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Autonomic Nervous
System
Keri Muma
Bio 6
Organization of the Nervous System
Efferent Division
Somatic Nervous System
Voluntary
control
= skeletal muscles
Muscles must be excited by a motor neuron or
they are inactive (On or off)
One motor neuron extends from the CNS all the
way to the effector muscle
Effector
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Efferent Division
Autonomic Nervous System
Involuntary control
Effectors = glands, smooth and cardiac muscles
Two neuron chain between the CNS and the
effector
Preganglionic neuron
Postganglionic neuron
Anatomy of the Sympathetic System
Origin sites
Thoracolumbar emerges from thoracic
and lumbar regions T1L2
Length of neurons
Short preganglionic
Long postganglionic
Location of Sympathetic Ganglia
Location of ganglia
Close to vertebral column
Paravertebral ganglia –
chain running parallel to the
vertebral column, extends
from C3 to S4
Collateral (prevertebral)
ganglia – lie anterior to the
vertebral column
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Path of Sympathetic Neurons
Cell bodies are located in the
lateral gray horns of spinal
segments T1 – L2
White ramus communicans
– pre neuron passes from the
spinal nerve to the
paravertebral ganglia
Gray ramus communicans –
post neuron exits the
paravertebral ganglia
Path of Sympathetic Neurons
Three different routes the preganglion neuron
can take once it enters the paravertebral ganglia
Anatomy of the Parasympathetic
System
Origin sites
Craniosacral emerges from
brainstem and sacral
regions
Length of neurons
Long preganglionic
Short postganglionic
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Location of Parasympathetic
Ganglia
Ganglia close to target
organ
Terminal ganglia – very
close to target organ
Intramural ganglia – within
the walls of the target organ
Structural Differences between
ANS Divisions
Division
Sympathetic
Origin of Fibers
Thoracolumbar
region of the
spinal cord
Parasympathetic Brain and sacral
spinal cord
Length of
Fibers
Location of
Ganglia
Short
preganglionic
and long
postganglionic
Close to the
spinal cord
Long
preganglionic
and short
postganglionic
Close to or in
the visceral
effector
organs
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Regulatory Functions of the ANS
Cardiovascular activities
Body fluid chemistry
Pulmonary activities
Gastrointestinal activities
Cardiac output, heart rate, blood pressure and distribution
pH, osmolarity, thirst, water content
Breathing rate, bronchiole diameter, O2 and CO2 content
Motility along digestive tract, mechanical and chemical
digestion
Visceral reflexes
Stress
Micturation, defecation, sexual reflexes
Stimulates various hormones to cope with situation
Autonomic Control Centers
Functions of the ANS
Sympathetic division – fight or flight
Enables body to cope rapidly during emergency
situations
Dominant when excited, frightened, or during exercise
Increase heart rate and blood pressure
Increase respiratory rate, dilates bronchioles
Blood shunted to skeletal muscles, brain, and heart away
from digestive organs and skin
Dilates pupils
Liver releases glucose to meet increased energy needs
Increased cellular metabolism
Initiates sweating to lower body temperature
Increased RBC production and clotting ability
Na+ absorption / K+ secretion, decreased urine production
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Functions of the ANS
Parasympathetic division – rest and digest
Dominant
in non-stressful situations
energy and directs maintenance
activities such as digestion and excretion
Conserves
Blood shunted to visceral organs
Constricts pupils
Increased digestive glandular secretions and activity
Respiratory and lacrimal secretions
Blood pressure, heart rate, respiratory rates at low
normal levels
Divisions of the ANS
Dual Innervation - most internal organs are
innervated by both autonomic divisions
Antagonistic control – the divisions counterbalance
each other by continuously making adjustments
The ANS either further excites or inhibits the organs
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Dual Innervation
There are exceptions to dual innervation:
Most
blood vessels, sweat glands, and
arrector pili muscles are controlled by
sympathetic fibers only
Sympathetic Tone:
The
sympathetic division controls blood
pressure and keeps the blood vessels in a
continual state of partial constriction
Sympathetic Tone
Increased
sympathetic activity
constricts blood
vessels
Decreased
sympathetic activity
dilates blood vessels
Sympathetic Effects
Some sympathetic metabolic effects are not
reversed by the parasympathetic division
Increases
the metabolic rate of body cells
Raises blood glucose levels
Mobilizes fat as a food source
Stimulates the reticular activating system (RAS)
of the brain, increasing mental alertness
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Sympathetic Effects
The sympathetic nervous system triggers the
release of epinephrine from the adrenal medulla
Some preganglion fibers travel to the adrenal medulla
where the synapse with hormone producing cells
These cells release epinephrine into the blood stream
ANS Neurotransmitters
Acetylcholine (ACh) and norepinephrine (NE) are
the two major neurotransmitters of the ANS
Neurons
Neurons
that release ACh are cholinergic fibers
that release NE are adrenergic fibers
Cholinergic Fibers
All parasympathetic pre and postganglionic
neurons
All sympathetic preganglionic neurons
Sympathetic postganglionic that innervate sweat
glands and arrector pili muscles
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Adrenergic Fibers
Most sympathetic postganglionic neurons
(except sweat glands and arrector pili)
Types of Receptors
Cholinergic receptors – bind Ach
Nicotinic
Muscarinic
Adrenergic receptors – binds NE and/or E
Alpha
Beta
ANS Receptors
Neurotransmitter effects can be excitatory or
inhibitory depending upon the receptor type they
bind to
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Nicotinic Receptors
Nicotinic receptors are found on:
Motor
end plates (somatic targets)
postganglionic neurons of both sympathetic
and parasympathetic divisions
The hormone-producing cells of the adrenal
medulla
All
The effect of ACh binding to nicotinic
receptors is always excitatory
Muscarinic Receptors
Muscarinic receptors are found on all effectors
stimulated by postganglionic cholinergic fibers
Parasympathetic effectors
Sweat glands and arrector pili
The effect of ACh binding to muscarinic receptors:
Can be either inhibitory or excitatory depending
on the receptor subtype of the target organ
Examples – slows cardiac muscle: inhibitory
smooth muscle of digestive: excitatory
Adrenergic Receptors
Alpha
receptors– greater sensitivity to NE
α1 – binding of NE is stimulatory
Constriction of blood vessels serving the skin
and abdominal viscera
α2 – binding of NE is inhibitory
Inhibits insulin secretion from the pancreas
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Adrenergic Receptors
Beta Receptors
β1 – found primarily in the heart, binding of NE
and E is excitatory
Increases cardiac output, release of renin from
kidneys to increase BP
β2 – binding of E is generally inhibitory
Dilates coronary blood vessels and bronchioles in
the lungs, relaxes digestive and urinary smooth
muscle
β3 – found in adipose tissue
Stimulates lipolysis
Localized vs. Wide Spread Effects
Parasympathetic – localized and short lived effect
Preganglionic synapse with one or a few
postganglionic
ACH is quickly destroyed
Sympathetic – longer lasting and body wide
mobilization
Preganglionic synapse with many posts at different
levels
NE is inactivated slower than ACH
NE and E are indirect neurotransmitters, using a
second-messenger system
Epinephrine is released into the blood and remains
there until destroyed by the liver
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Effects of Drugs on the ANS
Drugs are designed to obtain an inhibitory
or excitatory effect on a target organ by
either blocking or initiating desired effects
of neurotransmitters
Sympathomimetic
agents – enhance
sympathetic response
Sympatholytic agents – decrease sympathetic
response
Parasympathomimetic / Parasympatholytic
Effects of Drugs
Atropine – blocks muscarinic receptors therefore
blocks parasympathetic effects, increase heart
rate and fecal and urinary retention
Tricyclic antidepressants – prolong the activity of
NE on postsynaptic membranes
Over-the-counter drugs for colds, allergies, and
nasal congestion – stimulate α-adrenergic
receptors
parasympatholytic
sympathomimetic
sympathomimetic
Effects of Drugs
Beta-blockers – blocks cardiac B1 receptors,
decreases HR and BP
Alpha-blockers - interfere with vasomotor fibers
and are used to treat hypertension
Salbutamol – activates B2 receptors, dilates
bronchioles (asthma treatment)
sympatholytic
sympatholytic
sympathomimetic
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Age and the ANS
In old age, ANS efficiency decreases,
resulting in constipation, dry eyes, and
orthostatic hypotension
Orthostatic
hypotension is a form of low blood
pressure that occurs when sympathetic
vasoconstriction centers respond slowly to
positional changes
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