapter, we examine the structural and
functional features of the autonomic nervous
system (ANS) and compare the organization and
actions of its two major parts, the sympathetic and
parasympathetic divisions.
The autonomic nervous system contributes to
homeostasis by responding to subconscious
visceral sensations and exciting or inhibiting
smooth muscle, cardiac muscle, and many glands.
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Introduction to the ANS Structurally, the ANS includes autonomic sensory
neurons, integrating centers in the CNS, and
autonomic motor neurons.
The enteric division is a specialized network of
nerves and ganglia forming an independent
nerve network within the wall of the
gastrointestinal (GI) tract. The enteric division
will not be further discussed in this chapter, but
we will return to it in Chapter 24.
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Introduction to the ANS While both the ANS and the somatic nervous system
(SNS) include sensory and motor neurons, the ANS
has many distinctive features which set it apart.
Perhaps the biggest difference between these two
systems is the involvement of conscious control.
• In the SNS, feedback via tactile, thermal, pain, and
proprioceptive sensations are consciously
perceived, and skeletal muscle is the main tool
used to provide reflexive and voluntary movement.
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Introduction to the ANS If a somatic motor neuron ceases to stimulate a
muscle, the result is a paralyzed, limp muscle that has no
tone.
Although we are generally not conscious of breathing,
the muscles that generate respiratory movements are
skeletal muscles controlled by somatic motor neurons.• If the respiratory motor neurons become
inactive, breathing stops.
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Introduction to the ANS The ANS usually operates without conscious control,
though centers in the hypothalamus and brain stem
do provide regulation for ANS reflexes.
Sensory receptors called interoceptors located in
blood vessels, visceral organs, muscles, and the
nervous system monitor conditions in the internal
environment.
• Examples of interoceptors are chemoreceptors
that monitor blood CO2 level and
mechanoreceptors that detect the degree of
stretch in the walls of organs or blood vessels.
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ANS Motor Pathways Autonomic motor neurons regulate visceral activities
by either increasing (exciting) or decreasing
(inhibiting) ongoing activities in their effector tissues.
Because autonomic responses cannot be consciously
altered to any great degree, some autonomic
responses are the basis for polygraph (“lie detector”)
tests.
However, practitioners of yoga and biofeedback
techniques may learn how to regulate at least some
of their autonomic activities through long practice.
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Introduction to the ANS The anatomy of all autonomic pathways can
best be understood by picturing a double-
barrelled neuronal construct consisting of
a preganglionic neuron
leading to an intermediate
ganglion that contains
the cell bodies of post-
ganglionic neurons
(that innervate an effector).
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Introduction to the ANS
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Introduction to the ANSInteractions Animation
The ANS: An Introduction Animation
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Most body organs have dual ANS
innervation; that is, they receive impulses
from both sympathetic and
parasympathetic neurons.
Usually the nerve impulses from one
division stimulate an organ, while impulses
from the other division decrease activity.
Divisions of the ANS
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Divisions of the ANS
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Divisions of the ANS
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Divisions of the ANS Furthermore, the responses of the various
organs to ANS stimulation neatly group into
two functional categories :
Like children on a teeter-totter, the
sympathetic divisions “fight or flight”
response is balanced against the “rest
and relax” (or rest and digest)
activities of the parasympathetic
division.
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The Sympathetic Division The cell bodies of neurons which participate in motor
responses of the sympathetic nervous system are
located
in the lateral horns of the gray matter in the 12 thoracic
segments and the first two lumbar segments of the
cord.
Sympathetic preganglionic neurons exit the spinal cord
only between levels T1-L2 (hence the name
thoracolumbar division), though sympathetic ganglia
extend in the vicinity of the cord from the cervical to the
sacral region.
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Some of the major groups of sympathetic
ganglia include:
The sympathetic trunk
(vertebral chain) ganglia
Prevertebral ganglia
The celiac, superior
mesenteric, inferior
mesenteric, aorticorenal
and renal ganglia
The Sympathetic Division
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The Sympathetic Division Axons leave the sympathetic trunk in four possible ways:
They can enter and travel with spinal nerves.
They can form fine networks of periarterial
preganglionic traveling cephalad to synapse in the
cervical ganglia.
Postganglionic axons exiting the sympathetic trunk
can form sympathetic nerves to the heart and lungs.
Preganglionic axons can leave the sympathetic trunk
without synapsing and form splanchnic nerves.
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The Sympathetic Division
Major groups of sympathetic
ganglia.
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The Sympathetic Division A truism of the sympathetic division is that a
single sympathetic preganglionic fiber synapses
with many postganglionic branches (with 20 or
more) to create a diverging circuit.
The postganglionic axons typically terminate
in several different visceral effectors, making
the effects of sympathetic stimulation a
widespread massive response.
• This is why anger can be hard to control – it is
such a diffuse response.
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The Sympathetic Division This schematic
illustrates the
outflow of the
sympathetic
division of the ANS
via thoracolumbar
pathways to the
many organs of the
body.
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The Parasympathetic Division The cell bodies of preganglionic neurons which
participate in motor responses of the
parasympathetic nervous system are located in
nuclei of 4 cranial nerves in the brainstem (III, VII, IX
and X) and in the lateral gray matter of sacral areas
of the spinal cord (S2-S4).
The vagus nerve (CN X) carries nearly 80% of
the total parasympathetic flow to the organs of the
thorax and upper abdomen. Lower abdominal and
pelvic organs are innervated by the sacral output.
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The Parasympathetic Division Parasympathetic ganglia are called terminal
ganglia because they are located far from their
origin at the “terminal” ends of the pathways (near
the target organs).
Four pairs of cranial parasympathetic ganglia
innervate structures in the head: The ciliary,
pterygopalatine, submandibular, and otic ganglia.
• The cranial-sacral division also has the ganglia
associated with the vagus (X) nerve and the
sacral nerves.
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Most of the parasympathetic ganglia are located very close to the organs or intended action.
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The sacral preganglionic axons branch off of sacral
spinal nerves to form pelvic splanchnic nerves
which synapse with parasympathetic postganglionic
neurons located in terminal ganglia in the walls
of the innervated viscera.From the terminal ganglia,
postganglionic axons innervate
smooth muscle and glands in
the walls of the colon, ureters,
urinary bladder, and
reproductive organs
The Parasympathetic Division
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The Parasympathetic Division In contrast to the sympathetic system, the
parasympathetic response is more
controlled.
Presynaptic parasympathetic neurons usually
synapse with only 4–5 postsynaptic neurons, all
of which supply a single visceral effector.
Parasympathetic stimulation leads to a narrow,
focused action on specific organs.
• This is why it is possible to walk and chew
gum at the same time (not really!)
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The division of the sympathetic and parasympathetic divisions of the ANS are compared in Table 15.3
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ANS Neurotransmitters The total number of neurotransmitters used in the entire
nervous system is not known, but is well over 100.
Despite the variety of possible chemicals that could be
used to transmit chemical messages in the ANS, only
2, acetylcholine and norepinephrine, are used to
any great degree.
• Synapses at which ACh is used are termed
cholinergic.
• Synapses at which norepinephrine or epinephrine
are used are termed adrenergic.
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ANS Neurotransmitters The neurotransmitter used in all of the synapses of
sympathetic and parasympathetic ganglia
(between the synapses of the preganglionic and
postganglionic fibers) is acetylcholine.
Receptors that respond to Ach released by these
cholinergic neurons are called cholinergic
receptors and there are 2 subtypes: nicotinic
receptors (found in the ganglia) and
muscarinic receptors (found in the synapses
with the effector organs).
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ANS Neurotransmitters
Acetylcholine acts on a sub-type of cholinergic receptor (called nicotinic receptors) at ganglia of the ANS.
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ANS Neurotransmitters The neurotransmitter used at
most sympathetic
postganglionic synapses is
norepinephrine.
The exception to this rule
is that ACh is used at
sympathetic
postganglionic synapses
for sweat glands.
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ANS Neurotransmitters The neurotransmitter
used at all
parasympathetic
postganglionic
synapses
is Ach.
These are all a variety
of cholinergic
receptors
called muscarinic.
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ANS Neurotransmitters
Neurons and Neurotransmitters of the Parasympathetic Nervous System
Preganglionic Postganglionic
Cell body in brain or spinal cord
Cell body in intramural ganglion
Acetylcholine (ACh) Acetylcholine (ACh)
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ANS NeurotransmittersNeurons and Neurotransmitters ofthe Sympathetic Nervous System
Preganglionic Postganglionic
oCell body in lateral horn of ospinal cord
Cell body in sympathetico chain
gangliono
oAcetylcholine (ACh)
(norepinephrine, NE) ol
except sweat glands (Ach) o
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ANS NeurotransmittersInteractions Animation
ANS Neurotransmitters and Neurons Anima
tion
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Physiology of the ANS Sympathetic stimulation leads to secretion of
norepinephrine by the adrenal glands, an
increase in the rate and strength of the
heartbeat, constriction of blood vessels of non-
essential organs, dilation of vessels of essential
organs (skeletal muscle and the cerebral
cortex), an increase in the rate and depth of
breathing, hepatic conversion of glycogen to
glucose, and decrease in GI activity.
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Physiology of the ANS
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Physiology of the ANS
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Physiology of the ANS
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Physiology of the ANS SLUDD is as an acronym used to describe the
responses of the parasympathetic nervous system:
Salivation (increased)
Lacrimation (increased)
Urination (increased)
Digestion (increased)
Defecation (increased)
• … and 3 decreases (in the rate and force of the
heart beat, airway size and rate of breathing, and
pupil size)
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Physiology of the ANSInteractions Animation
The balance of autonomic sympathetic-parasympathetic
tone is regulated by feedback loops between the spinal
cord and brainstem, with input from the limbic system
and oversight by the hypothalamus.
Physiological Effects of the ANS Animation
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