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Biology 232
Human Anatomy and Physiology
Chapter 16 Lecture Outline
Autonomic Nervous System – regulates activities of smooth muscle, cardiac muscle,
and glands; operate at all times, usually without conscious control or perception
operates mainly via reflex arcs
Comparison of Somatic and Autonomic Nervous Systems
SOMATIC
stimuli – conscious sensations
higher brain functions
integration – cerebral cortex
other brain regions & spinal cord
conscious perception
voluntary control
motor – somatic motor neurons
1-neuron pathways from CNS
effectors – skeletal muscles
AUTONOMIC
stimuli – mainly visceral interoceptors
(some conscious sensations)
integration – hypothalamus
brain stem, spinal cord & limbic system
usually not perceived
little voluntary control
motor – autonomic motor neurons
2-neuron pathways from CNS
effectors – smooth muscles, cardiac
muscle, glands, adipocytes
neurotransmitters – ACh & NE
response – excitation or inhibition
neurotransmitter – ACh
response – excitation
Autonomic Reflex Pathways
sensory receptors and neurons – mainly visceral interoceptors
found in internal organs and blood vessels; sensory axons run to spinal
cord or brain stem
conscious sensations can also affect the ANS (eg. light, sound, taste)
integrating centers – hypothalamus, brain stem, spinal cord, and limbic system;
little conscious perception or voluntary control
autonomic motor neurons – 2-neuron pathways from CNS; have excitatory or
inhibitory effect on effector tissues
autonomic ganglion – site of synapse between 2 autonomic motor neurons
preganglionic neurons – myelinated
release ACh
cell bodies in brainstem or spinal cord
axons run to autonomic ganglia
postganglionic neurons – unmyelinated
release norepinephrine (NE) or Ach
cell bodies in autonomic ganglia
axons run to effectors
autonomic effectors – smooth muscle, cardiac muscle, glands, adipocytes
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dual innervation – many organs are innervated by 2 divisions of ANS
one division is usually excitatory, the other inhibitory
Sympathetic division – motor neurons that trigger fight-or-flight
responses due to physical or emotional stress
pupils dilate
increased heart rate and force
increased blood flow to skeletal muscles, lungs
airways dilate, respiratory rate increased
energy released from storage in liver and adipocytes
increased metabolism and sweating
inhibition of non-essential activities – digestive & urinary function
Parasympathetic division – motor neurons that trigger rest-and-digest
activities; function in production and storage of energy
pupils constrict
decreased heart rate and force
airway constriction
stimulation of digestive and urinary functions
increased nutrient absorption in cells
increased energy storage
sexual arousal
Sympathetic Division (Thoracolumbar Division)
preganglionic neurons – cell bodies in lateral gray horns of spinal cord segments
T1-L2
axon travels through ventral root of spinal nerve and rami communicantes;
axon terminal releases ACh at autonomic ganglion
always excitatory at postganglionic neuron
autonomic ganglion
near spinal cord = short preganglionic axon and long postganglionic axon
sympathetic chain ganglia – vertical row on either side of vertebral
column from base of skull to coccyx
innervate head, neck, body wall, limbs, thoracic organs
prevertebral (collateral) ganglia – within abdominal cavity anterior to
vertebral column
innervate abdominopelvic organs and tissues
sympathetic preganglionic axons diverge widely (many axon collaterals)
form many synapses at multiple autonomic ganglia
postganglionic neuron – most release NE (some ACh)
usually diverge and synapse with several visceral effectors
may be excitatory or inhibitory
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adrenal medulla – modified sympathetic ganglion in center of adrenal gland
neuroendocrine cells – modified postganglionic neurons
preganglionic neurons trigger release of NE (noradrenalin) and
epinephrine (E – adrenalin)
NE and epinephrine act as hormones – released into interstitial space and
diffuse into bloodstream
circulate throughout body and affect any tissue with receptors
Summary of Sympathetic Division:
thoracolumbar division
preganglionic neuron – short axon; releases ACh
arise at spinal cord segments T1-T12 and L1-L2
autonomic ganglia
sympathetic chain ganglia – lateral to spinal column
prevertebral ganglia – anterior to spinal column
postganglionic neuron – long axon; most release NE
has rapid, widespread affect
widely divergent axons
innervates most tissues and organs of the body
circulation of adrenal hormones through the blood
Parasympathetic Division (Craniosacral Division)
preganglionic neurons - cell bodies in nuclei of c.n.III, VII, IX, X, and
lateral gray horns of spinal cord segments S2-S4
axons travel through cranial nerves or ventral horn & rami communicantes
axon terminals release ACh at autonomic ganglia – always excitatory
preganglionic axons don’t diverge as widely as they do in the
sympathetic division
c.n.III,VII,IX innervate inner eye and glands of eye, nose, mouth
c.n. X (vagus) innervates throat, thoracic and abdominal organs
sacral segments innervate inferior abdominopelvic organs
autonomic ganglia
near or within effectors = long preganglionic axons and short
postganglionic axons
terminal ganglia – near effector (intramural=in wall)
postganglionic neurons – release ACh; may be excitatory or inhibitory
each postganglionic neuron synapses at a single effector
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Summary of Parasympathetic Division
craniosacral division
preganglionic neuron – long axon; releases ACh
arise at brainstem nuclei of c.n.III, VII, IX, X
and spinal cord segments S2-S4
autonomic ganglia
terminal ganglia – near or within effectors
postganglionic neuron – short axon; releases ACh
affect is less widespread, more specific
little divergence of axons
innervates only specific organs in head, neck and body cavity
no hormonal component as in sympathetic division
autonomic plexuses – complex networks of autonomic motor neurons and some
autonomic sensory neurons running between ganglia in thoracic and
abdominal cavities
sympathetic, splanchnic, and pelvic nerves – preganglionic and postganglionic
axons traveling to ganglia or effectors in body cavities
Neurotransmitters of the ANS
cholinergic neurons – release acetylcholine (ACh)
all preganglionic neurons
all parasympathetic postganglionic neurons
a few sympathetic postganglionic neurons (eg. sweat glands)
cholinergic receptors – postsynaptic integral membrane proteins
2 types:
nicotinic receptors – found on all postganglionic neurons and
skeletal muscles
always cause excitation (open Na+ channels)
nicotine is an agonist at these receptors
muscarinic receptors – on all parasympathetic effectors
(and a few sympathetic effectors)
may cause excitation or inhibition
(G-proteins – activate complex pathways that use
enzymes to alter ion permeability)
(muscarine (a mushroom toxin) is an agonist)
acetylcholinesterase – enzymatically inactivates ACh rapidly
ACh has a short duration of effect at cholinergic receptors
agonist – mimics neurotransmitter and activates receptor
(eg. nicotine, muscarine)
antagonist – binds or blocks receptor and prevents activation
(eg. atropine – blocks muscarinic receptors)
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adrenergic neurons – release norepinephrine (NE)
most sympathetic postganglionic neurons
adrenergic receptors – most sympathetic effectors
activated by NE and E (from adrenal medulla)
may cause excitation or inhibition due to different receptors
2 classes of adrenergic receptors:
alpha receptors – some constrict blood vessels
beta receptors – some increase heart rate,
others dilate airways
inactivated by reuptake into neurons or diffusing away
enzymes in cells and tissues break down NE and E
effect lasts longer than ACh
Effects of Sympathetic and Parasympathetic Division
EFFECTOR
adrenal medullae
lacrimal glands
pancreas
salivary glands
posterior pituitary
sweat glands
digestive glands
adipocytes
liver
iris
ciliary muscles of eye
lungs – bronchiole muscles
gall bladder and ducts
stomach and intestines
spleen
urinary bladder
arrector pili muscles
SYMPATHETIC
GLANDS
secretion of epinephrine
and norepinephrine
no known effect
inhibits secretion of
digestive enzymes
inhibits secretion
secretion of ADH
increases secretion
decreases secretion
breakdown of triglycerides
to release fatty acids and
glycerol into blood
release of glucose (various
mechanisms)
decreases bile secretion
SMOOTH MUSCLE
dilation of pupil
(radial muscle)
relaxation – distant vision
dilates airways
relaxation – no bile release
decreased motility and tone;
sphincter contraction
contraction – releases stored
blood into circulation
relaxation of muscle wall;
sphincter contraction
erection of hairs
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PARASYMPATHETIC
no known effect
secretion of tears
increases secretion of
digestive enzymes
increases secretion
no known effect
no known effect
increases secretion
no known effect
glucose storage
(as glycogen)
increases bile secretion
contraction of pupil
(circular muscle)
contraction – close vision
constricts airways
contraction – releases bile
increased motility and tone;
sphincter relaxation
no known effect
contraction of muscle wall;
sphincter relaxation
no known effect
VASCULAR SMOOTH MUSCLE – ARTERIOLES OF ORGANS
heart arterioles
variable
no known effect
skin and mucosal arterioles
variable
no known effect
skeletal muscle arterioles
dilation – increases flow
no known effect
abdominal viscera arterioles
variable
no known effect
brain arterioles
dilation – increases flow
no known effect
kidney arterioles
variable
no known effect
systemic veins
constriction – reduces flow
no known effect
increased heart rate and
decreased heart rate and
CARDIAC MUSCLE
force of contraction
force of contraction
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