Download the autonomic nervous system

THE AUTONOMIC NERVOUS SYSTEM
What is the general function of the autonomic nervous system?
The autonomic nervous system (ANS) regulates the activities of cardiac muscle,
smooth muscle, and glands.
Structurally, the ANS consists of two types of neurons. Name and give a brief
description of each.
Visceral afferent (sensory) neurons – provide a continual flow of information to
the CNS from the viscera and blood vessels
Visceral efferent (motor) neurons – provide motor innervation to the various
effectors (cardiac muscle, smooth muscle, glands)
How does the ANS operate? What controls it?
The ANS operates without conscious control, relying upon reflex arcs that are
dependent upon the hypothalamus and medulla for overriding control.
A.
COMPARISON OF SOMATIC AND AUTONOMIC NERVOUS SYSTEMS
Comparison of somatic and autonomic nervous systems
SOMATIC
cutaneous receptors
proprioceptors
special senses
AUTONOMIC
chemoreceptors
baroreceptors
mechanoreceptors
conscious?
may become conscious
unconscious
excitatory or
inhibitory?
excitatory for skeletal
muscles only
excitatory or inhibitory for
cardiac muscle, smooth,
and glands
number of motor
neurons?
single motor neuron
two motor neurons
receptor types?
Name the two separate divisions of the ANS.
sympathetic and parasympathetic nervous systems
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What is meant by the concept of dual innervation?
In general, if one division starts or increases the activity of an organ, the
other division will stop or decrease the activity. This is called dual
innervation.
B.
ANATOMY OF AUTONOMIC MOTOR PATHWAYS
1.
OVERVIEW
a.
PREGANGLIONIC NEURONS
b.
AUTONOMIC GANGLIA
c.
POSTGANGLIONIC NEURONS
Name the two types of visceral motor neurons by the divisions of
the ANS?
preganglionic and postganglionic
Where is the cell body of a preganglionic neuron located?
The cell body is located in gray matter of the spinal cord or
brain.
How does the axon of a preganglionic neuron pass from the CNS?
The preganglionic axon passes from the CNS in a spinal or a
cranial nerve.
Where does the preganglionic axon terminate?
The preganglionic axon terminates in a ganglion.
What is a ganglion?
A ganglion is a collection of nerve cell bodies located in a
specific site within the body, but outside the CNS.
Where is the cell body of a postganglionic neuron located?
The cell body is located in a specific ganglion. The location
of the ganglion is dependent upon the division of the ANS to
which the neuron belongs and which organ it will innervate.
Where does the axon of the postganglionic neuron terminate?
The postganglionic axon passes from the ganglion to the
effector.
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What effect does the postganglionic neuron have on the effector?
The peripheral effector (cardiac muscle, smooth muscle, or
gland) is either stimulated or inhibited, depending upon
which division of the ANS is being discussed.
What is the origin of the sympathetic division?
Sympathetic preganglionic neurons originate from the lateral
gray horns of spinal cord segments T1-L2. Their axons
travel in the spinal nerves of these segments. Therefore, the
sympathetic division is called the thoracolumbar division and
the preganglionic axons are called the thoracolumbar
outflow.
What is the origin of the parasympathetic system?
Parasympathetic preganglionic neurons arise from the nuclei
of cranial nerves III, VII, IX, and X, and the lateral gray horns
of spinal cord segments S2-4. For this reason, the parasympathetic division is also called the craniosacral division and
the preganglionic axons are the craniosacral outflow.
Describe the following:
Sympathetic chain ganglia – Sympathetic chain (vertebral
chain or paravertebral) ganglia are located in a series
that lie in a vertical row on either side of the vertebral
column, extending from the base of the skull to the
coccyx.
Prevertebral ganglia – Prevertebral (collateral) ganglia lie as
three clusters associated with the three unpaired
arteries arising from the abdominal aorta (celiac,
superior mesenteric, and inferior mesenteric). These
ganglia are used by the sympathetic division.
Terminal ganglia – The parasympathetic division uses
terminal (intramural) ganglia located very close to or
within the walls of the organ to be innervated.
Relative length of fibers – Because of the closeness of the
spinal cord to the sympathetic chain ganglia and the
collateral ganglia, sympathetic preganglionic neurons
are fairly short. Because of the distance between the
brain or spinal cord and the innervated organs,
parasympathetic preganglionic fibers are relatively
long.
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Describe the postganglionic neurons of the two systems.
Because of the locations of ganglia, sympathetic
postganglionic fibers are relatively long while
parasympathetic postganglionic fibers are relatively short.
Regardless, all postganglionic neurons of either system have
their axons pass from their respective ganglia to the effector
for innervation.
C.
PHYSIOLOGICAL EFFECTS OF THE ANS
1.
ANS NEUROTRANSMITTERS
Most effectors receive innervation from both divisions of the ANS, one
causing excitation and the other causing inhibition. How is this possible?
This is possible because the postganglionic neurons use different
neurotransmitters and the effectors bear different receptors.
What is the ultimate controller of autonomic tone?
The hypothalamus regulates the balance of sympathetic versus
parasympathetic activity or tone.
Which ANS division is usually in control of autonomic tone
In general, we are in parasympathetic tone, except during states of
emergency when we immediately switch to sympathetic tone.
ANS neurotransmitters
CHOLINERGIC
neurotransmitter? acetylcholine
ADRENERGIC
norepinephrine
Which neurons
all preganglionic neurons
use this
all postganglionic paraneurotransmitter?
sympathetic neurons
a very few postganglionic
sympathetic neurons
most postganglionic
sympathetic neurons
enzymes used to acetylcholinesterase
destroy
neurotransmitter?
catechol-Omethyltransferase
monoamine oxidase
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2.
PARASYMPATHETIC AND SYMPATHETIC RESPONSES
In general, describe the activities of the parasympathetic nervous system.
What is meant by the name “rest and recovery” system?
The parasympathetic nervous system is an energy conservationrestorative system. It regulates those activities that conserve and
restore body energy during times of rest and recovery.
The parasympathetic nervous system dominates over sympathetic
activity in the glands and smooth muscle of the gut, stimulating
glandular secretion and the gut movements necessary for food to
be digested and absorbed.
The acronym SLUD stands for salivation, lacrimation, urination, and
defecation, all controlled by the parasympathetic nervous system.
In general, describe the activities of the sympathetic nervous system.
What is meant by the name “fight or flight” system?
The sympathetic system prepares the body to meet emergency
demands and is primarily involved with processes that expend
energy. During physical or emotional stress, the sympathetic
division dominates the parasympathetic system, initiating a series
of activities known as the fight-or-flight response.
This includes increased heart rate, force of contraction, and blood
pressure; increased blood flow to essential structures (brain, lungs,
heart, skeletal muscles) and decreased activity in nonessential
ones; increased rate and depth of respiration.
In addition, there is activation of the adrenal medulla, causing
secretion of norepinephrine and epinephrine as hormones to
greatly heighten the response.
In general. describe why parasympathetic responses are short-lived and
very precise while sympathetic responses are much longer-lived and body
wide.
The parasympathetic system has very little divergence in the
connections between preganglionic and postganglionic fibers, so
that only very small areas of effectors are stimulated at any given
time. Also, acetylcholine is rapidly destroyed in the synaptic clefts,
so that the response to neurotransmitter is very short-lived.
The sympathetic system has a good deal of divergence between
preganglionic and postganglionic fibers (1:5-500), so that the
sympathetic message is sent body-wide. Secondly, norepinephrine
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is poorly removed from the synapse, so that it diffuses into the
bloodstream for delivery throughout the body. Lastly, activation of
the sympathetic nervous system results in secretion of norepinephrine and epinephrine, hormones that have stronger and body
wide effects.
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