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Chapter 18
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Chapter 18 Outline
• Comparison of the Somatic and Autonomic
Nervous Systems
• Overview of the Autonomic Nervous System
• Parasympathetic Division
• Sympathetic Division
• Other Features of the Autonomic Nervous
System
• CNS Control of Autonomic Function
• Development of the Autonomic Nervous
System
Autonomic Nervous System
• The autonomic nervous system (ANS)
is a complex system of nerves that
govern involuntary actions.
• The ANS works constantly with the
somatic nervous system (SNS) to
regulate body organs and maintain
normal internal functions.
Autonomic Nervous System
• The ANS and SNS are part of both the
central nervous system and the
peripheral nervous system.
• The SNS operates under our conscious
control. The ANS functions are
involuntary and we are usually unaware
of them.
Comparison of Somatic and
Autonomic Nervous Systems
Figure 18.1
Comparison of Somatic and
Autonomic Nervous Systems
• Both the SNS and the ANS use sensory and
motor neurons.
• In the SNS, somatic motor neurons innervate
skeletal muscle fibers, causing conscious
voluntary movement.
• ANS motor neurons innervate smooth muscle
fibers, cardiac muscle fibers, or glands.
• ANS motor neurons can either excite or
inhibit cells in the viscera.
Comparison of Somatic and
Autonomic Nervous Systems
• SNS—single lower motor neuron axon
extends uninterrupted from the spinal cord to
one or more muscle fibers
• ANS—two-neuron chain innervates muscles
and glands
Components of the Autonomic
Nervous System
Figure 18.2
Comparison of Somatic and
Autonomic Motor Nervous Systems
Two-Neuron Chain in ANS
• The first neuron in the ANS pathway is the
preganglionic neuron. Its cell body is in the
brain or spinal cord.
• A preganglionic axon extends to the second
cell body housed within an autonomic
ganglion in the peripheral nervous system.
• The second neuron in the pathway is called a
ganglionic neuron.
• A postganglionic axon extends from its cell
body to effector (target) cells.
Two-Neuron Chain in ANS
• Neuronal convergence—axons from
numerous preganglionic cells synapse
on a single ganglionic cell
• Neuronal divergence—axons from one
preganglionic cell synapse on numerous
ganglionic cells
Divisions of Autonomic
Nervous System
• Parasympathetic division—conservation of
energy and replenishment of nutrient stores
(“rest-and-digest”)
• Sympathetic division—preparation of body
for emergencies (“fight-or-flight”); increased
activity of this division results in increased
alertness and metabolic activity
Comparison of Parasympathetic
and Sympathetic Divisions
Figure 18.3
Anatomic Differences Between
Parasympathetic and Sympathetic Neurons
Figure 18.4
Comparison of Sympathetic
and Parasympathetic Divisions
Parasympathetic Division
• Termed craniosacral division because
preganglionic neurons are housed within
nuclei in the brainstem and within lateral gray
regions of the S2–S4 spinal cord segments
• Ganglionic neurons are found in either
terminal ganglia close to the target organ, or
intramural ganglia in the wall of the target
organ
Overview of Parasympathetic
Pathways
Figure 18.5
Four Cranial Nerves of
Parasympathetic Division
1.
2.
3.
4.
Oculomotor (CN III)
Facial (CN VII)
Glossopharyngeal (CN IX)
Vagus (CN X)
Oculomotor Nerve (CN III)
• Preganglionic cell bodies housed in
nuclei in mesencephalon
• Postganglionic cell bodies located in
ciliary ganglion within the orbit
• Postganglionic axons project to the
ciliary muscle and to the pupillary
constrictor muscle of iris
Facial Nerve (VII)
Two branches of preganglionic axons in
the facial nerve (VII) exit the pons:
1. greater petrosal nerve
2. chorda tympani
Facial Nerve (VII)
Greater petrosal nerve:
• terminates at the pterygopalatine
ganglion
• postganglionic axons project to
lacrimal glands and small glands of
nasal cavity, oral cavity, and palate
Facial Nerve (VII)
Chorda tympani:
• terminates at the submandibular
ganglion
– postganglionic axons project to
submandibular and sublingual
salivary glands in the floor of the
mouth
Glossopharyngeal
Nerve (CN IX)
• Preganglionic axons exit brainstem
• Preganglionic axons branch and
synapse on ganglionic neurons in the
otic ganglion
• Postganglionic axons cause increase in
secretion from parotid salivary glands
Vagus Nerve (CN X)
• Projects inferiorly through neck to
supply innervation to thoracic organs
and most abdominal organs
• Multiple branches extend to thoracic
organs to increase mucous production
and decrease diameter of airways, and
to decrease heart rate and force of
heart contraction
Vagus Nerve (CN X)
• Vagal trunks pass through diaphragm,
associate with the abdominal aorta, and
project branches to ganglia located
adjacent to or within wall of target
organs
• Increases smooth muscle motility and
secretory activity in digestive tract
organs
Spinal Sacral Nerves
• Preganglionic neuron cell bodies are housed
within lateral gray regions of S2–S4 spinal
cord segments.
• Preganglionic axons branch to form pelvic
splanchnic nerves, which contribute to the
superior and inferior hypogastric plexus.
• Preganglionic fibers project to ganglionic
neurons within terminal or intramural ganglia
of large intestine, rectum, reproductive
organs, urinary bladder, and distal ureter.
Parasympathetic Division
Outflow
Sympathetic Division
• Also termed thoracolumbar division because
preganglionic neuron cell bodies are housed
in lateral horn between first thoracic (T1) and
second lumbar (L2) spinal segments
• Preganglionic axons travel with somatic
motor neuron axons to exit the spinal cord
and enter the anterior roots and then the T1–
L2 spinal nerves
Right and Left
Sympathetic Trunks
• Sympathetic trunks are located anterior to
spinal nerves and immediately lateral to
vertebral column.
• Sympathetic trunk ganglia (paravertebral or
chain ganglia) house sympathetic ganglionic
neuron cell bodies. One sympathetic ganglion
is approximately associated with each spinal
nerve.
• Cervical portion of each sympathetic trunk is
partitioned into three ganglia: the superior,
middle, and inferior ganglia.
Overview of Sympathetic
Pathways
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Preganglionic
Postganglionic
Eye
Blood vessels and
sweat glands of head
Salivary glands
Blood vessels
Heart
Right
Cardiac and
pulmonary
plexuses
Left
Superior cervical ganglion
Middle cervical ganglion
Inferior cervical ganglion
T1
T1
T1
Lesser thoracic splanchnic nerve
T2
T2
T3
T3
T4
T4
T5
T5
T6
T6
T7
T7
T8
T8
T9
T9
T10
T10
T11
T11
T12
T12
L1
L1
Lung
Celiac ganglion
Greater thoracic splanchnic nerve
Liver and
gallbladder
Stomach
Spleen
Postganglionic fibers
to skin, blood vessels
L2
L2
Adrenal medulla
Kidney
Superior
mesenteric
ganglion
Ureter (proximal)
Pancreas
Large intestine
Small intestine
Inferior
mesenteric
ganglion
Rectum
Ureter (distal)
L2
Least thoracic splanchnic nerve
Lumbar splanchnic nerves
Spinal cord
Sacral splanchnic nerves
Sympathetic chain ganglia
L3
Hypogastric plexus
L5
Vas deferens
Seminal vesicle
Prostate
S1
S2
Ovary
Figure 18.6
Bladder
L4
Uterus
Testis
Sympathetic Trunk
Figure 18.7
Cervical Sympathetic Ganglia
• Postganglionic axons from cell bodies in the
superior cervical ganglion distribute to
structures in the head and neck. Axons
innervate sweat glands, smooth muscle in
blood vessels, dilator pupillae muscle of the
eye, and the superior tarsal muscle of the
eye.
• Middle and inferior cervical ganglia house
neuron cell bodies that extend postganglionic
axons to the thoracic viscera.
Rami Communicantes
• Connect sympathetic trunk to each
spinal nerve
• Preganglionic sympathetic axons of T1–
L2 spinal nerves are carried by white
rami communicantes (or white rami)
Rami Communicantes
• Postganglionic sympathetic axons are carried
from the sympathetic trunk to the spinal nerve
by gray rami communicantes (or gray
rami). Gray rami connect to all spinal nerves
including cervical, sacral, and coccygeal
spinal nerves.
• Preganglionic axons of white rami are
myelinated and postganglionic axons of gray
rami are unmyelinated.
Splanchnic Nerves
• Composed of preganglionic sympathetic
axons that did not synapse in a
sympathetic trunk ganglion
• Run anteriorly from sympathetic trunk to
most of viscera
Splanchnic Nerves
• Larger nerves are preganglionic axons that
extend from sympathetic trunk ganglia:
–
–
–
–
–
Greater thoracic splanchnic nerves: T5–T9
Lesser thoracic splanchnic nerves: T10–T11
Least thoracic splanchnic nerves: T12
Lumbar splanchnic nerves: L1 and L2
Sacral splanchnic nerves: sacral sympathetic
ganglia
Prevertebral (Collateral)
Ganglia
• Splanchnic nerves typically terminate in
prevertebral ganglia.
• Ganglia are unpaired and immediately
anterior to the vertebral column on the
anterolateral surface of the aorta in the
abdominopelvic cavity.
Prevertebral (Collateral)
Ganglia
• Ganglia typically cluster around the
major abdominal arteries and are
named for the arteries.
• Sympathetic postganglionic axons
extend away from the ganglionic neuron
cell bodies in the ganglia and innervate
many of the abdominal organs.
Prevertebral Ganglia
Prevertebral ganglia include:
• Celiac ganglion
• Superior mesenteric ganglion
• Inferior mesenteric ganglion
Celiac Ganglion
• Location—adjacent to origin of celiac
artery
• Preganglion axons—greater thoracic
splanchnic nerves (T5–T9 segment of
spinal cord)
• Postganglionic axons—innervate
stomach, spleen, liver, gallbladder,
proximal duodenum, part of pancreas
Superior Mesenteric Ganglion
• Location—adjacent to origin of superior
mesenteric artery
• Preganglionic axons—lesser and least
thoracic splanchnic nerves (T10–T12
segment of spinal cord)
• Postganglionic axons—innervate distal
duodenum, part of pancreas, remainder of
small intestine, proximal large intestine,
kidneys, proximal part of ureters
Inferior Mesenteric Ganglion
• Location—adjacent to the origin of the
inferior mesenteric artery
• Preganglionic axons—lumbar
splanchnic nerves (L1–L2 segment of
spinal cord)
• Postganglionic axons—innervate the
distal colon, rectum, urinary bladder,
distal ureter, and most of reproductive
organs
Sympathetic Pathways
• All preganglionic neurons originate in
lateral gray horns of T1–L2 regions of
the spinal cord.
• Preganglionic axons travel with T1–L2
spinal nerves.
• Preganglionic axons immediately leave
spinal nerve and travel through white
rami to enter sympathetic trunk.
Sympathetic Pathways
• Once inside the sympathetic trunk
preganglionic axons may remain at the level of
entry or travel superiorly or inferiorly within the
sympathetic trunk.
• Axons exit sympathetic trunk ganglia by:
–
–
–
–
spinal nerve pathway
postganglionic sympathetic nerve pathway
splanchnic nerve pathway
adrenal medulla pathway
Spinal Nerve Pathway
Figure 18.8
Postganglionic Sympathetic
Nerve Pathway
Figure 18.8
Splanchnic Nerve Pathway
Figure 18.8
Adrenal Medulla Pathway
Figure 18.8
Adrenal Medulla Pathway
• Adrenal medulla—the internal region
of adrenal gland that releases
hormones within the bloodstream to
help promote fight-or-flight response
– The hormones are epinephrine and, to a
lesser degree, norepinephrine
Sympathetic Outflow
Autonomic Plexuses
• Collections of sympathetic
postganglionic axons, parasympathetic
preganglionic axons, and visceral
sensory axons
• Sympathetic and parasympathetic
axons are close to one another
Autonomic Plexuses
•
•
•
•
Cardiac plexus
Pulmonary plexus
Esophageal plexus
Abdominal aortic plexus consists of
celiac plexus, superior mesenteric
plexus, inferior mesenteric plexus,
• Hypogastric plexus
Autonomic Plexuses
Figure 18.9
Cardiac Plexus
• In mediastinum of thoracic cavity
• Consists of postganglionic sympathetic axons
from thoracic sympathetic trunk ganglia and
preganglionic axons of vagus nerve
• Increased sympathetic activity increases
heart rate and blood pressure
• Increased parasympathetic activity decreases
heart rate
Pulmonary Plexus
• Consists of postganglionic sympathetic axons
from thoracic sympathetic trunk ganglia and
preganglionic axons from the vagus nerve
• Axons project to bronchi and blood vessels of
the lungs
• Parasympathetic stimulation reduces bronchial
diameter (bronchoconstriction) and increases
mucous gland secretion in bronchial tree
• Sympathetic innervation causes
bronchodilation (increase bronchial diameter)
Esophageal Plexus
• Consists of preganglionic axons from
the vagus nerve
• Parasympathetic activity coordinates
smooth muscle activity during
swallowing reflex in inferior wall and
cardiac sphincter in inferior esophagus
Abdominal Aortic Plexus
• Consists of celiac plexus, superior
mesenteric plexus, and inferior
mesenteric plexus
• Composed of postganglionic axons
projecting from the prevertebral ganglia
and preganglionic axons from the vagus
nerve
Hypogastric Plexus
• Complex meshwork of postganglionic
sympathetic axons (from the aortic plexus
and the lumbar region of the sympathetic
trunk) and preganglionic parasympathetic
axons from the pelvic splanchnic nerve
• Axons innervate viscera within the pelvic
region
Neurotransmitters and
Receptors
• All preganglionic axons release acetylcholine
(ACh), which has an excitatory effect on the
ganglionic cell.
• All postganglionic parasympathetic axons
and a few postganglionic sympathetic axons
release ACh on the effector.
• Depending on the receptor, ACh from
parasympathetic axons may have either an
inhibitory or excitatory effect on the effector.
Neurotransmitters and
Receptors
• Ach released from sympathetic axons is
excitatory only.
• Most postganglionic sympathetic axons
release norepinephrine on the effector.
• Depending on the receptor, the effects of
norepinephrine has an inhibitory or an
excitatory effect.
Comparison of Neurotransmitters
in the Autonomic Nervous System
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Parasympathetic Pathway
Sympathetic Pathways
Preganglionic axon releases ACh
Ganglionic neuron cell body and
dendrites always contain receptors
for ACh
ACh
ACh
ACh
ACh
receptors
ACh
receptors
ACh
receptors
ACh
ACh
NE
ACh
receptors
ACh
receptors
Postganglionic axon releases
ACh or NE
Target cells contain either ACh
receptors (bind ACh) or NE
receptors (bind NE)
Figure 18.10
Target cell
Target cell
NE
receptors
Target cell
Dual Innervation
• Many visceral effectors have dual
innervation, meaning innervation by
postganglionic axons from both ANS
divisions.
• The actions of the divisions usually
oppose each other (antagonistic
effects).
Autonomic Reflexes
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Ureters
Urinary
bladder
stretches
as it fills
with urine
2 Nerve impulse travels through sensory
neuron to integration center in the spinal cord
3 Nerve impulse is processed
in the integration center
Interneuron
1 Stimulus
Spinal cord
activates
receptor
Pelvic splanchnic nerve
4 Motor impulses are
conducted through
motor neurons
Postganglionic
axon
Ureter
Urinary bladder
Detrusor muscle contracts
5 Effector responds to
impulse from motor neuron
(smooth muscle contraction
occurs in the bladder wall
and relaxation in the internal
urethral sphincter)
Figure 18.11
Internal urethral sphincter
relaxes
CNS Control of Autonomic
Function
Autonomic function is influenced by the:
• cerebrum
• hypothalamus
• brainstem
• spinal cord
Control of Autonomic Functions
by Higher Brain Centers
Figure 18.12
CNS Control of Autonomic
Function
Cerebrum:
• ANS activities are affected by conscious
activities in cerebral cortex and subconscious
communication between association areas in
the cortex with centers of sympathetic and
parasympathetic control in the hypothalamus.
• Sensory processing in the thalamus and
emotional states controlled in the limbic
system directly affect the hypothalamus.
CNS Control of Autonomic
Function
Hypothalamus:
• Integration and command center for
autonomic functions
• Contains nuclei that control visceral functions
in both divisions of the ANS
• Communicates with other CNS regions,
including cerebral cortex, thalamus,
brainstem, cerebellum, and spinal cord
• Central brain structure involved in emotions
and drives that act through the ANS
CNS Control of Autonomic
Function
Brainstem:
• Nuclei in the mesencephalon, pons, medulla
oblongata mediate visceral reflexes.
• Reflex centers control accommodation of the
lens, blood pressure changes, blood vessel
diameter changes, digestive activities, heart
rate changes, and pupil size.
• Centers for cardiac, digestive, and vasomotor
functions are housed within the brainstem.
CNS Control of Autonomic
Function
Spinal Cord:
• Processes some autonomic responses,
notably the parasympathetic activities
associated with defecation and urination,
without involvement of the brain
• Higher centers in the brain may consciously
inhibit these reflex activities
Neural Crest Cell Derivatives
Figure 18.13