Transcript Lecture 33 (ANS)

BIO 132
Neurophysiology
Lecture 33
Autonomic Nervous
System
Lecture Goals:
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Understanding the advantage of having an ANS
Understanding the differences between the
Sympathetic and Parasympathetic nervous
systems.
Understanding the structure and role of the
Enteric NS.
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There is no voluntary control of the ANS (its
'automatic')
The ANS has both somatic (body) and visceral
(organs) sensory input to regulate autonomic
motor output.
Autonomic motor output goes to the muscle of
the heart and to smooth muscle (surrounds
blood vessels, GI tract, airways, glands, etc).
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The function of the ANS is to keep you alive by
taking care of vital tasks so that you don't have
to spend any conscious time, effort or thought
on those tasks.
ANS Branches
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The ANS has two branches that function to maintain
your survival:
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Sympathetic branch (SNS) - for short-term survival ('fight-orflight')
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The SNS branch is activated in stressful or emergency situations.
It activates some organs and inhibits others in order to prepare your
body for taking immediate action (like fighting or running away from
danger).
Parasympathetic branch (PNS) - for long-term survival ('rest
and digest').
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The PNS branch is activated during low stress situations and
functions to ensure the body has sufficient nutrients for normal
growth.
It activates some organs and inhibits others to increase digestion and
absorption.
SNS/PNS Duality
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Because the two branches have somewhat
opposite functions, when one is activated the
other is inhibited.
The 'default' branch (activated at rest) is the
PSN.
As stress levels increase SNS activation is
increased and PNS activation is decreased.
The periventricular zone of the hypothalamus
has direct control over both the SNS and PNS.
Sympathetic NS
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Neurons of the SNS descend from the hypothalamus
to make connections with SNS motor neurons located
in the spinal cord.
The motor pathway from the spinal cord to the target
organ involves two neurons.
Hypothalamus
spinal cord
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Target organ
Sympathetic NS
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All SNS motor neurons synapsing with the target organ
are long and have their cell bodies just outside the
spinal cord.
The cell bodies of these neurons coming from the same
level of the spinal cord (exiting the spinal cord between
the same two vertebrae) are clustered together.
The cluster of these cell bodies is called a ganglion
(pleural: ganglia).
Sympathetic NS
To make the referencing of the two SNS motor
neurons easier, the first is called the preganglionic
neuron and the second is called the postganglionic
neuron.
Hypothalamus
Ganglion
spinal cord
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Preganglionic
Postganglionic
Target organ
Sympathetic NS
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The preganglionic neuron is short and releases
acetylcholine (ACh) onto the postganglionic neuron.
The postganglionic neuron is long and releases
norepinephrine (NE) onto the target organ.
spinal cord
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ACh
Preganglionic
NE
Postganglionic
Target organ
Sympathetic NS
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The NE binds to noradrenergic receptors on the target
organ.
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There are two types of noradrenergic receptors: alpha and
beta.
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Each of those has sub-types (i.e. alpha-1, alpha-2, beta-1, beta-2, etc.)
The actions of each activated receptor can be different, and many
times opposite.
Because NE has different receptors with different
actions, it can cause vasoconstriction in some blood
vessels and vasodilation in others.
Having different actions on different organs allows for
NE to coordinate all the body’s organs for one goal.
Sympathetic NS
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Goal of SNS: Prepare the body for surviving a shortterm emergency at the expense of long-term survival.
Physiological effects of SNS:
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Increase MAP (increase heart rate & force of contraction)
Increase blood flow to the skeletal muscle and skin
Decrease blood flow to GI tract, kidneys, liver, etc.
Inhibit salivation and release of digestive enzymes
Increase diameter of airways
Dilate pupils
Increase release of sweat
Increase metabolism and glucose levels in the blood
Parasympathetic NS
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Neurons of the PNS descend from the hypothalamus
to make connections with SNS motor neurons located
in the spinal cord.
The motor pathway from the spinal cord to the target
organ involves two neurons.
Hypothalamus
spinal cord
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Target organ
Parasympathetic NS
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All PNS motor neurons synapsing with the target organ
are long and have their cell bodies just outside the target
organ.
The cell bodies of these neurons going to the same
target organ are clustered together.
The cluster of these cell bodies is called a ganglion
(pleural: ganglia).
Parasympathetic NS
To make the referencing of the two PNS motor
neurons easier, the first is called the preganglionic
neuron and the second is called the postganglionic
neuron.
Hypothalamus
Ganglion
spinal cord
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Preganglionic
Postganglionic Target organ
Parasympathetic NS
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The preganglionic neuron is long and releases
acetylcholine (ACh) onto the postganglionic neuron.
The postganglionic neuron is short and releases ACh
onto the target organ which binds to cholinergic
receptors of the muscarinic subtype.
spinal cord
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ACh
Preganglionic
ACh
Postganglionic Target organ
Parasympathetic NS
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Goal of PNS: Prepare the body for long-term survival.
Physiological effects of PNS:
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Decrease MAP (decrease heart rate)
Decrease blood flow to the skeletal muscle and skin
Increase blood flow to GI tract, kidneys, liver, etc.
Enhance salivation and release of digestive enzymes
Decrease diameter of airways
Constrict pupils
Decrease metabolism
The Enteric Nervous System
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The Enteric NS is another branch of the ANS.
It is a large nervous system (network of neurons)
involved in the digestion of food.
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Has as many neurons as the entire spinal cord.
Has both sensory and motor neurons in the esophagus,
stomach, intestines, pancreas, and gall bladder.
Operates with a large degree of independence
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Doesn’t need the brain
Can be influenced by the SNS and PNS
Decrease metabolism
The Enteric NS Morphology
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Most of the GI tract is hollow tubes surrounded by two
layers of smooth muscle.
The smooth muscle layers coordinate the rhythmic
contractions to move food through.
The inside of the tube (lumen) is lined with an epithelial
layer, around which is a circular smooth muscle layer,
around which is a transverse smooth muscle layer.
The Enteric NS Morphology
Exploded cross section view of intestinal layers
Epithelial layer
Circular muscle layer
lumen
Longitudinal muscle layer
The Enteric NS Morphology
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There are two network of neurons that comprise the
enteric NS.
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One network is called the Submucosal Plexus and is
sandwiched between the epithelial layer and the circular
muscle layer.
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This plexus is primarily sensory: monitoring stretch of the walls of the
GI tubes, chemical composition and pH of the lumen, and the
hormone levels in the local blood supply.
The other network is called the Myenteric Plexus and lies
between the circular and the longitudinal muscle layers.
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This plexus is primarily motor: coordinating muscle contractions for
moving food, controlling mucus and digestive secretions, and altering
the diameter of local blood vessels.
The Enteric NS Morphology
Exploded cross section view of intestinal layers
Epithelial layer
Circular muscle layer
Longitudinal muscle layer
lumen
Submucosal Plexus
Myenteric Plexus
The Enteric NS
The myenteric plexus receives sensory
input from the submucosal plexus and
makes the appropriate motor responses.
 The Enteric NS can operate without any
input from the brain.
 The SNS tends to inhibit Enteric NS
function while the PNS enhances it.
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