Autonomic Nervous System
ANS
Honors Anatomy & Physiology
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Comparison Of Autonomic &
Somatic Nervous Systems
2 Neuron Chain
Preganglionic Neurons
Sympathetic
• 1st neuron in spinal
cord: thoracic and
lumbar spinal nerves
Parasympatheitc
• 1st neuron in 4 cranial
nerves in the brain
stem (III, VII, IX, X)
or in S2 –S4 of spinal
cord
Postganglionic Neurons
Sympathetic
• 2nd neuron in
sympathetic trunk
ganglion, a chain of
ganglion that is found
on either side lateral
to spine
• Parasympathetic
2nd neuron in
individual ganglion
closer to organ it serves
• both sympathetic &
parasympathetic neurons
lie outside CNS
Axons of Autonomic Nerves
• Preganglionic:
– in cranial or spinal n. (from CNS 
ganglion)
– myelinated
• Postganglionic:
– from ganglion  visceral effector
– unmyelinated
ANS Neurotransmitters
• based on the neurotransmitter they
produce & release autonomic
neurons are classified as either:
1. Cholinergic
• release acetylcholine (ACh)
1. Adrenergic
• release norepinephrine (NE) aka
noradrenalin
Cholinergic Neurons &
Receptors
• Cholinergic neurons include:
1. all Sympathetic & Parasympathetic
preganglionic neurons
2. Sympathetic postganglionic
neurons that innervate most sweat
glands
3. all Parasympathetic postganglionic
neurons
Receptors
• that bind ACh called cholinergic
receptors
• 2 types:
1. Nicotinic receptors
•
in plasma membranes & dendrites of
symp. & parasymp postganglionic neurons
& in NMJ
1. Muscarinic receptors
•
•
in plasma membrane of all effectors
(smooth muscle, cardiac muscle, glands)
Muscarine: mushroom poisonmimics
actions of ACh
ACh
• when activates nicotinic receptors 
depolarization (excitation)
• when activates muscarinic receptors
 sometimes depolarization,
sometimes hyperpolarization
(inhibition) depending on the cell
NE
• most sympathetic postganglionic
neurons are adrenergic
• Adrenergic receptors bind both NE
& Epinephrine (Epi)
• 2 types receptors:
1. Alpha receptors (α)
– subtypes: α1, α2
2. Beta receptors (β)
– subtypes: β1, β2, β3
αlpha & βeta Receptors
• α1 & β1 produce excitation when
activated
• α2 & β2 receptors cause inhibition of
effector tissues
• β3 found only on cells of brown
adipose where activation causes
thermogenesis (heat production)
αlpha & βeta Receptors
• cells of most effectors have either α
or β receptors
• some visceral effectors contain both
• NE stimulates α more strongly than
β
• Epi is potent stimulator of both
MAO
• MonoAmine Oxidase: inactivates NE
in synaptic cleft
• group of pharmaceuticals that are
MAO inhibitors so prolong effect of
NE
Agonists
• substance that binds to & activates a
receptor
• in the process mimicking the effect of
a natural neurotransmitter or
hormone
• example: phenylephrine is an
adrenergic agonist @ α1 receptors;
used in cold remedies
– constricts blood vessels in nasal mucosa
 reduces production of mucus
Antagonists
• substance that blocks receptors so
prevents the natural
neurotransmitter or hormone from
exerting its effect
• example: propanolol a β1 blocker,
used to treat HTN  decreases heart
rate & force of contraction  lowers
BP
– side effects: hypoglycemia, mild
bronchoconstriction, decreases
frequency & severity of migraines
Autonomic Tone
• balance between sympathetic &
parasympathetic activity
• regulated by hypothalamus
– if turns up sympathetic tone, turns
down parasympathetic tone @ same
time
Sympathetic Responses
• dominate during physical or
emotional stress
• occur during “E situations”
– Exercise
– Emotions
– Emergency
– Excitement
Sympathetic Responses
• Fight or Flight Response
pupils dilate
HR, force of contraction, & BP increase
airways dilate
vessels to kidneys & GI tract constrict slowing
down digestion & urine production
– vessels  muscles (skeletal & cardiac), liver, &
adipose tissue  dilate
– hepatocytes increase glycogenolysis & adipose
increase lipolysis  blood glucose increases
– anything nonessential slowed down
–
–
–
–
Sympathetic Stimulation
• effects longer lasting than
parasympathetic responses (NE
lasts longer in synaptic cleft than
ACh)
• effects are more widespread (more
tissues activated)
Parasympathetic Responses
• enhance “rest & digest” activities
• remeber SLUDD:
– Salivation
– Lacrimation
– Urination
– Digestion
– Defecation
Disorders of the ANS
• Raynaud’s phenomena:
• digits become ischemic after exposure to
cold or w/ emotional stress
• due to excessive sympathetic stimulation
of smooth muscle in arterioles in digits &
increased response to stimuli that cause
vasoconstriction
Raynaud’s Phenomenon
• treatment options:
• Ca++ channel blockers to relax smooth
muscle
• Prazosin: blocks α receptors which blocks
smooth muscle contractions