Chapter 14
The Autonomic Nervous System
Autonomic system
System of nerves mediating involuntary actions
Regulates body organ activity
Maintains normal internal functions
Divided into sympathetic and parasympathetic divisions
Allows for varied nervous system responses in times of
stress and rest
A schematic of the somatic nervous system
(SNS), which provides conscious and subConscious control over skeletal muscles
Upper motor
neurons in
primary motor
cortex
BRAIN
Somatic motor
nuclei of brain
stem
Skeletal
muscle
Lower
motor
neurons
Spinal cord
Somatic
motor
nuclei of
spinal cord
Skeletal
muscle
Figure 14 Section 1 1
Comparison of the Somatic and Autonomic
Nervous Systems: Functional Organization
• Somatic nervous system (SNS)
– Includes processes perceived or controlled consciously
– Somatic sensory portion
• detects stimuli from special senses, skin, and proprioceptors
• sends information to CNS
– Somatic motor portion
• transmits nerve signals from CNS to control skeletal muscles
Comparison of the Somatic and Autonomic
Nervous Systems: Functional Organization
• Autonomic nervous system (ANS)
– Includes processes regulated below conscious level
– Visceral sensory portion
• detects stimuli from blood vessels and internal organs
– Autonomic motor portion (visceral motor)
• transmits nerve signals to cardiac muscle, smooth muscle, and glands
– Functions to maintain homeostasis
• constant internal environment
– Regulates:
• heart rate and blood pressure
• respiratory rate, sweating, and digestion
– Keeps these variables within optimal ranges
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Somatic Nervous System
Posterior root ganglion
Anterior root
Figure 15.1a
Somatic motor neuron
sends nerve signals to
skeletal muscle.
Somatic sensory
neuron detects stimuli
and transmits nerve
signals from skin,
skeletal muscle, joints,
and special senses
(vision, hearing, etc.).
Skeletal muscle
Sensory receptor
in skin
(a)
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Autonomic Nervous System
Autonomic
ganglion
Preganglionic autonomic motor neuron
transmits nerve signals to a ganglionic
motor neuron.
Figure 15.1b
Ganglionic autonomic
motor neuron transmits
nerve signals to smooth
muscle, cardiac muscle,
and glands.
Smooth muscle
in trachea
Sensory receptor
in viscera
(b)
Visceral sensory
neuron detects
stimuli within blood
vessels and smooth
muscle in the
viscera.
Divisions of the Autonomic Nervous Systems:
Functional Differences
• Motor component of ANS
– Subdivided into parasympathetic and sympathetic divisions
– Parasympathetic division
• functions to maintain homeostasis at rest
• energy conservation and replenishing stage
• “rest-and-digest” division
– Sympathetic division
•
•
•
•
prepares the body for emergencies
“fight-or-flight” division
increased alertness and metabolic activity
“three E’s”: emergency, exercise, or excitement
Autonomic Nervous System
Sympathetic Division
Parasympathetic Division
In the sympathetic division, or thoracolumbar
(thor-a-kō-LUM-bar) division, axons emerge from
the thoracic and superior lumbar segments of the
spinal cord and innervate ganglia relatively close
to the spinal cord.
In the parasympathetic division, or craniosacral (krā-nē-ō-SĀ-krul) divions, axons
emerge from the brain stem and the sacral
segments of the spinal cord, and they innervate
ganglia very close to (or within) target organs.
Cranial nerves
(III, VII, IX, and
X)
The two main divisions
of the ANS: the sympathetic
and parasympathetic
divisions
Thoracic
nerves
Lumbar nerves
(L1, L2 only)
T1
T2
T3
T4
T5
T6
T7
T8
T9
T10
T11
T12
L1
L2
S2
S3
S4
Sacral nerves
(S2, S3, S4 only)
Figure 14.1
1
Divisions of the Autonomic Nervous Systems:
Anatomic Differences
• Parasympathetic
– Preganglionic neuron in
brainstem or S2-S4 spinal cord
– Termed craniosacral division
– Ganglionic neuron innervating
muscles or glands
– Preganglionic axons longer
– Postganglionic axons shorter
– Few preganglionic axons
– Ganglia close to or within
effector
• Sympathetic
– Preganglionic neuron in lateral
horns of T1-L2
– Termed thoracolumbar
division
– Ganglionic neuron innervating
muscles or glands
– Preganglionic axons shorter
– Postganglionic axons longer
– Many preganglionic axons
– Ganglia relatively close to
spinal cord (in sympathetic
trunk ganglia or prevertebral
ganglia)
Figure 15.3
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Autonomic Motor Nervous System
Parasympathetic Division
Origin:
Preganglionic neurons located
in brainstem nuclei and S2–S4
segments of spinal cord
(craniosacral)
Functions:
• “Rest-and-digest” response
• Brings body to homeostasis
Sympathetic Division
Origin:
Preganglionic neurons
located in lateral horns of
T1– L2 segments of spinal
cord (thoracolumbar)
CN III (oculomotor)
CN VII (facial)
CN IX (glossopharyngeal)
Sympathetic trunk
CN X (vagus)
T1–L2 segments
of spinal cord
S2–S4 segments
of spinal cord
Pelvic splanchnic
nerves
Functions:
• Activated in emergency
situations
• “Fight-or-flight” response
• Also involved with
homeostasis
Figure 15.4
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Sympathetic Division
Parasympathetic Division
Ganglionic
Preganglionic
neuron
neuron
Long preganglionic axon
Short
postganglionic
axon
Short, branching
preganglionic axon
Long postganglionic axon
Preganglionic
neuron
Ganglionic neuron
Autonomic ganglion
(close to or within effector organ wall)
(a)
Autonomic ganglion
(close to the vertebral column)
(b)
The parasympathetic division
has short axons with relatively
few branches and ganglia
located close to or within the
wall of the organ.
The sympathetic division has long
axons with many branches and
ganglia located in the sympathetic
trunk or prevertebral ganglia
Parasympathetic Division:
Cranial Components
• Cranial components
– Oculomotor(lens & iris), facial(lacrimal glands), and glossopharyngeal
nerves(parotid)
• convey parasympathetic information to the head
– Vagus nerve
•
•
•
•
parasympathetic information for thoracic and most abdominal organs
increased mucus production and decreased airway diameter
decrease in heart rate and force
causes increased smooth muscle motility and secretory activity
Parasympathetic Division:
Cranial Components
Which four cranial nerves have a
parasympathetic component? What organs are
innervated by each?
The oculomotor nerve innervates the ciliary and constrictor
muscles of the eye.
The facial nerve innervates the lacrimal, submandibular,
and sublingual glands.
The glossopharnygeal nerve innervates the parotid salivary
gland.
The vagus nerve innervates the thoracic organs, most of the
abdominal organs, and the gonads.
Parasympathetic Division:
Pelvic Splanchnic Nerves
• Target organs
–
–
–
–
Distal portion of large intestine, rectum
Bladder, distal ureter
Most reproductive organs, and others
Causes:
•
•
•
•
smooth muscle motility
secretory activity in digestive tract
contraction in bladder wall
erection of clitoris and penis
See Table 15.3: Parasympathetic Division Outflow
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Preganglionic
Ciliary ganglion
Postganglionic
Lacrimal gland
Pterygopalatine
ganglion
CN III
Parotid salivary gland
Submandibular salivary gland
Sublingual salivary gland
CN VII
CN IX
Submandibular ganglion
Otic ganglion
Pons
Heart
CN X
Cardiac plexus
Trachea
Figure 15.5
Pulmonary plexus
Esophageal plexus
Lung
Esophagus
Liver
Abdominal aortic
plexus
Gallbladder
Stomach
Spleen
Spinal cord
Kidney
Ureter
Pancreas
Small intestine
Hypogastric plexus
Testis
Ovary
Descending colon
Rectum
S2
S3
S4
Bladder
Pelvic splanchnic nerves
Penis
Uterus
Vagina
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Superior vena
cava
Figure 15.7
Intercostal nerve
Sympathetic trunk
Gray ramus
White ramus
Sympathetic trunk
ganglia
Descending
thoracic aorta
Azygos vein
Greater thoracic
splanchnic nerve
Diaphragm
© The McGraw-Hill Companies, Inc./Photo and Dissection by Christine Eckel
Sympathetic Division:
Clinical View: Horner Syndrome
–
–
–
–
Injury of cervical sympathetic trunk or T1 trunk ganglion
Symptoms on same side of head
Impairment of sympathetic signaling
Ptosis
• drooping of superior eyelid
• due to paralysis of superior tarsal muscle
– Miosis
• constricted pupil
• due to paralysis of pupil dilator muscle
– Anhydrosis
• lack of sweating
• sweat glands not receiving sympathetic innervation
– Facial flushing
• due to vasodilation due to lack of sympathetic innervation
Sympathetic Division:
Sympathetic Pathways
– adrenal medulla pathway
• internal region of adrenal gland, adrenal medulla
• directly innervated by preganglionic sympathetic axons
• preganglionic neuron
– extends through sympathetic trunk and prevertebral ganglia
• upon stimulation, epinephrine and norepinephrine produced in
medulla
– circulate within blood
– help prolong fight-or-flight response
– prolong effects of sympathetic stimulation
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White ramus
Figure 15.8d
Gray ramus
Splanchnic nerve
Adrenal
medulla
Prevertebral ganglion
(no synapse occurs)
(d) Adrenal medulla pathway
Overview of
ANS Neurotransmitters
• Neurotransmitters
– Acetylcholine (ACh) and norepinephrine (NE)
– Bind to specific receptors on postsynaptic cell
– Cause stimulation or inhibition, depending on receptor
• Acetylcholine
– Synthesized and released by cholinergic neurons
• all sympathetic and parasympathetic preganglionic neurons
• all parasympathetic ganglionic neurons
• neurons innervating sweat glands and blood vessels of skeletal muscle
• Norepinephrine
– Synthesized and released by adrenergic neurons
• most other sympathetic ganglionic neurons
• form network of swellings at target organ
– termed varicosities
– contain stored NE
– NE released along length of axon
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Preganglionic axon
Autonomic ganglion
Ganglionic neuron cell body
Postganglionic axon
Figure 15.10
Varicosities
Smooth muscle cells
Mitochondrion
Synaptic vesicle
containing NE
Varicosity
Cholinergic Receptors
• Nicotinic receptors cholinergic receptor
– Sensitive to the drug nicotine
– Found on all ganglionic neurons and adrenal medulla
– When bound: always produces a stimulatory response
• open ion channels greater movement of Na+ into cell than K+ out of cell
• Muscarinic receptors cholinergic receptor
– Responsive to muscarine, a mushroom toxin
– Found in: all target membranes in parasympathetic division selected sympathetic
cells
• e.g., sweat glands in skin, blood vessels in skeletal muscle
• decreases heartbeat rate
• Biogenic amines (monoamines)
– Bind adrenergic receptors
– Catecholamines, subcategory
• have catechol ring structure in molecule
• include dopamine, norepinephrine, and epinephrine
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Parasympathetic Pathway
Sympathetic Pathways
Preganglionic axon
releases ACh.
ACh
ACh
ACh
Nicotinic
receptors
Nicotinic
receptors
Nicotinic
receptors
ACh
ACh
NE
Figure 15.9
Ganglionic neuron
cell body and
dendrites always
contain receptors
for ACh.
Postganglionic axon
releases ACh or NE.
Muscarinic
receptors
Target cells contain
either ACh receptors
(bind ACh) or NE
receptors (bind NE).
Target cell
Muscarinic
receptors
Target cell
(e.g., sweat glands
and blood vessels
in skeletal muscle)
Adrenergic
receptors
Target cell
(e.g., most other
body structures)
Interactions Between the Parasympathetic
and Sympathetic Divisions: Dual Innervation
Antagonistic Effects
– Parasympathetic and sympathetic effects usually antagonistic
– E.g., control of heart rate
• parasympathetic stimulation slowing heart rate
• sympathetic stimulation increasing heart rate
– E.g., control of muscular activity in GI tract
• parasympathetic stimulation accelerating rate of contraction and motility
• sympathetic stimulation decreasing motility
– E.g., control of pupil diameter in the eye
• parasympathetic stimulation of circular muscle layer of iris
– causes pupil constriction
• sympathetic stimulation of radial muscle layer of iris
– causes pupil dilation
Interactions Between the Parasympathetic
and Sympathetic Divisions: Dual Innervation
Cooperative Effects
– When both parasympathetic and sympathetic produce single result
– E.g., male sexual function
• penis erect due to parasympathetic innervation
• ejaculation due to sympathetic innervation
Parasympathetic and Sympathetic Interactions:
Systems Controlled Only by Sympathetic Division
• Opposing effects without dual innervation
– E.g., blood vessels innervated by sympathetic axons only
• cause increased smooth muscle contraction and blood pressure
• vasodilation achieved by decreasing stimulation below autonomic tone
– E.g., sweat glands in the trunk and arrector pili muscles in the skin
• cause sweating and “goosebumps”
– E.g., neurosecretory cells of adrenal medulla
• release epinephrine and norepinephrine, prolonging fight-or-flight effects
Figure 14.6
1
Parasympathetic and Sympathetic Interactions:
Systems Controlled Only by Sympathetic Division
Clinical View:
Raynaud
Syndrome
– Sudden constriction of
small arteries of digits
– Results in loss of normal
hue of distal skin
– Accompanied by pain
– Triggered by cold or
emotional stress
– Due to exaggerated local
sympathetic response
– More common in women
Control and Integration of Autonomic
System Function: Autonomic Reflexes
Clinical View: Autonomic Dysreflexia
– Causes blood pressure to rise profoundly
– Stimulates a sympathetic reflex
• causes systemic vasoconstriction
• marked increase in blood pressure
– Caused by hyperactivity of ANS after a spinal cord injury
– Initial response to injury is spinal shock, with loss of autonomic reflexes
– Abnormal response to lack of innervation, denervation hypersensitivity
• e.g., involuntary relaxation of internal urethral sphincter
• due to spinal cord reflex
Involvement of CNS
ANS is a regulated nervous system, not independent
Influenced by four CNS regions:
cerebrum, hypothalamus, brainstem, and spinal cord
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Cerebrum
Conscious activities in the
cerebrum affect hypothalamus
control of the ANS
Hypothalamus
Integration and command center
for autonomic functions; involved
in emotions
Brainstem
Contains major ANS reflex
centers
Spinal cord
Contains ANS reflex centers for
defecation and urination
Figure 14.6
1
Figure 14.6
1
Sympathetic
• Adapts body for
physical activity
• Fight or flight
• Subtle effects
Parasympathetic
• Calming effect on
functions
• Reduced energy
expenditure and
normal bodily
maintenance