Download Chapter 20 The Autonomic Nervous System

11/27/2012
Chapter 20
The Autonomic Nervous System
Copyright 2009 John Wiley & Sons
Introduction
Autonomic nervous system (ANS)- division of the peripheral
nervous system that consists of:
Autonomic sensory neurons in visceral organs and in blood
vessels
convey information (interoceptors)
Integrating centers in the central nervous system (CNS)
Autonomic motor neurons- propagate from the CNS to various
effectors
regulate the activity of smooth muscle, cardiac muscle, and
many glands.
The enteric division, a specialized network of nerves and ganglia
forming an independent nerve network within the wall of the
gastrointestinal (GI) tract.
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Anatomy of Autonomic Motor Pathway
Preganglionic neuron- first of the two motor neurons in any
autonomic motor pathway
Cell body is in the brain or spinal cord, the axon exits the CNS as a
small-diameter, myelinated fiber extending to an autonomic
ganglion.
Postganglionic neuron lies outside the CNS, the cell body and
dendrites are in an autonomic ganglion
Forms one or more synapses with preganglionic neurons
Axon of a postganglionic neuron is a small-diameter, unmyelinated
Type C fiber that terminates in a visceral effector.
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Motor Neuron Pathways in the
Somatic Nervous System
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Motor Neuron Pathways in the ANS
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Preganglionic Neurons
Sympathetic division- thoracolumbar division Preganglionic neurons have cell bodies in the lateral horns of the
gray matter in the12 thoracic segments and the lumbar 1 and 2
segments of the spinal cord.
Sympathetic preganglionic axons are relatively short
Parasympathetic division- craniosacral division
Cell bodies of preganglionic neurons in the nuclei of 4 cranial
nerves in the brain stem (III, VII, IX, and X) and in the lateral gray
horns of the sacral segments 2-4 of the spinal cord.
Parasympathetic preganglionic axons are relatively long
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3 General Groups of Autonomic Ganglia
Sympathetic Ganglia- sites of synapses between
sympathetic preganglionic and postganglionic neurons.
Sympathetic trunk ganglia- lie in a vertical row on
either side of the vertebral column, skull to coccyx
Postganglionic axons innervate areas above the diaphragm
Neck innervations called: Superior, Middle, and Inferior
Cervical Ganglia
Prevertebral (collateral) ganglia- lies anterior to the
vertebral column and close to the large abdominal
arteries.
Postganglionic axons innervate organs below the diaphragm.
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Autonomic Ganglia
4 major Prevertebral Ganglia:
1. Celiac ganglion- on either side of the celiac artery just inferior to the
diaphragm.
2. Superior mesenteric ganglion- near the beginning of the superior
mesenteric artery in the upper abdomen.
3. Inferior mesenteric ganglion- near the beginning of the inferior
mesenteric artery in the middle of the abdomen.
4. Aorticorenal ganglion- near the renal artery as it branches from the
aorta.
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Sympathetic Division of the ANS
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Parasympathetic ganglia
Preganglionic axons of the parasympathetic division synapse with
postganglionic neurons in terminal ganglia.
Most are located close or within the wall of a visceral organ
They are longer than axons of sympathetic preganglionic
neurons.
Terminal ganglia in the head have specific names:
ciliary ganglion
pterygopalatine ganglion
submandibular ganglion
otic ganglion
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Parasympathetic Division of the ANS
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Autonomic Plexuses
Autonomic plexuses- in the thorax, abdomen, and pelvis
axons of both sympathetic and parasympathetic
preganglionic neurons forming tangled networks
Many of which lie along major arteries.
May contain sympathetic ganglia and axons of autonomic
sensory neurons.
Major plexuses in the thorax:
Cardiac plexus- supplies the heart
Pulmonary plexus- supplies the bronchial tree
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Autonomic Plexuses
Abdominal and Pelvic Plexuses
Celiac Plexus- supplies the liver, gall bladder,
stomach, pancreas, spleen, kidneys, adrenal medulla,
testes, ovaries
Superior Mesenteric Plexus- supplies the small
intestine to the large intestine
Inferior Mesenteric Plexus- supplies the large
intestine
Hypogastric Plexus- supplies the pelvic organs
Renal Plexuses- supplies the kidneys and ureters
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Autonomic Plexus in the Thorax,
Abdomen, and Pelvis
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Postganglionic Neurons
Postganglionic autonomic fibers do not end in a single terminal
swelling like a synaptic knob or end plate.
Terminal branches of autonomic fibers contain swellings, called
varicosities
Simultaneously release neurotransmitter over a large area of the organ
rather than on single cells.
This extensive release of neurotransmitter and the greater number
of postganglionic neurons means that entire organs, rather than
discrete cells, are typically influenced by autonomic activity.
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Sympathetic Postganglionic Neurons
Sympathetic preganglionic neurons pass to sympathetic trunk ganglia,
they may connect with postganglionic neurons in one of the
following ways:
➊ Axon may synapse with postganglionic neurons in the first ganglion it
reaches.
➋ Axon may ascend or descend to a higher or lower ganglion before
synapsing with postganglionic neurons.
➌ Axon may continue through the sympathetic trunk ganglion to end at
a prevertebral ganglion and synapse with postganglionic neurons.
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Ganglia and Postglanglionic Neurons
Process Diagram Step-by-Step
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Posterior horn
Posterior ramus
of spinal nerve
Anterior ramus
of spinal nerve
Posterior root
Posterior
root
ganglion
2
Skin
Lateral
horn
Spinal
nerve
Anterior
horn
Spinal cord
Anterior root
1
Sympathetic
trunk ganglion
Gray ramus
communicans
3
2
Prevertebral
ganglion
(celiac ganglion)
To visceral effectors:
smooth muscle of
blood vessels,
arrector pili muscles,
sweat glands of skin
White ramus
communicans
Visceral effector: intestine
Preganglionic neuron
view
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Postganglionic neuronsCopyright 2009 John Anterior
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Parasympathetic postganglionic
neurons
Parasympathetic division neurons pass to terminal
ganglia near or within a visceral effector.
In the ganglion, presynaptic neuron usually
synapses with only four or five postsynaptic
neurons, all of which supply a single visceral effector.
Thus, parasympathetic responses can be localized to
a single effector.
Parasympathetic postganglionic axons are relatively
short.
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Structure of the Sympathetic Division
Cell bodies of sympathetic preganglionic neurons- part of the lateral
horns of all thoracic segments and of the lumbar 1 and 2 segments
of the spinal cord.
Preganglionic axons leave the spinal cord along with the somatic
motor neurons at the same segmental level.
Myelinated preganglionic sympathetic axons pass into the anterior
root of a spinal nerve and enter a white ramus before passing to
the nearest sympathetic trunk ganglion on the same side.
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Cervical Ganglion of the Sympathetic
Trunk
Superior cervical ganglion serve the head and heart.
Distributed to sweat glands, smooth muscle of the eye, blood
vessels of the face, lacrimal glands, nasal mucosa, salivary
glands, and the heart.
Gray rami communicantes pass to the upper two to four
cervical spinal nerves.
Structures containing the postganglionic axons that connect the
ganglia of the various portions of the sympathetic trunk ganglion to
spinal nerves.
Postganglionic neurons leaving the middle cervical ganglion and
inferior cervical ganglion innervate the heart.
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Thoracic Ganglion of the Sympathetic
Trunk
Thoracic portion of each sympathetic trunk ganglion lies anterior to
the necks of the corresponding ribs.
Receives most of the sympathetic preganglionic axons, and its
postganglionic neurons
Innervate the heart, lungs, bronchi, and other thoracic viscera.
In the skin, these neurons also innervate sweat glands, blood
vessels, and arrector pili muscles of hair follicles.
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Lumbar region of Sympathetic truck
Unmyelinated postganglionic axons from the lumbar and
sacral sympathetic trunk ganglia enter a gray ramus
Then merge with a spinal nerve or join the hypogastric
plexus via direct visceral branches.
Lead to 31 pairs of spinal nerves producing sympathetic
output to smooth muscle and glands
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Prevertebral Ganglia
Preganglionic axons extend from a white ramus
communicans into the sympathetic trunk ganglion, they
give off several axon collaterals (branches).
Splanchnic nerves extend to and terminate in the
outlying prevertebral ganglia.
Splanchnic nerves from Thoracic region become the
Greater, Lesser, Least (Lowest) Splanchnic nerves,
and the Lumbar Splanchnic nerve from the Lumbar
region
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Structure of the Parasympathetic Division
Parasympathetic preganglionic neuron cell bodies are found in
nuclei in the brain stem and in the lateral horns of sacral segments
2-4 of the spinal cord.
Axons emerge as part of a cranial nerve or anterior spinal nerve root
Cranial parasympathetic outflow consists of preganglionic axons
extending from the brain stem in 4 cranial nerves.
Sacral parasympathetic outflow consists of preganglionic axons in
anterior roots of the sacral nerves 2-4.
Both preganglionic axons outflows end in terminal ganglia, where
they synapse with postganglionic neurons.
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The Cranial Outflow
1. Ciliary
ganglia- lateral to each optic (II) nerve near the
posterior aspect of the orbit.
Preganglionic axons pass with the oculomotor (III)
nerves to the ciliary ganglia.
Postganglionic axons from the ganglia innervate smooth
muscle fibers in the eyeball.
2. Pterygopalatine ganglia- lateral to the sphenopalatine
foramen between the sphenoid and palatine bones.
Preganglionic axons from the facial nerve (VII).
Postganglionic axons innervate the nasal mucosa,
palate, pharynx, and lacrimal glands.
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The Cranial Outflow
3. Submandibular ganglia- near the ducts of the
submandibular salivary glands.
Receive preganglionic axons from the facial nerves
Postganglionic axons innervate the submandibular and
sublingual salivary glands.
4. Otic ganglia- just inferior to each foramen ovale.
Receive preganglionic axons from the glossopharyngeal
(IX) nerves
Postganglionic axons innervate the parotid salivary
glands.
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Sacral Outflow
Sacral parasympathetic outflow
Receive preganglionic axons from the anterior nerve root
from S2-S4 forming the Pelvic Splanchnic nerve
Postganglionic axons innervate the smooth muscle and
glands of the colon, ureters, bladder, and reproductive
organs
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Structure of the Enteric Division
Enteric division of the autonomic nervous system forms
an extensive area of contact with the environment.
Specialized network of nerves and ganglia forming an
integrated neuronal network.
System of nerves makes possible the normal motility and
secretory functions of the gastrointestinal tract
Myenteric plexus- between the outer longitudinal and
circular muscle layers from the upper esophagus to the
anus.
Submucous plexus- occupies the gut wall between the
circular muscle layer and the muscularis mucosae and runs
from the stomach to the anus.
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Cholinergic Neurons and Receptors
Release the neurotransmitter acetylcholine.
In the ANS, the cholinergic neurons include
(1) all sympathetic and parasympathetic preganglionic neurons
(2) sympathetic postganglionic neurons that innervate sweat glands
(3) all parasympathetic postganglionic neurons
ACh is stored in synaptic vesicles and released by exocytosis.
Nicotinic receptors- present in plasma membranes of dendrites and cell
bodies of both sympathetic and parasympathetic postganglionic neurons
and in the motor end plate at the neuromuscular junction.
Muscarinic receptors- present in plasma membranes of all effectors
innervated by parasympathetic postganglionic axons. Most sweat glands,
which receive innervation from cholinergic sympathetic postganglionic
neurons, possess muscarinic receptors
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Adrenergic Neurons and Receptors
In the ANS, adrenergic neurons release norepinephrine or NE,
also known as noradrenalin.
Most sympathetic postganglionic neurons are adrenergic.
NE is synthesized and stored in synaptic vesicles and released by
exocytosis.
Adrenergic receptors bind both NE and epinephrine, a hormone.
The main types of adrenergic receptors are alpha (a) receptors
and beta (b) receptors, which are found on visceral effectors
innervated by most sympathetic postganglionic axons.
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Cholinergic Neurons and Adrenergic
Neurons (Fig. 20.6)
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Fight or Flight Response
1. The pupils of the eyes dilate.
2. Heart rate, force of heart contraction, and blood pressure
increase.
3. The airways dilate, allowing faster movement of air into and
out of the lungs.
4. Blood vessels that supply organs that are involved in exercise
or fighting off danger dilate.
5. Liver cells perform glycogenolysis and adipose tissue cells
perform lipolysis
6. Release of glucose by the liver increases blood glucose level.
7. Processes that are not essential for meeting the stressful
situation are inhibited.
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Effects of Sympathetic Stimulation
The effects of sympathetic stimulation are longer lasting and
more widespread than of parasympathetic stimulation for 3
reasons:
(1) Sympathetic postganglionic axons diverge more
extensively so many tissues are activated simultaneously.
(2) AChE quickly inactivates ACh, but NE lingers in the
synaptic cleft for a longer period.
(3) Epinephrine and NE secreted as hormones into the
blood from the adrenal medulla intensify and prolong the
responses caused by NE released as a neurotransmitter
from sympathetic postganglionic axons.
In time, blood-borne NE and epinephrine are inactivated by
enzymatic destruction in the liver.
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Parasympathetic Responses
The parasympathetic division enhances “rest-and-digest”
activities.
Parasympathetic responses support body functions that conserve
and restore body energy during times of rest and recovery.
In the quiet intervals between periods of exercise, parasympathetic
impulses to the digestive glands and the smooth muscle of the
gastrointestinal tract predominate over sympathetic impulses.
SLUDD-Salivation, Lacrimation, Urination, Digestion, Defecation
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Autonomic Reflexes
Autonomic reflexes- responses that occur when nerve
impulses pass over an autonomic reflex arc.
Key role in regulating controlled conditions in the body such
as:
blood pressure, by adjusting heart rate, force of ventricular
contraction, and blood vessel diameter
digestion, by adjusting the motility (movement) and muscle
tone of the gastrointestinal tract
defecation and urination, by regulating the opening and
closing of sphincters.
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Autonomic Reflex Components
Receptor: Distal end of a sensory neuron (interoceptors), which responds to
a stimulus and produces a change that will ultimately trigger nerve impulses
Sensory neuron: Conducts nerve impulses from receptors to the CNS.
Integrating center: Interneurons within the CNS relay signals from sensory
neurons to motor neurons. The main integrating centers for most autonomic
reflexes are located in the hypothalamus and brain stem.
Motor neurons: Two motor neurons connect the CNS to an effector:
The preganglionic neuron conducts motor impulses from the CNS to an
autonomic ganglion
The postganglionic neuron conducts motor impulses from an autonomic
ganglion to an effector.
Effector: Smooth muscle, cardiac muscle, and glands
The reflex is called an autonomic reflex.
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