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AUTONOMIC NERVOUS SYSTEM
Larry J. Ream, Ph.D.
Department of Neuroscience, Cell Biology and Physiology
105A White Hall
775-3188
[email protected]
READING ASSIGNMENT
Preview pages 128-129 in Netter’s and Chapter 6 in Rhoades & Bell.
OBJECTIVES
• To understand the general organization of the nervous system (central vs. peripheral
nervous system) and the role of the autonomic nervous system as an efferent pathway in
the peripheral nervous system.
• To understand the organization of the autonomic nervous system, specifically in terms
of how the sympathetic and parasympathetic nervous systems are organized and which
neurotransmitters and receptors they use.
• Be able to discuss the general functional roles of the craniosacral vs. the thoracolumbar
division of the autonomic nervous system.
• To recognize that autonomic function is under control by higher centers in the cortex,
limbic system, and hypothalamus.
COMPARISON OF SOMATIC AND AUTONOMIC NERVOUS SYSTEMS
The peripheral nervous system (PNS) may be subdivided further into a somatic
nervous system (SNS), an autonomic nervous system (ANS) and an enteric nervous
system (ENS).
703
The ANS, like the SNS, is organized on the basis of a reflex arc.
Somatic Nervous System
Autonomic Nervous System
Dorsal root
ganglion
IN
OUT
Spinal
nerve
Ventral root
Primary somatic efferents of the PNS
Effector…Skeletal muscle
Motoneurons are activated by:
Local reflexes; e.g., knee jerk
Higher brain centers; e.g.,
planned, voluntary motor acts
Function: Our awareness of the external
environment and in directing our
skeletal muscle activity to cope with it
autonomos
self
law, governing
Primary autonomic efferents of the PNS
Function: Govern involuntary functions
of the body that do not affect
consciousness; e.g.,
GI tract motility
Cardiac activity
Contraction of the urinary bladder
Constriction of the iris of the eye
Effectors include…
Smooth muscle
Cardiac muscle
Glands
…but not skeletal muscle
autonomic
autonomic
704
• The SNS consists of (1) sensory neurons that convey information from somatic
receptors in the head, body wall, and limbs and from receptors for special senses of
vision, hearing, taste, and smell (exteroceptors and proprioceptors) to the CNS and (2)
motor neurons that conduct impulses from the CNS to skeletal muscles only. Because
these motor responses can be consciously controlled, the action of this part of the PNS is
voluntary.
> Each of its efferent pathways consists of a single motoneuron and the skeletal
muscle fibers it innervates. When a somatic motoneuron stimulates a skeletal
muscle, the muscle contracts; the effect is always excitation.
> The soma of the motoneuron is located in either the brainstem or spinal cord,
and its axon synapses directly on skeletal muscle. Acetylcholine is released from
presynaptic terminals of the motoneurons and activates nicotinic receptors located
on the motor end plates of the skeletal muscle.
GSE
excitation
ACh
A
α-motoneuron
GENERAL PLAN:
GSA
R
exteroceptor or
proprioceptor
e.g., muscle spindle
naked nerve ending
Meissnerʼs corpuscle
Golgi tendon organ
GSE
CNS
E
monosynaptic or
polysynaptic
SPINAL CORD
skeletal muscle
BRAINSTEM
Spinal nerves
GSE
Cranial nerves
GSE
705
• The ANS consists of (1) sensory neurons that convey information from autonomic
sensory receptors, located primarily in visceral organs to the CNS, and (2) motor neurons
that conduct nerve impulses from the CNS to smooth muscle, cardiac muscle, and glands.
Because its motor responses are not normally under conscious control, the action of the
ANS is involuntary.
> Autonomic sensory neurons are associated with interoceptors, such as
chemoreceptors that monitor blood CO2 level and mechanoreceptors that detect
the stretch in the walls of organs or blood vessels. These sensory signals are not
consciously perceived most of the time, although intense activation of
interoceptors may produce conscious sensations. Three examples of visceral
sensations that are perceived are sensations of pain from damaged viscera, angina
pectoris (chest pain) from inadequate blood flow to the heart, and fullness of the
urinary bladder.
> Each efferent pathway consists of two neurons: a preganglionic and a
postganglionic neuron.
> The soma of the preganglionic neuron is located in the CNS. Axons from these
neurons synapse on the soma of postganglionic neurons in one of several
autonomic ganglia.
> The axons of postganglionic neurons then travel to the periphery, where they
synapse on visceral effector organs.
> All preganglionic neurons release acetylcholine. Postganglionic neurons release
either acetylcholine or norepinephrine, or, in some cases, neuropeptides.
GVE
excitation or
inhibition
1
ACh
or NE
2
B
C
GENERAL PLAN:
GVA
R
interoceptor
SPINAL CORD
GVE…two neurons in series
CNS
E
neuron 1
BRAINSTEM
Cranial nerves
GVE
Spinal nerves
GVE
706
neuron 2
Summary of Somatic and Autonomic Nervous Systems
Characteristic
Sensory input
Control of motor
output
Motor neuron
pathway
Somatic Nervous System
Special senses and somatic
senses.
Voluntary control from cerebral
cortex, with contributions from
basal ganglia, cerebellum,
brainstem, and spinal cord.
One-neuron pathway: Somatic
motoneurons extending from
CNS synapse directly with
effector.
Neurotransmitters
and hormones
All somatic motoneurons release
ACh.
Effectors
Skeletal muscle.
Responses
Contraction of skeletal muscle.
707
Autonomic Nervous System
Mainly from interoceptors; some from special
senses.
Involuntary control from limbic system,
hypothalamus, brainstem, and spinal cord;
limited control from cerebral cortex.
Usually two-neuron pathway: Preganglionic
neurons extending from CNS synapse with
postganglionic neurons in an autonomic
ganglion, and postganglionic neurons
extending from ganglion synapse with a
visceral effector. Alternatively, preganglionic
neurons may extend from CNS to synapse
with cells of the adrenal medullae.
All preganglionic axons release ACh; most
sympathetic postganglionic neurons release
NE; those to most sweat glands release ACh;
all parasympathetic postganglionic neurons
release ACh; adrenal medullae release
epinephrine and NE.
Smooth muscle, cardiac muscle, and glands.
Visceral effector organs such as the heart,
bronchioles, vascular smooth muscle, GI tract,
bladder, and genitalia.
Contraction or relaxation of smooth muscle;
increased or decreased rate and force of
contraction of cardiac muscle; increased or
decreased secretions of glands.
ORGANIZATION AND GENERAL FEATURES OF THE ANS
• The motor part of the ANS consists of two branches, the sympathetic division and the
parasympathetic division.
• With a few exceptions, effectors receive nerves from both divisions, and usually the two
divisions have opposing actions, i.e., act reciprocally in their effect on end organs.
> For example, cardiovascular activity:
SNS stimulation ➔ ↑HR, ↑SV (↑HR x ↑SV = ↑CO)
PNS stimulation ➔ ↓HR, ↓SV (↓HR x ↓SV = ↓CO)
> Not true for all organs, however. For example, sweat glands and limb
vasculature only receive sympathetic innervation.
• A third division of the ANS, the enteric nervous system, is located in plexuses of the
gastrointestinal tract.
VARIATIONS:
P
S
long
short
P
S
short
long
ACh
neuron 1
P
S
ACh or NE
neuron 2
synapse
preganglionic
P: cell body in
nucleus in
brainstem or
spinal cord
S: cell body in
spinal cord
only
postganglionic
cell body
in ganglion
synapse
708
smooth muscle
heart
gland
excitation or
inhibition
EXCEPTIONS:
1. sympathetics to adrenal medullae:
ACh
adrenal medulla
preganglionic
sympathetic
general plan — sympathetics to eccrine sweat glands:
NE
hands and feet
preganglionic
sympathetic
postganglionic
sympathetic
2. sympathetics to most sweat glands:
ACh
preganglionic
sympathetic
postganglionic
sympathetic
most body
regions
thermoregulatory
sweat glands
709
Organization of the Autonomic Nervous System
Characteristic
Origin of
preganglionic
neurons
Location of
autonomic ganglia
Length of
preganglionic axons
Length of
postganglionic
axons
Neuroeffector
junctions
Neurotransmitter
and receptor type in
ganglion
Neurotransmitter in
effector organs
Receptor types in
effector organs
Sympathetic
Division
Spinal cord
segments T1-L2
(thoracolumbar)
Paravertebral and
prevertebral
Short
Parasympathetic
Division
Nuclei of CN III,
VII, IX and X;
spinal cord
segments S2-S4
(craniosacral)
In or near effector
organs
Long
Somatic Nervous
System
Long
Short
Diffuse, branching;
receptors not
concentrated in one
region
ACh/nicotinic
receptor
Diffuse, branching;
receptors not
concentrated in one
region
ACh/ nicotinic
receptor
Discrete, organized;
ACh receptors
localized on motor
end plate
NE (except for most
sweat glands)
α1, α2, β1, β2
ACh
ACh
Muscarinic
Nicotinic
Somatic
Nicotinic
receptor
Sympathetic
Nicotinic
receptor
α 1, α 2, β 1, β 2
receptors
Nicotinic
receptor
Muscarinic
receptor
Parasympathetic
Adrenal
Nicotinic
receptor
710
ANATOMICAL COMPONENTS OF AUTONOMIC MOTOR PATHWAYS
Preganglionic neurons (myelinated, type B)
1. Sympathetic division
Thoracolumbar outflow (T1-L2)
Preganglionic cell bodies are
located at thoracic levels 1 through 12
and lumbar 1 through 2 (or 3)…in the
intermediolateral cell column of spinal cord
Short fibers in the white rami
Synapse in paravertebral (chain) ganglia…
OR synapse in prevertebral ganglia via
splanchnic nerves
• Celiac ganglion → stomach, spleen, liver and kidney
• Superior mesenteric ganglion → small intestine and colon
• Inferior mesenteric ganglion → colon, rectum, bladder, genital organs
Note: All three supply the enteric nervous system of the GI tract
711
2. Parasympathetic division
Craniosacral outflow (CN III, VII, IX, X [80%], S2-S4)
Preganglionic neurons are located in brainstem cranial nerve nuclei and the
sacral spinal cord levels 2 through 4
Long preganglionic fibers
Synapse in terminal ganglia…two types:
• cranial nerve head ganglia:
• intramural ganglia:
MEDULLA → Dorsal motor nucleus of the vagus n. (X)
712
Postganglionic neurons (unmyelinated, type C)
1. Sympathetic division
Long postganglionic fibers in the gray rami
One preganglionic fiber → 20+ postganglionic fibers
Extensive branching of both pre- and
postganglionic neurons
Divergence…sympathetic responses tend to be
widespread throughout the body
2. Parasympathetic division
Short postganglionic fibers
One preganglionic fiber → 4-5 postganglionic fibers
Parasympathetic effects tend to be localized
Exception:
Preganglionic sympathetic fibers to adrenal medullae
LONG
LONG
713
SYMPATHETIC GANGLIA
Sympathetic ganglia lie in two chains on either side of the vertebral column
(sympathetic chain ganglia) and near large abdominal arteries anterior to the vertebral
column (prevertebral ganglia).
White ramus…contains myelinated, type B axons
Gray ramus…contains unmyelinated, type C axons
Gray rami outnumber the white rami…
• only thoracic and first two or three lumbar nerves have white rami
• gray rami lead to each of the 31 pairs of spinal nerves
714
AUTONOMIC MOTOR PATHWAYS:
STRUCTURE OF THE SYMPATHETIC DIVISION
Thoracolumbar outflow
Cell bodies of sympathetic preganglionic neurons are located in the lateral horns of gray
matter in the 12 thoracic and first two (and sometimes three) lumbar segments of the
spinal cord.
715
AUTONOMIC MOTOR PATHWAYS:
STRUCTURE OF THE PARASYMPATHETIC DIVISION
Craniosacral outflow
Cell bodies of parasympathetic preganglionic neurons are located in brainstem nuclei and
in the lateral horns of gray matter in the second through fourth sacral segments of the
spinal cord.
716
AUTONOMIC PLEXUSES
• In the thorax, abdomen, and pelvis, axons of both sympathetic and parasympathetic
neurons form tangled networks of autonomic plexuses, many of which lie along major
arteries. These plexuses also may contain sympathetic ganglia and axons of autonomic
sensory neurons.
• The major plexuses in the thorax are the cardiac plexus, which supplies the heart, and
the pulmonary plexus, which supplies the bronchial tree.
• The abdomen and pelvis also contain major autonomic plexuses and often the plexuses
are named after the artery along which they are distributed. The celiac (solar) plexus is
the largest autonomic plexus and surrounds the celiac and superior mesenteric arteries. It
contains two large celiac ganglia and a dense network of autonomic axons and is
distributed to the liver, gallbladder, stomach, pancreas, spleen, kidneys, adrenal medullae,
testes, and ovaries.
• Other important autonomic plexuses are the superior mesenteric plexus, inferior
mesenteric plexus, the hypogastric plexus, and the renal plexuses.
ANS NEUROTRANSMITTERS
• Based on the neurotransmitter they produce and release, autonomic neurons are
classified as either cholinergic or adrenergic.
• Cholinergic neurons release acetylcholine; receptors for ACh are called
cholinoreceptors. (Note: A third term is non-adrenergic, non-cholinergic, which
describes some postganglionic parasympathetic neurons of the GI tract that release
peptides, e.g., substance P, or other substances, e.g., nitric oxide, as their
neurotransmitter.) In the ANS, cholinergic neurons include:
- all sympathetic and parasympathetic preganglionic neurons,
- sympathetic postganglionic neurons that innervate most sweat glands, and
- all parasympathetic postganglionic neurons.
• Adrenergic neurons release norepinephrine; receptors for NE on the effector organs
are adrenoreceptors. (Note: Adrenoreceptors may also be activated by epinephrine from
the adrenal medulla.) Most sympathetic postganglionic neurons are adrenergic.
NEUROEFFECTOR JUNCTIONS OF THE ANS
• In contrast to the SNS, in the ANS, the postganglionic neurons
that innervate target tissues form diffuse, branching networks.
Varicosities line these branches and are the sites of
neurotransmitter synthesis, storage, and release. (The varicosities
are analogous to the presynaptic nerve terminals of the
neuromuscular junction in the SNS.)
• There is overlap in the branching networks from different
postganglionic neurons, such that target tissues may be innervated
by many postganglionic neurons.
• Postganglionic receptors are widely distributed on the target
tissues, and there is no specialized region of receptors analogous
to the motor end plate of skeletal muscle.
717
CHOLINORECEPTORS
• The two types of cholinergic receptors, both of which bind ACh, are nicotinic receptors
and muscarinic receptors.
• Nicotinic receptors are present in the plasma membrane of dendrites and cell bodies of
both sympathetic and parasympathetic postganglionic neurons (i.e., in the autonomic
ganglia), in the motor end plate at the neuromuscular junction, and in the adrenal
medulla.
α 1, α 2, β 1, β 2
Sympathetic
Smooth
muscle,
glands
Parasympathetic
Motoneuron
Skeletal muscle
Preganglionic
Adrenal medulla
Somatic
Adrenal
medulla
• Nicotinic receptors produce excitation, and they are blocked by ganglionic blockers
(e.g., hexamethonium) in the autonomic ganglia, but not at the neuromuscular junction.
• Mechanism of action: ACh binds to the α subunits of the nicotinic ACh receptor. The
nicotinic ACh receptors are also ion channels for Na+ and K+.
IONOTROPIC RECEPTORS
i.e., chemically-gated ion channels
718
• Muscarinic receptors are present in the plasma membrane of all effectors (smooth
muscle, cardiac muscle, and glands) innervated by parasympathetic postganglionic axons.
• Most sweat glands receive their innervation from cholinergic sympathetic
postganglionic neurons and possess muscarinic receptors.
…all but hands and feet
• Activation of nicotinic receptors by ACh causes depolarization and thus excitation of
the postsynaptic cell…either a postganglionic neuron, an autonomic effector in smooth
muscle and glands), or a skeletal muscle fiber.
• Activation of muscarinic receptors by ACh sometimes causes depolarization and
sometimes causes hyperpolarization, depending on which particular cell bears the
muscarinic receptors.
> For example, binding of ACh to muscarinic receptors inhibits (relaxes) smooth
muscle sphincters in the GI tract.
> By contrast, ACh excites smooth muscle fibers in the circular muscles of the iris
of the eye, causing them to contract.
• Note: Because ACh is quickly inactivated by acetylcholinesterase, effects triggered by
cholinergic neurons are brief.
• Muscarinic receptors are blocked by atropine.
• Mechanism of action:
> Heart SA node. Inhibition of adenylate cyclase, which leads to the opening of
K+ channels, slowing the rate of spontaneous Phase 4 depolarization, and
decreased heart rate.
> Smooth muscle and glands. Formation of IP3 and increase in intracellular Ca++.
METABOTROPIC RECEPTORS
i.e., elicits 2nd-messenger cascades
719
ADRENORECEPTORS
• Adrenergic receptors bind both NE (both neurotransmitter and hormone) and
epinephrine (hormone).
• The two main types of adrenoreceptors are alpha (α) receptors and beta (β)
receptors, which are found on visceral effectors innervated by most sympathetic
postganglionic axons.
• These receptors are further classified into subtypes—α 1, α 2, β 1, β 2—based on the
specific responses they elicit and by their selective binding of drugs that activate or block
them. Although there are some exceptions…
> activation of α1 and β1 receptors generally produces excitation, whereas
> activation of α2, and β2 receptors causes inhibition of effector tissues.
• β3 receptors are present only on cells of brown adipose tissue, where their activation
causes thermogenesis.
• Cells of most effectors contain either alpha or beta receptors; some visceral effector
cells contain both. NE stimulates alpha receptors more strongly than beta receptors,
whereas epinephrine is a potent stimulator of both alpha and beta receptors.
• The activity of NE at a synapse is terminated when the NE is taken up by the axon that
released it or when the NE is inactivated by catechol-O-methyltransferase (COMT) or
monoamine oxidase (MAO). Compared to ACh, NE lingers in the synaptic cleft for a
longer time. Thus, effects triggered by adrenergic neurons typically are longer than
those triggered by cholinergic neurons.
• Mechanisms of action:
Receptor
α1
α2
Location
On vascular smooth muscle of the skin and
splanchnic regions, GI and bladder sphincters,
and the radial muscle of the iris
In presynaptic nerve terminals, platelets, fat
cells, walls of the GI tract
β1
In the SA node, AV node, and ventricular
muscle of the heart
β2
On vascular smooth muscle of skeletal
muscles, bronchial smooth muscle, and in the
walls of the GI tract and bladder
Mechanism of Action
Formation of IP3 and
increase in intracellular
Ca++
Inhibition of adenylate
cyclase and decrease in
cAMP
Activation of adenylate
cyclase and increase in
cAMP
Activation of adenylate
cyclase and increase in
cAMP
METABOTROPIC RECEPTORS
i.e., elicits 2nd-messenger cascades
720
Locations and Responses of Adrenergic and Cholinergic Receptors
Receptor
Cholinergic
Major Locations
Integral proteins in postsynaptic plasma membranes;
activated by ACh
Effects of Receptor Activation
Nicotinic
Plasma membrane of postganglionic sympathetic and
parasympathetic neurons
Excitation → impulses in postganglionic
neurons
Chromaffin cells of adrenal medullae
Epinephrine and NE secretion
Motor end plate of skeletal muscle
Excitation → contraction
Effectors innervated by parasympathetic postganglionic
neurons
In some receptors, excitation; in others,
inhibition
Sweat glands innervated by cholinergic sympathetic
postganglionic neurons
Increased sweating
Skeletal muscle blood vessels innervated by cholinergic
postganglionic sympathetic fibers
Integral proteins in postsynaptic plasma membranes;
activated by the neurotransmitter epinephrine, and by
hormonal NE and epinephrine
Inhibition → relaxation → vasodilation
Smooth muscle fibers in blood vessels that serve salivary
glands, skin, mucosal membranes, kidneys, and abdominal
viscera; radial muscle in iris of eye; sphincter muscles of
stomach and urinary bladder
Excitation → contraction, which causes
vasoconstriction, dilation of pupil, and
closing of sphincters
Salivary gland cells
Secretion of K+ and water
Sweat glands on palms and soles
Increased sweating
Smooth muscle fibers in some blood vessels
Inhibition → relaxation → vasodilation
Beta cells of pancreatic islets that secrete insulin
Decreased insulin secretion
Pancreatic acinar cells
Inhibition of digestive enzyme secretion
Platelets in blood
Aggregation to form platelet plug
Muscarinic
Adrenergic
α1
α2
β1
Cardiac muscle fibers
Excitation → increased force and rate of
contraction
Juxtaglomerular cells of kidneys
Renin secretion
Posterior pituitary
Secretion of ADH
Adipocytes
β2
Breakdown of triglycerides → release of
fatty acids into blood
Smooth muscle in walls of airways; in blood vessels that
serve the heart, skeletal muscle, adipose tissue, and liver;
and in walls of visceral organs, such as urinary bladder
Hepatocytes in liver
β3
Inhibition → relaxation, which causes
dilation of airways, vasodilation, and
relaxation of organ walls
Glycogenolysis
Brown adipose tissue
Thermogenesis
721
Effects of Sympathetic and Parasympathetic Divisions on Blood Vessels
Vascular smooth muscle
Salivary gland arterioles
Gastric gland arterioles
Intestinal gland arterioles
Coronary arterioles
Skin and mucosal arterioles
Skeletal muscle arterioles
Abdominal viscera arterioles
Brain arterioles
Kidney arterioles
Systemic veins
Effect of Sympathetic Stimulation
(α or β adrenergic receptors, except
as noted)
Vasoconstriction, which decreases
secretion (α1)
Vasoconstriction, which inhibits
secretion (α1)
Vasoconstriction, which inhibits
secretion (α1)
Relaxation → vasodilation (β2)
Contraction → vasoconstriction (α1,
α 2)
Contraction → vasoconstriction
(muscarinic ACh receptors)
Constriction: vasoconstriction (α1)
Contraction → vasoconstriction (α1)
Relaxation → vasodilation (β2)
Relaxation → vasodilation (muscarinic
ACh receptors)
Contraction → vasoconstriction (α1,
β 2)
Slight contraction → vasoconstriction
(α1)
Constriction of blood vessels →
decreased urine volume (α1)
Contraction → constriction (α1)
Relaxation → dilation (β2)
722
Effect of Parasympathetic
Stimulation (muscarinic ACh
receptors)
Vasodilation, which increases K+
and water secretion
Secretion of gastric juice
Secretion of intestinal juice
Contraction → vasoconstriction
Vasodilation, which may not be
physiologically significant
Drugs that Affect Autonomic Activity
Type of Receptor
Adrenergic
α1
Agonist
Antagonist
Norepinephrine
Phenylephrine (NeoSynephrine)
Phenoxybenzamine
Phentolamine
Prazosin (Minipress)
α2
Clonidine
Yohimbine
β1
Norepinephrine
Isoproterenol (Isuprel)
Dobutamine
Propranolol
Metoprolol
β2
Isoproterenol (Isuprel)
Albuterol
Propranolol (Inderal)
Butoxamine
Acetylcholine
Nicotine
Carbachol
Curare
Hexamethonium (ganglion,
Cholinergic
Nicotinic
Muscarinic
Acetylcholine
Muscarine
Carbachol
but not neuromuscular junction)
Atropine
NE
Beta2
Beta-blocker drug
Most sympathetic pathways
Beta1
Alpha
Alpha-blocker drug
723
SYMPATHETIC ADRENERGIC VARICOSITIES
• Sympathetic adrenergic varicosities contain both the classic neurotransmitter (NE) and
non-classic neurotransmitters (ATP and neuropeptide Y).
• The classic neurotransmitter, norepinephrine, is synthesized from tyrosine in the
varicosities and stored in small dense-core vesicles, ready for release; these small densecore vesicles also contain dopamine β-hydroxylase, which catalyzes the conversion of
dopamine to NE (the final step in the synthetic pathway), and ATP.
• A separate group of large dense-core vesicles contain neuropeptide Y.
Small dense-core vesicle
Clear vesicle
Large dense-core vesicle
PARASYMPATHETIC CHOLINERGIC VARICOSITIES
• The parasympathetic cholinergic varicosities release both the classic neurotransmitter
—ACh—and non-classic neurotransmitters—e.g., vasoactive intestinal peptide (VIP) and
nitric oxide (NO).
• The classic neurotransmitter, acetylcholine, is synthesized in the varicosities from
choline and acetyl CoA and stored in small clear vesicles. A separate group of large
dense-core vesicles contain peptides such as VIP. The varicosities also contain nitric
oxide synthase and can synthesize NO on demand.
724
SYMPATHETIC VS. PARASYMPATHETIC RESPONSES
• Most body organs receive innervation from both divisions of the ANS, which typically
work in opposition to one another.
• The balance between sympathetic and parasympathetic activity or “tone” is regulated by
the HYPOTHALAMUS.
• A few structures receive only sympathetic innervation—sweat glands, arrector pili
muscles, the kidneys, the spleen, most blood vessels, and the adrenal medullae.
Sympathetic
During physical or emotional stress, the sympathetic division dominates over the
parasympathetic system
Fight-or-flight response…prepares the body for emergency situations…expenditure of
energy
[Along with hormones from the adrenal medullae]
“four E situations”
Exercise
Emergency
Excitement
Embarrassment
Includes increased heart rate, cardiac output and blood pressure; redistribution of blood
flow away from skin and splanchnic regions and toward active muscles; increased
metabolic rate; increased blood glucose concentration; increased ventilation, with dilation
of the airways; decreased GI motility and secretions; and increased mental activity
More widespread and longer lasting than parasympathetic responses:
• divergence is more extensive; as a result, many tissues are activated
simultaneously
• slower break down of NE compared to ACh
• NE (and epinephrine) also from adrenal medullae
Parasympathetic
The parasympathetic division enhances “rest-and-digest” activities
Energy conservation-restorative system…conserve and restore body energy during
times of rest or recovery
“SLUDD”
Salivation
Lacrimation
Urination
Digestion
Defecation
Also “three decreases”
- decreased heart rate
- decreased airway diameter (bronchoconstriction)
- decreased pupil diameter (pupillary constriction)
725
Effects of the ANS on Organ Systems
Sympathetic
receptor
β1
β1
β1
Parasympathetic
Action*
↓ heart rate
↓ contractility (atria)
↓AV node conduction
Organ
Heart
Sympathetic Action
↑ heart rate
↑ contractility
↑ AV node conduction
Vascular
smooth muscle
Constricts blood vessels in
skin; splanchnic
Dilates blood vessels in
skeletal muscle
α1
GI tract
↓ motility
Constricts sphincters
α2, β2
α1
↑ motility
Relaxes sphincters
Bronchioles
Dilates bronchiolar smooth
muscle
β2
Constricts bronchiolar
smooth muscle
Male sex organs
Ejaculation
α
Erection
Bladder
Relaxes bladder wall
Constricts sphincter
β2
α1
Contracts bladder wall
Relaxes sphincter
Sweat glands
↑ sweating
Muscarinic**
Kidney
↑ renin secretion
β1
Adipocytes
↑ lipolysis
β1
*Receptors are muscarinic
** Sympathetic cholinergic
726
β2
HYPOTHALAMIC AND BRAINSTEM CENTERS
• Centers in the hypothalamus and brainstem coordinate the autonomic regulation of
organ system functions.
• Axons extend from the hypothalamus to sympathetic and parasympathetic nuclei in the
brainstem and spinal cord.
• Through the ANS, the hypothalamus is a major regulator of visceral activities,
including temperature regulation, thirst, food intake (satiety), micturition, breathing, and
cardiovascular (vasomotor) activity.
• The medulla contains several nuclei that control vital body functions.
The cardiovascular center regulates the rates and the force of the heartbeat and
the diameter of blood vessels.
The medullary rhythmicity area of the respiratory center adjusts the basic rhythm
of breathing.
Other nuclei in the medulla control reflexes for vomiting, coughing, swallowing,
hiccupping, and sneezing.
• The pons contains the pneumotaxic and the apneustic areas. Together with the
medullary rhythmicity area, these areas help control breathing.
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Autonomic Regulation of Heart Rate
INPUT TO CARDIOVASCULAR CENTER
From higher brain centers: cerebral
cortex, limbic system, and hypothalamus
From sensory receptors:
Proprioceptors—monitor joint movements
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Chemoreceptors—monitor blood H , CO2, O2
Baroreceptors—monitor blood pressure
OUTPUT TO HEART
Increased rate of spontaneous
depolarization in SA node (and AV node)
increases heart rate
Cardiovascular center
Increased contractility of atria and
ventricles increases stroke volume
Decreased rate of spontaneous
depolarization in SA node (and AV node)
decreases heart rate
1. Cardiac accelerator nerves (sympathetic)
2. Vagus (X) nerves (parasympathetic)
• Even before physical activity begins, especially in competitive situations, heart rate may
climb. This anticipatory increase occurs because the limbic system sends nerve impulses
to the CV center. As physical activity begins, proprioceptors that are monitoring the
position of the limbs and muscles send nerve impulses at an increased frequency to the
CV center. Proprioceptor input is a major stimulus for the quick rise in heart rate that
occurs at the onset of physical activity.
• Other sensory receptors that provide input to the CV center include chemoreceptors,
which monitor the chemical changes in the blood, and baroreceptors, which monitor the
stretching of major arteries and veins caused by the pressure of the blood flowing through
them. Important baroreceptors are the carotid sinus and the aortic sinus, which detect
changes in blood pressure and provide input to the CV center when it changes. The
vasomotor center compares this information to a blood pressure set point. If corrections
are necessary, the vasomotor center orchestrates changes in output of both the
sympathetic and the parasympathetic innervation of the heart and blood vessels to bring
about the necessary change in blood pressure.
• Sympathetic neurons extend from the medulla into the spinal cord. From the thoracic
region of the spinal cord, sympathetic cardiac accelerator nerves extend out to the SA
node, AV node, and most portions of the myocardium. Impulses in these nerves trigger
the release of NE, which binds to β1 receptors on cardiac muscle fibers. This interaction
has two specific effects: (1) In SA (and AV) node fibers, NE speeds the rate of
spontaneous depolarization so that these pacemakers fire impulses more rapidly and heart
rate increases; (2) in contractile fibers throughout the atria and ventricles, NE enhances
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Ca++ entry through voltage-gated slow Ca++ channels, thereby increasing contractility.
As a result, a greater volume of blood is ejected during systole. With a moderate increase
in heart rate, SV does not decline because the increased contractility offsets the decreased
preload. With maximal sympathetic stimulation, however, heart rate may reach 200
beats/min in a 20-year-old person. At such a high heart rate, SV is lower than at rest due
to the very short filling time. The maximal heart rate declines with age; as a rule,
subtracting one’s age from 220 provides a good estimate of maximal heart rate in beats
per minute.
• Parasympathetic nerve impulses reach the heart via the right and left vagus (X) nerves.
Vagal axons terminate in the SA node, AV node, and atrial myocardium. They release
ACh, which decreases heart rate by slowing the rate of spontaneous depolarization in
autorhythmic fibers. As only a few vagal fibers innervate ventricular muscle, changes in
parasympathetic activity have little effect on the contractility of the ventricles.
• A continually shifting balance exists between sympathetic and parasympathetic
stimulation of the heart. At rest, parasympathetic stimulation predominates. The resting
heart rate—about 75 beats per minute—is usually lower than the autorhythmic rate of the
SA node (~100 beats per minute). With maximal stimulation by the parasympathetic
division, the heart can slow to 20 or 30 beats/min., or can even stop momentarily.
REVIEW QUESTIONS
1. The ANS innervates all of the following EXCEPT
A. muscles in the wall of the stomach
B. biceps muscle
C. cardiac muscle
D. pituitary gland
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E. muscles responsible for vasoconstriction
2. Cell bodies of preganglionic sympathetic neurons are located
A. in the brain
B. in the sacral spinal cord
C. in the thoracic spinal cord
D. throughout the CNS
E. in autonomic ganglia only
3. Which autonomic receptor is activated by low concentrations of epinephrine released
from the adrenal medulla and causes vasodilation?
A. Adrenergic α receptors
B. Adrenergic β1 receptors
C. Adrenergic β2 receptors
D. Cholinergic muscarinic receptors
E. Cholinergic nicotinic receptors
4. Which of the following autonomic drugs acts by stimulating adenylate cyclase?
A. Atropine
B. Clonidine
C. Curare
D. Norepinephrine
E. Propranolol
5. Which autonomic receptor mediates secretion of epinephrine by the adrenal medulla?
A. Adrenergic α receptors
B. Adrenergic β1 receptors
C. Adrenergic β2 receptors
D. Cholinergic muscarinic receptors
E. Cholinergic nicotinic receptors
6. Which autonomic receptor mediates an increase in heart rate?
A. Adrenergic α receptors
B. Adrenergic β1 receptors
C. Adrenergic β2 receptors
D. Cholinergic muscarinic receptors
E. Cholinergic nicotinic receptors
7. Which adrenergic receptor produces its stimulatory effects by the formation of IP3 and
an increase in intracellular Ca++?
A. α1 receptors
B. α2 receptors
C. β1 receptors
D. β2 receptors
E. Muscarinic receptors
8. Which of the following responses is mediated by parasympathetic muscarinic
responses?
A. Dilation of bronchiolar smooth muscle
B. Erection
C. Ejaculation
D. Constriction of GI sphincters
E. Increased cardiac contractility
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9. Which autonomic receptor is blocked by hexamethonium at the ganglia, but not at the
neuromuscular junction?
A. Adrenergic α receptors
B. Adrenergic β1 receptors
C. Adrenergic β2 receptors
D. Cholinergic muscarinic receptors
E. Cholinergic nicotinic receptors
10. NE and epinephrine both increase the heart rate and produce vasoconstriction in the
skin. On the other hand, epinephrine produces vasodilation in skeletal muscle where NE
produces vasoconstriction. These observations are consistent with the view that
A. NE stimulates only α receptors
B. NE stimulates only β receptors
C. NE stimulates α receptors of the heart and β receptors of the arterioles
D. NE stimulates α receptors of arterioles and β receptors of the heart, but not β
receptors of the arterioles
E. epinephrine stimulates only β receptors
11. Atropine is a drug that blocks the action of ACh on smooth muscles, glands, and the
heart. Which of the following actions would you expect atropine to have?
A. loss of control over the diaphragm
B. Cardiac asystole
C. Pupillary constriction
D, Decreased bronchial secretions
12. Which of the following is a feature of the sympathetic, but not the parasympathetic,
nervous system?
A. Ganglia located in the effector organs
B. Long preganglionic fibers
C. Preganglionic neurons release NE
D. Preganglionic neurons release ACh
E. Preganglionic neurons originate in the thoracolumbar spinal cord
F. Postganglionic neurons synapse on effector organs
G. Postganglionic neurons release epinephrine
H. Postganglionic neurons release ACh
13. The only action below not caused by stimulation of postganglionic sympathetic
neurons is
A. an increase in heart rate and stroke volume
B. vasodilation in the gastrocnemius muscle
C. erection of the penis
D. ejaculation in the male
E. renin secretion
14. The only action below not caused by stimulation of postganglionic parasympathetic
neurons is
A. release of ACh
B. a decreased conduction time in the AV node of the heart
C. relaxation of the internal anal sphincter
D. contraction of the circular muscle of the iris
E. secretion by the sweat glands
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15. What type of receptor does epinephrine stimulate in veins to produce venous
constriction?
A. Nicotinic
B. Muscarinic
C. α1
D. α2
E. β1
16. What type of receptor does epinephrine stimulate in the arterioles to produce
arteriolar constriction?
A. Nicotinic
B. Muscarinic
C. α1
D. α2
E. β1
17. What type of receptor does epinephrine stimulate in arterioles to decrease arteriolar
constriction?
A. Nicotinic
B. Muscarinic
C. α2
D. β1
E. β2
18. Terminal ganglia are where
A. the cell bodies for sympathetic preganglionic fibers are located.
B. preganglionic parasympathetic fibers synapse with postganglionic parasympathetic
fibers.
C. the cell bodies of sensory neurons are located.
D. preganglionic sympathetic fibers synapse with postganglionic sympathetic fibers.
E. preganglionic sympathetic fibers synapse with postganglionic parasympathetic fibers.
19. Most autonomic sensory neurons are associated with
A. exteroceptors
B. interoceptors
C. proprioceptors
D. special senses
E. somatic efferent neurons
20. To say that most organs served by the ANS have “dual innervation” means that
A. these organs release either ACh or NE when stimulated.
B. it takes two postganglionic neurons to achieve the desired response.
C. the organs are innervated by both sympathetic and parasympathetic neurons.
D. the organs have both α and β receptors.
E. both a preganglionic and postganglionic neuron go to the organ.
21. An adrenergic neuron produces the neurotransmitter
A. serotonin.
B. GABA.
C. NE.
D. ACh.
E. glycine.
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22. The term “craniosacral outflow” refers to
A. release of ACh from preganglionic neurons.
B. axons of postganglionic parasympathetic neurons.
C. axons of postganglionic sympathetic neurons.
D. axons of preganglionic parasympathetic neurons.
E. axons of preganglionic sympathetic neurons.
23. In which of the following would synapses between preganglionic and postganglionic
sympathetic fibers occur?
A. Ciliary ganglion
B. Superior mesenteric ganglion
C. Celiac ganglion
D. Otic ganglion
24. Which of the following responses is initiated by the sympathetic nervous system?
A. Decreased heart rate
B. Constriction of the pupils
C. Splitting glycogen to glucose by the liver
D. Constriction of the bronchioles
E. Decreased blood pressure
25. Parasympathetic stimulation to the nasal mucosa, pharynx, and lacrimal glands is
provided by fibers arising from the
A. superior cervical ganglion.
B. middle cervical ganglion.
C. inferior cervical ganglion.
D. sphenopalatine ganglion.
E. submandibular ganglion.
26. A major organ that receives sympathetic stimulation, but not parasympathetic
stimulation is the
A. heart.
B. liver.
C. stomach.
D. lung.
E. kidney.
27. ACh exerts its effects on postsynaptic cells when it
A. is broken down by enzymes in the synaptic cleft, and the end-products diffuse into the
postsynaptic cell.
B. binds to specific receptors on the postsynaptic cell membrane and changes the
permeability of the membrane to particular ions.
C. diffuses into the postsynaptic cell and changes the pH of the intracellular fluid.
D. actively transports ions from the synaptic cleft into the postsynaptic cell.
E. blocks specific receptors to which other neurotransmitters could attach.
28. Because the principal active ingredient in tobacco is nicotene, you might expect
smoking to enhance the effects of
A. ACh of parasympathetic visceral effectors.
B. ACh on any postganglionic neurons.
C. NE on the heart and blood vessels.
D. NE on most sympathetic visceral effectors.
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29. Cholinergic postganglionic sympathetic neurons stimulate the
A. adrenal medullae.
B. heart.
C. sweat glands.
D. walls of the GI tract.
30. You have just discovered that your pants were unzipped the entire time you gave a
speech to your small group session in our CaTOS class. Which of the following
responses would you likely experience?
A. Increased parasympathetic stimulation to the iris.
B. Increased parasympathetic stimulation to the stomach.
C. Increased sympathetic stimulation to the heart.
D. Decreased sympathetic stimulation to the bronchioles.
E. Decreased sympathetic stimulation to the small intestine.
31. Which of the following is stimulated by the parasympathetic nervous system?
A. Dilation of the bronchioles
B. Erection of the penis
D. Dilation of the pupil
D. Increased secretion by sweat glands in the palms and soles
32. You are just about to perform a certain “sensitive” clinical procedure for the first
time on a real patient in your ICM class and your palms begin to sweat. This is due to
A. increased sympathetic stimulation of sweat glands possessing α receptors.
B. increased sympathetic stimulation of sweat glands possessing β receptors.
C. increased parasympathetic stimulation of sweat glands possessing nicotinic receptors.
D. increased parasympathetic stimulation of sweat glands possessing muscarinic
receptors.
E. increased sympathetic stimulation of sweat glands possessing muscarinic receptors.
F. an allergic reaction to latex gloves.
33. A patient with chronic hypertension is treated with prazosin by his physician. The
treatment successfully decreases the patient’s blood pressure to within the normal range.
What is the mechanism of the drug’s action?
A. Inhibition of β1 receptors in the SA node
B. Inhibition of β2 receptors in the SA node
C. Stimulation of muscarinic receptors in the SA node
D. Stimulation of nicotinic receptors in the SA node
E. Inhibition of β1 receptors in ventricular muscle
F. Stimulation of β1 receptors in ventricular muscle
G. Inhibition of α1 receptors in ventricular muscle
H. Stimulation of α1 receptors in the SA node
I. Inhibition of α1 receptors in the SA node
J. Inhibition of α1 receptors on vascular smooth muscle
K. Stimulation of α1 receptors on vascular smooth muscle
L. Stimulation of α2 receptors on vascular smooth muscle
34. Prevertebral ganglia receive preganglionic parasympathetic fibers.
A. True
B. False
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35. Axons of preganglionic parasympathetic neurons synapse with 20 or more
postganglionic neurons.
A. True
B. False
36. Cholinergic neurons release NE and epinephrine.
A. True
B. False
37. All preganglionic neurons release ACh.
A. True
B. False
38. Nicotinic receptors bind epinephrine.
A. True
B. False
39. Atropine is a drug that blocks muscarinic receptors.
A. True
B. False
40. Monoamine oxidase inactivates epinephrine (and NE).
A. True
B. False
41. Activation of β3 receptors causes thermogenesis.
A. True
B. False
42-52. Write S if the description applies to the sympathetic division of the ANS, P if it
applies to the parasympathetic division, and PS if it applies to both.
_____ 42. Also called thoracolumbar outflow
_____ 43. Has long preganglionic fibers leading to terminal ganglia and very short
postganglionic fibers
_____ 44. Celiac and superior mesenteric ganglia are sites of postganglionic neuron cell
bodies
_____ 45. Sends some preganglionic fibers through cranial nerves
_____ 46. Has some preganglionic fibers synapsing in vertebral chain (trunk)
_____ 47. Has more widespread effect in the body, affecting more organs
_____ 48. Has fibers running in gray rami communicates to supply sweat glands,
arrector pili muscles, and blood vessels
_____ 49. Has fibers in white rami
_____ 50. Contains fibers that supply viscera with motor impulses
_____ 51. The greater and lesser splanchnic nerves contain axons of this division
_____ 52. The pelvic splanchnic nerves contain axons of this division
53-65. Use arrows to show whether parasympathetic (P) or sympathetic (S) fibers
stimulate (↑) or inhibit (↓) each of the following activities. Use a dash (—) to indicate
that there is no (or virtually no) parasympathetic innervation.
53. P_____ S_____ Dilation of pupil
54. P_____ S_____ Heart rate and blood flow to coronary blood vessels
55. P_____ S_____ Constriction of blood vessels of skin and abdominal viscera
56. P_____ S_____ Salivation and digestive organ contractions
57. P_____ S_____ Erection or engorgement of genitalia
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58. P_____ S_____ Dilation of bronchioles for easier breathing
59. P_____ S_____ Contraction of bladder and relaxation of internal urethral sphincter
causing urination
60. P_____ S_____ Contraction of arrector pili of hair follicles causing “goose bumps”
61. P_____ S_____ Contraction of spleen which transfers some of its blood to general
circulation, causing increase in blood pressure
62. P_____ S_____ Release of epinephrine and NE from adrenal medulla
63. P_____ S_____ Secretion of insulin and digestive enzymes from the pancreas
64. P_____ S_____ Increase in blood glucose level by pancreatic secretion of the
hormone glucagon and by formation of glucose in the liver
65. P_____ S_____ Coping with stress, fight-or-flight response
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66. Complete the sympathetic pathways shown in the diagram below. A preganglionic
neuron located at T5 of the cord has its axon drawn as far as the white ramus. Finish this
pathway by showing how the axon may branch and synapse to eventually innervate these
three organs: the heart, a sweat gland in the skin of the thoracic region, and the stomach.
Draw the preganglionic fibers in solid lines and the postganglionic fibers in broken lines.
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Answers: 1B, 2C, 3C, 4D, 5E, 6B, 7A, 8B, 9E, 10D, 11D, 12E, 13C, 14E, 15C, 16C,
17E, 18B, 19B, 20C, 21C, 22D, 23BC, 24C, 25D, 26E, 27B, 28B, 29C, 30C, 31B, 32A,
33J, 34B, 35B, 36B, 37A, 38B, 39A, 40A, 41A, 42S, 43P, 44S, 45P, 46S, 47S, 48S, 49S,
50PS, 51S, 52P, 53P↓S↑, 54P↓S↑, 55P—S↑, 56P↑S↓, 57P↑S↓, 58P↓S↑, 59P↑S↓,
60P—S↑, 61P—S↑, 62P—S↑, 63P↑S↓, 64P—S↑, 65P↓S↑, 66 see below
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