Download Autonomic Ganglia

AUTONOMIC
NERVOUS
SYSTEM
Will Kleinelp
Associate Professor
Department of Biology
AUTONOMIC NERVOUS SYSTEM
The autonomic nervous system (ANS) operates via reflex arcs.
Structurally, the ANS includes
autonomic sensory neurons,
integrating centers in the CNS, and
autonomic motor neurons.
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A continual flow of nerve impulses
from autonomic sensory neurons in visceral organs and blood vessels
to integrating centers in the central nervous system (CNS).
Then, impulses in autonomic motor neurons propagate to various effector tissues
regulating the activity of smooth muscle, cardiac muscle, and many glands.
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The ANS usually operates without conscious control.
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The system was originally named autonomic because it was thought to function
autonomously or in a self-governing manner, without control by the CNS.
However, centers in the hypothalamus and brain stem do regulate ANS reflexes.
Comparison: Autonomic vs Somatic Nervous Systems
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The somatic nervous system includes both sensory and motor neurons.
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Sensory neurons convey input from receptors for the special senses (vision, hearing, taste, smell, and equilibrium
and from receptors for somatic senses (pain, thermal, tactile, and proprioceptive sensations). All these sensations
normally are consciously perceived.
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In turn, somatic motor neurons innervate skeletal muscle—the effector tissue of the somatic nervous system—
and produce voluntary movements.
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When a somatic motor neuron stimulates the muscle, it contracts and your arm flexes; the effect always is
excitation. If somatic motor neurons cease to stimulate a muscle, the result is a paralyzed, limp muscle that has no
muscle tone.
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The muscles that generate respiratory movements are also skeletal muscles controlled by somatic motor
neurons. If the respiratory motor neurons become inactive, breathing stops. A few skeletal muscles, such as those
in the middle ear, are controlled by reflexes and cannot be contracted voluntarily.
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The main input to the ANS comes from autonomic sensory neurons.
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These neurons are associated with interoceptors, which are sensory receptors located in blood vessels, visceral
organs, muscles, and the nervous system that monitor conditions in the internal environment.
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Examples of interoceptors are
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chemoreceptors that monitor blood CO2 level and mechanoreceptors that detect the degree of stretch in the
walls of organs or blood vessels.
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These sensory signals are not consciously perceived most of the time, although intense activation of interoceptors
may produce conscious sensations.
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Two examples of perceived visceral sensations are
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pain sensations from damaged viscera and angina pectoris (chest pain) from inadequate blood flow to the
heart.
Comparison: Autonomic vs Somatic Nervous Systems
Autonomic motor neurons regulate visceral activities by either increasing (exciting) or decreasing (inhibiting) ongoing activities in
their effector tissues (cardiac muscle, smooth muscle, and glands). Examples are:
•Changes in the diameter of the pupils,
•dilation and constriction of blood vessels, and
•adjustment of the rate and force of the heartbeat
Unlike skeletal muscle, tissues innervated by the ANS often function to some extent even if their nerve supply is damaged.
•The heart continues to beat when it is removed for transplantation into another person,
•smooth muscle in the lining of the gastrointestinal tract contracts rhythmically on its own, and
•glands produce some secretions in the absence of ANS control.
Most autonomic responses cannot be consciously altered or suppressed to any great degree.
Signals from the general somatic and special senses, acting via the limbic system, also influence responses of autonomic motor
neurons.
•Seeing a bike about to hit you,
•hearing squealing brakes of a nearby car, or
•being grabbed by an attacker would all increase the rate and force of your heartbeat.
Recall the axon of a single, myelinated somatic motor neuron extends from the CNS all the way to the skeletal muscle fibers in its
motor unit
Comparison: Autonomic vs Somatic Nervous Systems
Most autonomic motor pathways consist of two motor neurons in series, one following the other.
The first neuron has its cell body in the CNS; its myelinated axon extends from the CNS to an autonomic
ganglion.
The cell body of the second neuron is also in that autonomic ganglion; its unmyelinated axon extends
directly from the ganglion to the effector (smooth muscle, cardiac muscle, or a gland).
In some autonomic pathways, the first motor neuron extends to the adrenal medullae rather than an
autonomic ganglion. In addition, all somatic motor neurons release only acetylcholine (ACh) as their
neurotransmitter, but autonomic motor neurons release either ACh or norepinephrine (NE).
Comparison: Autonomic vs Somatic Nervous Systems
The output (motor) part of the ANS has two principal branches:
•the sympathetic division
and
•the parasympathetic division.
Most organs have dual innervation: They receive impulses from both sympathetic and parasympathetic
neurons.
In general, nerve impulses from one division of the ANS stimulate the organ to increase its activity
(excitation), and impulses from the other division decrease the organ's activity (inhibition).
For example, an increased rate of nerve impulses from the sympathetic division increases heart rate, and
an increased rate of nerve impulses from the parasympathetic division decreases heart rate.
Motor Pathway Anatomy
A. Anatomical Components
The first of the two motor neurons in any autonomic motor pathway is called a preganglionic neuron
Its cell body is in the brain or spinal cord, and its axon exits the CNS as part of a cranial or spinal nerve.
The axon of a preganglionic neuron is a small-diameter, myelinated type B fiber that usually extends to an autonomic
ganglion, where it synapses with a postganglionic neuron, the second neuron in the autonomic motor pathway
Notice that the postganglionic neuron lies entirely outside the CNS. Its cell body and dendrites are located in an
autonomic ganglion, where it forms synapses with one or more preganglionic axons.
The axon of a postganglionic neuron is a small-diameter, unmyelinated type C fiber that terminates in a visceral
effector.
Thus, preganglionic neurons convey nerve impulses from the CNS to autonomic ganglia, and postganglionic neurons
relay the impulses from autonomic ganglia to visceral effectors.
Sympathetic Division
Preganglionic Axons
Postganglionic Axons
vagus
nerve
In the sympathetic division, the
preganglionic neurons have their
cell bodies in the lateral horns of
the gray matter in the 12 thoracic
segments and the first two lumbar
segments of the spinal cord
For this reason, the sympathetic
division is also called the
thoracolumbar division
cranial
nerves
cervical
nerves
celiac
ganglion
thoracic
nerves
the axons of the sympathetic
preganglionic neurons are known as
the thoracolumbar outflow.
The preganglionic fibers are short
and the postganglionic fibers are
long
lumbar
nerves
ost ganglia
spinal cord
pelvic
nerve
sacral
nerves
Sympathetic Division
In the sympathetic division, the
preganglionic neurons have their
cell bodies in the lateral horns of
the gray matter in the 12 thoracic
segments and the first two lumbar
segments of the spinal cord
For this reason, the sympathetic
division is also called the
thoracolumbar division
the axons of the sympathetic
preganglionic neurons are known as
the thoracolumbar outflow.
The preganglionic fibers are short
and the postganglionic fibers are
long
Parasympathetic Division
Preganglionic Axons
Postganglionic Axons
Cell bodies of preganglionic
neurons of the parasympathetic
division are located in the nuclei of
four cranial nerves in the brain
stem (III, VII, IX, and X) and in the
lateral gray horns of the second
through fourth sacral segments of
the spinal cord .
Hence, the parasympathetic division
is also known as the craniosacral
division , and the axons of the
parasympathetic preganglionic
neurons are referred to as the
craniosacral outflow.
vagus
nerve
cranial
nerves
cervical
nerves
celiac ganglion
solar plexus
thoracic
nerves
The preganglionic fibers are long
and the postganglionic fibers are
short.
lumbar
nerves
ost ganglia
spinal cord
sympathetic
chain ganglia
pelvic
nerve
sacral
nerves
Parasympathetic Division
Cell bodies of preganglionic neurons of
the parasympathetic division are located
in the nuclei of four cranial nerves in the
brain stem (III, VII, IX, and X) and in the
lateral gray horns of the second through
fourth sacral segments of the spinal
cord .
Hence, the parasympathetic division is
also known as the craniosacral division ,
and the axons of the parasympathetic
preganglionic neurons are referred to as
the craniosacral outflow.
The preganglionic fibers are long and the
postganglionic fibers are short.
Parasympathetic Division
Autonomic Ganglia
The autonomic ganglia may be divided into three
general groups:
Two of the groups are components of the
sympathetic division,
and one group is a component of the
parasympathetic division.
Sympathetic Ganglia The sympathetic ganglia
are the sites of synapses between sympathetic
preganglionic and postganglionic neurons. The two
groups of sympathetic ganglia are
sympathetic trunk ganglia and
prevertebral ganglia.
Sympathetic trunk ganglia (also called vertebral
chain ganglia or paravertebral ganglia) lie in a
vertical row on either side of the vertebral column.
These ganglia extend from the base of the skull to
the coccyx ).
Because the sympathetic trunk ganglia are near the
spinal cord, most sympathetic preganglionic
axons are short. Postganglionic axons from
sympathetic trunk ganglia mostly innervate organs
above the diaphragm. Examples of sympathetic
trunk ganglia are the superior , middle, and inferior
cervical ganglia (Figure 15.2).
Autonomic Ganglia
The second group of sympathetic ganglia, the
prevertebral (collateral) ganglia, lies anterior to
the vertebral column and close to the large
abdominal arteries. In general, postganglionic
axons from prevertebral ganglia innervate organs
below the diaphragm.
There are three major prevertebral ganglia:
(1) The celiac ganglion is on either side of the
celiac artery just inferior to the diaphragm.
(2) The superior mesenteric ganglion is near
the beginning of the superior mesenteric artery in
the upper abdomen.
(3) The inferior mesenteric ganglion is near
the beginning of the inferior mesenteric artery in
the middle of the abdomen
Autonomic Ganglia
Parasympathetic Ganglia
Preganglionic axons of the parasympathetic division
synapse with postganglionic neurons in terminal
(intramural) ganglia. Most of these ganglia are located
close to or actually within the wall of a visceral organ.
Because the axons of parasympathetic preganglionic
neurons extend from the CNS to a terminal ganglion in
an innervated organ, they are longer than most of the
axons of sympathetic preganglionic neurons.
Examples of terminal ganglia include
the ciliary ganglion,
pterygopalatine ganglion,
submandibular ganglion, and
otic ganglion
Sympathetic Division Connections
Once axons of sympathetic
preganglionic neurons pass to
sympathetic trunk ganglia, they may
connect with postganglionic neurons in
one of the following ways
•An axon may synapse with
postganglionic neurons in the ganglion it
first reaches.
•An axon may ascend or descend to a
higher or lower ganglion before
synapsing with postganglionic neurons.
The axons of incoming sympathetic
preganglionic neurons that pass up or
down the sympathetic trunk collectively
form the sympathetic chains, the fibers
on which the ganglia are strung.
•An axon may continue, without
synapsing, through the sympathetic
trunk ganglion to end at a prevertebral
ganglion and synapse with
postganglionic neurons there.
A single sympathetic preganglionic fiber has many axon collaterals (branches) and may synapse with 20 or more postganglionic
neurons. This pattern of projection is an example of divergence and helps explain why many sympathetic responses affect almost
the entire body simultaneously. After exiting their ganglia, the postganglionic axons typically terminate in several visceral effectors
Axons of preganglionic neurons of the parasympathetic division pass to terminal ganglia near or within a visceral effector. In the
ganglion, the presynaptic neuron usually synapses with only four or five postsynaptic neurons, all of which supply a single visceral
effector, allowing parasympathetic responses to be localized to a single effector.
Autonomic Plexuses
• In the thorax, abdomen, and pelvis, axons of both
sympathetic and parasympathetic neurons form
tangled networks called autonomic plexuses,
many of which lie along major arteries.
• The autonomic plexuses also may contain
sympathetic ganglia and axons of autonomic
sensory neurons.
• The major plexuses in the thorax are the
• cardiac plexus, which supplies the heart, and
• the pulmonary plexus, which supplies the
bronchial tree
• The abdomen and pelvis also contain major
autonomic plexuses and often the plexuses are
named after the artery along which they are
distributed.
• The celiac (solar) plexus is the largest autonomic
plexus and surrounds the celiac and superior
mesenteric arteries. It contains two large celiac
ganglia and a dense network of autonomic axons
and is distributed to the liver, gallbladder, stomach,
pancreas, spleen, kidneys, adrenal medullae,
testes, and ovaries.
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The superior mesenteric plexus contains the
superior mesenteric ganglion and supplies the small
and large intestine.
• The inferior mesenteric plexus contains the
inferior mesenteric ganglion, which innervates the
large intestine.
• The hypogastric plexus is anterior to the fifth
lumbar vertebra and supplies pelvic viscera.
• The renal plexuses, located near the kidneys,
contain the renal ganglia and supply the renal
arteries within the kidneys and the ureters.
ANS Neurotransmitters & Receptors
Based on the neurotransmitter they produce and release, autonomic neurons are classified as either cholinergic or adrenergic. The
receptors for the neurotransmitters are integral membrane proteins located in the plasma membrane of the postsynaptic neuron or
effector cell.
Cholinergic Neurons and Receptors
Cholinergic neurons release the neurotransmitter acetylcholine
(ACh). In the ANS, the cholinergic neurons include
(1) all sympathetic and parasympathetic preganglionic neurons,
(2) sympathetic postganglionic neurons that innervate most sweat
glands, and
(3) all parasympathetic postganglionic neurons
ACh is stored in synaptic vesicles and released by exocytosis. It then
diffuses across the synaptic cleft and binds with specific cholinergic
receptors, integral membrane proteins in the postsynaptic plasma
membrane.
The two types of cholinergic receptors, both of which bind ACh, are
nicotinic receptors and
muscarinic receptors.
Nicotinic receptors are present in the plasma membrane of dendrites
and cell bodies of both sympathetic and parasympathetic
postganglionic neurons and in the motor end plate at the
neuromuscular junction. They are so named because nicotine mimics
the action of ACh by binding to these receptors.
Muscarinic receptors are present in the plasma membranes of all
effectors (smooth muscle, cardiac muscle, and glands) innervated by
parasympathetic postganglionic axons.
In addition, most sweat glands receive their innervation from
cholinergic sympathetic postganglionic neurons and possess
muscarinic receptors
These receptors are so named because a mushroom poison called
muscarine mimics the actions of ACh by binding to them. Nicotine
does not activate muscarinic receptors, and muscarine does not
activate nicotinic receptors, but ACh does activate both types of
cholinergic receptors
Adrenergic Neurons and Receptors
In the ANS, adrenergic neurons release norepinephrine (NE), also known as noradrenalin. Most sympathetic postganglionic neurons are
adrenergic. Like ACh, NE is synthesized and stored in synaptic vesicles and released by exocytosis. Molecules of NE diffuse across the synaptic cleft
and bind to specific adrenergic receptors on the postsynaptic membrane, causing either excitation or inhibition of the effector cell.
Adrenergic receptors bind both norepinephrine and epinephrine. The norepinephrine can be either released as a neurotransmitter by sympathetic
postganglionic neurons or released as a hormone into the blood by the adrenal medullae; epinephrine is released as a hormone.
The two main types of adrenergic receptors are
alpha (α) receptors and
beta (β) receptors, which are found on visceral effectors innervated by most sympathetic postganglionic axons.
These receptors are further classified into subtypes— α 1, α 2, β 1, β 2, and β 3—based on the specific responses they elicit and by their selective
binding of drugs that activate or block them. Although there are some exceptions, activation of α 1 and β 1 receptors generally produces excitation,
and activation of α 2 and β 2 receptors causes inhibition of effector tissues. β 3 receptors are present only on cells of brown adipose tissue,
where their activation causes thermogenesis (heat production).
The activity of norepinephrine at a synapse is terminated either when the NE is taken up by the axon that released it or when the NE is enzymatically
inactivated by either catechol-O-methyltransferase (COMT) or monoamine oxidase (MAO). Compared to ACh, norepinephrine lingers in the
synaptic cleft for a longer time. Thus, effects triggered by adrenergic neurons typically are longer lasting than those triggered by cholinergic neurons.
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