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Autonomic Nervous System
The Autonomic Nervous System and Visceral
Sensory Neurons
Autonomic Nervous System
• Concerned with the innervation and control of visceral
organs, smooth muscles and glands
• Along with the endocrine system, its primary function is
homeostasis of the internal environment
• The majority of the activities of the autonomic system do not
impinge on consciousness
• The control exerted by the system is extremely rapid and
widespread
• The visceral receptors include chemoreceptors,
baroreceptors, and osmoreceptors. Ischemia or stretch can
cause extreme pain
• Distributed both in the central and peripheral nervous system
• Like the somatic nervous system, it has sensory (afferent) &
motor (efferent) neurons and interneurons
• The afferent impulses originate in the visceral receptors, travel via
afferent pathways to the CNS and terminate on the sensory
neurons at different levels
• Like the somatic system, the cell bodies of the sensory neurons
are located in the sensory ganglia
sensory ganglia
Somatic
Autonomic
• The efferent pathway is made up of preganglionic and postganglionic
neurons
• The cell bodies of the preganglionic neurons are located in the brain
and spinal cord. Their axons synapse with the postganglionic neurons
whose cell bodies are located in the autonomic ganglia
Comparison of Autonomic and
Somatic Motor Systems
• Autonomic nervous system
– Chain of two motor neurons
• Preganglionic neuron
• Postganglionic neuron
– Conduction is slower due to thinly or
unmyelinated axons
Pre-ganglionic
Post-ganglionic
Ganglion
Autonomic and Somatic Motor
Systems
nicotinic
receptors
always
cholinergic
cholinergic or adrenergic
muscarinic
or
adrenergic
receptors
Somatic motor system
Autonomic motor system
Effector
Skeletal muscle
Cardiac muscle, smooth muscle,
glands
Action on effectors
Always excitatory
May be excitatory or inhibitory
Neurotransmitter
Acetylcholine
Acetylcholine or norepinephrine
Rate of conduction
Rapid due to myelinated
axons
Slower due to thinly myelinated
or unmyelinated axons
Visceral
motor
system
is
different
Type of control
Voluntary
Involuntary
from
somatic
motor
system
in
Chain of two motor neurons:
Neural pathway
One motor neuron extends
Preganglionic &
from
the
CNS
to
skeletal
many
respectsPostganglionic neuron
muscle
• Based on the anatomical, physiological and
pharmacological characteristics, the autonomic nervous
system is divided into:
 Sympathetic: Activated during exercise, excitement,
and emergencies. “fight, flight, or fright”
 Parasympathetic: Concerned with conserving energy.
“rest and digest”
 Both divisions operate in conjunction with one
another (have antagonistic control over the viscera) to
maintain a stable internal environment
Sympathetic
Division
Forms thoracolumbar
outflow: Issues from T1L2 segments of spinal
cord
Ganglia are close to the
CNS (longer
postganglionic fiber)
Each preganglionic fiber
synapses with many
postganglionic neurons
that pass to many
visceral effectors
Parasympathetic Division
• Forms craniosacral
outflow: Issues from brain
& S2-S4 segments of spinal
cord
• Ganglia are near or within
the viscera (longer
preganglionic fiber)
• Each preganglionic fiber
usually synapses with four
or five postganglionic
neurons that pass to a
single visceral effector
Parasympathetic Division
Cranial Outflow
• Emerges from brain
• Preganglionic neurons located in nuclei
of the 3rd,7th, 9th & 10th cranial nerves,
in the brain stem
• Postganglionic fibers are carried by
3rd,7th, 9th & 10th cranial nerves and
innervate organs of the head, neck,
thorax, and abdomen
Sacral Outflow
• Emerges from S2-S4
• Preganglionic neurons located in lateral
horn of spinal gray matter
• Postganglionic fibers carried by pelvic
splanchnic nerves to innervate organs of
the pelvis and lower abdomen
Parasympathetic Ganglia
• Multiple, small, located
nearer the viscera
• Ganglia related to
innervation of:
head & neck: (ciliary,
otic, pterygopalatine
& submandibular).
thoracic, abdominal &
pelvic viscera.
Sympathetic Division
• Thoracolumbar outflow: Emerges from T1-L2
segments of spinal cord
• Preganglionic neurons located in the lateral
gray horn.
• Regardless of target, all begin same
• Preganglionic axons exit spinal cord through
ventral root and enter spinal nerve
• Exit spinal nerve via communicating ramus
• Enter sympathetic trunk/chain where
postganglionic neurons :• Has three options…
Options of preganglionic axons in sympathetic
trunk
1. Synapse on postganglionic neuron in chain
ganglion then return to spinal nerve and follow
its branch to the skin
2. Ascend or descend within sympathetic trunk,
synapse with a postganglionic neuron within a
chain ganglion, and return to spinal nerve at
that level and follow branches to skin
3. Enter sympathetic chain, pass through without
synapsing, form a splanchnic nerve that passes
toward thoracic or abdominal organs
–
–
These synapse in prevertebral ganglion in front of
aorta
Postganglionic axons follow arteries to organs
Sympathetic Pathways to Periphery:Synapse in
chain ganglia at same level or different level
Preganglionic fibers run in the
ventral roots of the spinal
nerve
Travel through the spinal
nerve, and then join the
sympathetic chain via the
white rami communicans.
(myelinated axons) (WRC)
The postganglionic fibers
enter back into the spinal
nerve through grey rami
communicans (GRC)
(nonmyelinated axons)
Copyright © 2005 Pearson Education,
Inc., publishing
as Benjamin
Figure
15.9
Cummings
Pass through ganglia and synapse in
prevertebral ganglion
Sympathetic Ganglia
• Multiple, large in size
• Located nearer the
central nervous system:
• Based on their relation
to the vertebral
column, they are
grouped into:
 Prevertebral
 Paravertebral
Prevertebral Ganglia
• Unpaired, not segmentally
arranged
• Located in abdomen,
anterior to the vertebral
column
• Main ganglia
 Celiac
 Superior mesenteric
 Inferior mesenteric
 Aorticorenal
Paravertebral Ganglia
• Consist of the right and left
sympathetic chains or trunks.
• The chains lie next to the
vertebral column throughout
its length
• There is approximately one
ganglion associated with each
spinal cord segment, except in
the cervical and the sacral
regions.
• The chains end into a common
‘ganglion impar’ in front of
coccyx
 T1 to L2 ventral rami are
connected to the
sympathetic chain via
white rami
communicantes, which
carry preganglionic
sympathetic fibers to the
sympathetic chain
 All the ventral rami
receive postganglionic
sympathetic fibers from
sympathetic chain by a
gray ramus
The Role of the Adrenal Medulla in the
Sympathetic Division
• Major organ of the sympathetic nervous
system
• Secretes great quantities epinephrine (a little
norepinephrine)
• Stimulated to secrete by preganglionic
sympathetic fibers
The Adrenal Medulla
Neurotransmitter of the Autonomic Nervous System
• preganglionic axons
 Acetylcholine for
both divisions
(cholinergic)
• postganglionic axons
 Sympathetic:
mostly
norepinephrine
 Parasympathetic:
acetylcholine
Distribution of Autonomic Fibers
• Both divisions innervate
mostly the same structures &
operate in conjunction with
one another (have
antagonistic control over the
viscus) to maintain a stable
internal environment
Some viscera do not possess
dual control e.g. sweat
glands, adrenal medulla,
erector pili muscles and many
blood vessels have only
sympathetic innervation
Higher Control of the Autonomic Nervous
System
• The hypothalamus has a controlling influence
on the autonomic system and regulates
balance between sympathetic and
parasympathetic activity levels
• The hypothalamus integrates the autonomic
and neuroendocrine systems to preserve body
homeostasis