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ANS Review
ANS maintains you & allows you to
respond to the environment
SYMPATHETIC
• Mobilizes body to expend energy
• Targets widely distributed organ systems
Blood vessels, sweat glands,
piloerector muscles
FEAR
Sympathetic & adrenal medulla:
Fight or flight response
mobilizes the body to expend energy for
extreme sustained physical exertion .
• Cardiovascular System
– Increase in heart rate & force of contraction  increased
cardiac output (5L/min  25-40L/min)
– Increase in BP
– Redistribution of blood flow
• Respiratory System
– Airways are dilated
• Digestive System
– GI function (motility and secretions) inhibited
• Metabolism
– Glucose is mobilized
– Lipolysis is stimulated
– Metabolic rate increases
Stimuli that release adrenal
medullary catecholamines
• Physiological stressors
– Psychological: emotional states - fear
– Metabolic
– Hypothermia
• Pathological stressors
– Severe hemorrhage
– Heart attack
– Pain
NOTE: Asthmatic attacks do not release CAs
ANS maintains you & allows you to
respond to the environment
SYMPATHETIC
• Mobilizes body to expend energy
• Targets widely distributed organ systems
PARASYMPATHETIC
• Restorative role; conserves energy
• Discrete targeted control
General feature of ANS
• Tonically active system
• Always have basal level of end organ
activity
• Often modulates ongoing spontaneously
generated activity (heart; GI tract)
• Activity controlled reflexly & most
autonomic reflexes are readily affected by
emotional state
post
pre
Parasympathetic
Cranial
Spinal cord
T1
pre
post
Sympathetic
Thoraco-lumbar
pre
Collateral ganglia
Adrenal
Splanchnic nerve
L3
pre
Parasympathetic
Sacral
post
S2
S3
S4
Schematic ANS: Organization
1:1/1:2
pre
Parasympathetic
Cranial
Spinal cord
T1
Targeted control
Divergence/convergence
Diffuse targets
Sympathetic
pre
Collateral ganglia
Adrenal
Splanchnic nerve
L3
pre
S2
S3
S4
Parasympathetic
Sacral
1:1/1:2
Targeted control
Schematic ANS: Wiring of ganglia
Neuroeffector junction: Sympathetic postganglionic nerve
terminal is a diffusely distributed complex branching network
with varicosities (swellings)
Synaptic vesicle
ANS TRANSMITTERS
Ach
Ach
Spinal cord
T1
Sympathetic postganglionics
to sweat glands
Ach
Ach
Ach
Adrenal
Biological inactivation:
Breakdown: acetylcholinesterase
Resupply: De novo synthesis in terminal
L3
Ach
S2
S3
S4
Ach
• All preganglionic neurons release Ach
• All postganglionic parasympathetic neurons
release Ach
• Sympathetic postganglionics  general sweat
glands (except sweat gland on palms, soles of feet, &
underarms) release Ach
ANS TRANSMITTERS
Ach
Ach
Spinal cord
T1
Ach
NE
Ach
NE
EPI – 80%
Adrenal
L3
Biological inactivation of NE:
REUPTAKE
Resupply of NE: recycling &
new synthesis
Ach
S2
S3
S4
Ach
Sympathetic postganglionic neurons release NE
(except to generalized sweat glands; Adrenal
medulla releases NE and EPI (80%)
Peptides also in LDCV in terminals along with Ach
& NE
Peptides
• Stored in LDCV
• Never located at plasma membrane
• Released only by high frequency
stimulation
• Peptides augment actions of low
molecular weight transmitter (ACh or NE)
• Biological inactivation: breakdown by
peptidases
ANS RECEPTORS – ganglion - nicotinic
Ach
Ach
nicotinic
Spinal cord
nicotinic
T1
Ach
NE
Ach
nicotinic
L3
Differences between ganglionic
& skeletal NMJ nicotinic receptors:
• Subunit composition
• Sensitivity to antagonists
Ach
S2
S3
S4
Ach
nicotinic
Somatic motor nerve
All postganglionic neurons, cells of the adrenal medulla
& skeletal NMJ have nicotinic cholinergic receptors
Skeletal NMJ
ANS RECEPTORS
Ach
Ach
(+)
(-)
Spinal cord
muscarinic
T1
Ach
Sweat gl.
(+)
Sympathetic postganglionics to
generalized sweat glands release Ach
Ach
muscarinic
L3
Ach
S2
S3
S4
Ach
(+)
Cholinergic receptors of the muscarinic type are on
effector tissues innervated by parasympathetic
postganglionic neurons and on generalized sweat
glands
ANS RECEPTORS -Adrenergic
post
pre
Parasympathetic
Cranial
Spinal cord
T1
pre
NE
post
Sympathetic
pre
Adrenal
Splanchnic nerve
L3
pre
EPI
NE
Parasympathetic
Sacral
Alpha-1 & 2
Beta-1 & 2
post
S2
S3
S4
Schematic ANS
Receptors for NE
Alpha-1 receptors
• Excitatory  contraction of smooth muscle
• Most important location -- smooth muscle of blood
vessels
• Insensitive - require high concentrations of agonist; NE &
EPI ~ equipotent
• Under physiological conditions, a-1 receptors are
activated by NE released from sympathetic nerves.
• EPI released from adrenal in response to stress never
reaches high enough concentrations in the circulation to
activate a-1 receptors
• Effects mediated by IP3 production
Receptors for NE
Alpha-2 receptors are presynaptic
• Inhibitory receptors located on nerve terminals.
– Autoreceptors on sympathetic postganglionic
terminals;
– Hetero-receptors on terminals of parasympathetic
nerves of the GI tract bind NE, released from adjacent
postganglionic sympathetic nerves.
– Activation of a-2 receptor reduces evoked transmitter
release by reducing Ca2+ entry into presynaptic terminal
 a-2 heteroreceptors mediate sympathetic inhibition of GI tract
BETA-1 RECEPTORS
• Excitatory response to both NE & EPI
• Location – heart: beats faster and more
strongly
• Beta-1 receptor is very sensitive to both
NE & EPI – so heart can be stimulated by
NE released from sympathetic nerves or
EPI released from the adrenal medulla
• Beta-1 effects mediated by stimulation of
cAMP production
BETA-2 RECEPTORS
• inhibitory  relaxation of smooth muscle
• very sensitive to EPI, but not NE
• activated in response to stress (when adrenal
medulla secretes EPI into the circulation)
• found on smooth muscle of airways, GI tract, &
blood vessels supplying skeletal muscle (along
with alpha-1 receptors)
– Beta-2 agonists used for acute treatment of asthmatic
attacks
• stimulate production of cAMP
Blood vessels of skeletal muscle
Transmitter
Receptor
Action Target
ACh
nicotinic
+++
Postganglionic Ligand
neurons
gated ion
channel
depolarization
ACh
muscarinic
+++
Smooth m
Glands
Heart
GPCR
IP3 induced
Ca release
Opens K+
channel
Smooth m
Glands
Nerve
varicosities
GPCR
---
Signalling
Mechanism
GPCR
Action
a-1
+++
a-2
----
NE
EPI
b-1
+++
Heart
GPCR
cAMP
EPI
b-2
----
Smooth m
GPCR
cAMP
NE
GPCR
IP3 induced
Ca release
Ca entry
Examples of ANS Reflexes
• Pupillary light reflex
• Stress activation of the adrenal medulla
• Reflex regulation of the GI tract (CNS & LOCAL)
– Parasympathetic Nervous System
• Vago-vagal activation of stomach (CNS)
• Gastro-colic reflex (CNS)
– Enteric Nervous System - Locally mediated
stereotyped movements (e.g. peristalsis)
– Sympathetic Nervous System
• Global inhibition (stress response - CNS)
• Enterogastric reflex: selective local inhibition:
• Tonic inhibition (local)
ENS
PARASYMPATHETIC
SYMPATHETIC