David A. Morton, PhD
Lecture 03 – PNS and ANS
Lecture 03: PNS and ANS
Objectives:
• To gain an understanding of the autonomic nervous system (ANS)
Conceptual overview of the ANS
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The autonomic nervous system (ANS) (or visceral nervous system) is part of the
PNS and the involuntary branch of the nervous system.
• The ANS controls homeostasis, cardiovascular, digestive and respiratory
functions, as well as salivation, perspiration, pupil diameter, urination, and
reproductive functions through innervation of smooth muscle, cardiac muscle
and glands
The ANS consists of the sympathetic division and parasympathetic division which
typically function in opposition to each other:
• Sympathetic division typically functions in actions requiring quick responses;
consider sympathetic as "fight or flight"
• Parasympathetic division functions with actions that do not require immediate reaction; consider parasympathetic as "rest and digest"
However, many instances of sympathetic and parasympathetic activity cannot be
ascribed to "fight" or "rest" situations. These two systems should be seen as permanently modulating vital functions, in usually antagonistic fashion, to achieve homeostasis. Some typical actions of the sympathetic and parasympathetic systems
are listed on the following page.
The ANS possesses both motor and sensory neurons:
• Sensory neurons monitor changes in the viscera (organs)
• Motor nerves innervate smooth and cardiac muscle and glands; ANS motor
response contain a two-neuron distribution with pre-ganglionic neuronal cell
bodies in the CNS and post-ganglionic neuronal cell bodies in a ganglion in
the PNS
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David A. Morton, PhD
Lecture 03 – PNS and ANS
Overview of the ANS
CNS
PNS
Sensory
Motor
Visceral
Afferent
Visceral
Efferent
Smooth muscle, glands,
and modified
cardiac muscle
Return information
(reflexes, pain) concerning hollow organs and blood vessels
Organ, Tract, or System Sympathetic
Pupil •Dilates
Skin •Arrector pili muscle contraction
•Vessels- vasoconstriction
•Sweat glands- sweating
Lacrimal and salivary glands •Decreases secretion
Sympathetic
T1-L2
Parasympathetic
CN III,VII,IX,X
S2-S4
Parasympathetic
•Constricts
•No effect
•Increase secretion
Heart •↑ rate and strength of contraction
•Dilates coronary vessels
•↓ rate and strength of contraction
•Constricts coronary vessels
Lung •Bronchodilation
•Bronchoconstriction
Digestive tract •Inhibits peristalsis
•Constricts blood vessels
•Stimulates peristalsis
Genital system •Ejaculation
•Erection
Suprarenal medulla •Release of adrenaline
Overview •Fight or Flight (E)
•Exercise
•Excitement
•Emergency
•No effect
•Rest and Digest (D)
•Digestion
•Defecation
•Diuresis
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Lecture 03 – PNS and ANS
David A. Morton, PhD
ANS - Sympathetics
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Sympathetic neurons are responsible for the “fight or flight” response (pp. 78-84);
just as in the parasympathetic division, sympathetic pathways in the PNS require a
two neuron circuit of pre- and post-ganglionic sympathetic motor neurons.
Pre-ganglionic sympathetic neuron - the cell bodies originate in the lateral horns
of the gray matter at spinal cord levels T1-L2 (thoracolumbar division)
• Once pre-ganglionic sympathetic neurons exit the spinal cord via the anterior
root, the axons pass through a white ramus communicans to enter an adjoining paravertebral ganglion forming part of the sympathetic trunk; the sympathetic trunks flank each side of the vertebral column and look like strands
of glistening white beads
• Although the sympathetic trunks extend from the neck to the pelvis, remember sympathetic fibers arise only from spinal cord segments T1 to L2. The
ganglia vary in size, position, and number, but typically there are 23 ganglia
in each sympathetic chain: 3 cervical sympathetic ganglia, 11 thoracic, 4
lumber, 4 sacral, ad 1 coccygeal.
Once a pre-ganglionic sympathetic neuron reaches a paravertebral ganglion via the
white ramus communicans, one of three things happen … the pre-ganglionic sympathetic neuron can:
1. Synapse with a post-ganglionic neuron within the same para-vertebral ganglion and exits via the gray ramus communicans and travels along the anterior ramus to innervate blood vessels, arrector pili muscles and/or sweat
glands associated with that dermatome.
2. Ascends or descends the sympathetic chain to synapse in either a supra or
infra-adjancent paravertebral ganglion (It is these fibers running from one
ganglion to another that connect the ganglia into the sympathetic trunk). The
post-ganglionic sympathetic neuron will exit at the level of synapse via a
gray ramus communicantes and travel along the anterior ramus or exit the
paravertebral ganglion in some type of visceral ramus (i.e., cardiac fibers) to
course to an end organ.
3. Course through the ganglion without a synapse and the pre-ganglionic sympathetic neuron will enter a splanchnic nerve to synapse in a pre-vertebral
ganglion (i.e., celiac ganglion) in the abdominal cavity. Once the preganglionic sympathetic fiber synapses in the pre-vertebral ganglion the postganglionic sympathetic fiber will course along nerves to innervate end organs such as the kidney
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David A. Morton, PhD
Lecture 03 – PNS and ANS
2.
1.
3.
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Lecture 03 – PNS and ANS
David A. Morton, PhD
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Lecture 03 – PNS and ANS
David A. Morton, PhD
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Lecture 03 – PNS and ANS
David A. Morton, PhD
ANS - Parasympathetics
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The parasympathetic division of the ANS is concerned with vegetative functions,
e.g., encourages secretory activity on the body’s mucous and serous membranes,
promotes digestion by increased peristalsis and glandular secretion, and induces
contraction of the urinary bladder.
• Pre-ganglionic parasympathetic neuron - the cell bodies originate in the
brain stem associated with certain cranial nerves. They leave the brain stem
with their cranial nerve, and synapse at one of the cranial ganglia. The postganglionic parasympathetic neurons tend to be short, terminating in salivary glands and other glands of the nasal and oral cavities. The preganglionic fibers associated with the vagus (CN X cranial nerve) are unusually long, descending the neck, the esophagus, and through the esophageal
hiatus to the gastrointestinal tract. The axons of these neurons extend as far
as the descending colon. The ganglia are in the muscular walls of the organ
they supply (intramural ganglia); the post-ganglionic axons are very short,
terminating in smooth muscle and glands.
• The cell bodies of the sacral pre-ganglionic neurons are located in the lateral
horns of sacral spinal cord segments 2,3 and 4. Their axons leave the cord
via the anterior rami but form their own nerves called the pelvic splanchnic
nerves (nervi erigentes). These nerves project to the pelvis, mix with sympathetic post-ganglionics in the pelvic plexus, and depart for their target organs. They synapse with the post-ganglionic neurons in the intramural ganglia in the walls of the organ supplied. These fibers stimulate contraction of
rectal and bladder musculature, and induce vasodilation of vessels to the penis and clitoris (erection).
The parasymphathetic and sympathetic divisions of the ANS are not antagonistic.
Their respective activities are coordinated and synchronized to achieve dynamic
stability of body function during a broad range of life functions such as eating, running, fear, relaxation and so forth.
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Lecture 03 – PNS and ANS
David A. Morton, PhD
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Lecture 03 – PNS and ANS
David A. Morton, PhD
Comparison of Pre- versus Postganglionic Neurons of the ANS
Sympathetic
Parasympathetic
Short preganglionic fibers
Long preganglionic fibers
Long postganglionic fibers
Short postganglionic fibers
Ganglia located relatively far
from their target organ
Ganglia located on or near their
target organ
Sympathetic
Parasympathetic
Preganglionic cell bodies: T1
to L2
Preganglionic cell bodies:
brainstem and S2-S4
Postganglionic cell bodies:
chain or collateral ganglia
Postganglionic cell bodies:
cranial or enteric ganglia
Ganglia run entire length of
spinal cord
Ganglia are at one end or the
other of spinal cord
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David A. Morton, PhD
Lecture 03 – PNS and ANS
Overview of the Nervous System
CNS
PNS
Sensory
General
Afferent
Visceral
Afferent
Motor
Special
Afferent
Somatic
Efferent
Skeletal muscle of the body
wall
Touch
Pain
Temperature
Vibration
Proprioception
Return information
(reflexes, pain) concerning hollow organs and blood vessels
Sight,
Taste,
Sound
Organ, Tract, or System Sympathetic
Pupil •Dilates
Skin •Arrector pili muscle contraction
•Vessels- vasoconstriction
•Sweat glands- sweating
Lacrimal and salivary glands •Decreases secretion
Branchial
Efferent
Visceral
Efferent
Skeletal muscle derived
from pharyngeal arches
Smooth muscle, glands,
and modified
cardiac muscle
Sympathetic
T1-L2
Parasympathetic
CN III,VII,IX,X
S2-S4
Parasympathetic
•Constricts
•No effect
•Increase secretion
Heart •↑ rate and strength of contraction
•Dilates coronary vessels
•↓ rate and strength of contraction
•Constricts coronary vessels
Lung •Bronchodilation
•Bronchoconstriction
Digestive tract •Inhibits peristalsis
•Constricts blood vessels
•Stimulates peristalsis
Genital system •Ejaculation
•Erection
Suprarenal medulla •Release of adrenaline
Overview •Fight or Flight (E)
•Exercise
•Excitement
•Emergency
•No effect
•Rest and Digest (D)
•Digestion
•Defecation
•Diuresis
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