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A & P II LECTURE NOTES
The Autonomic Nervous System
CHAPTER 15
1. Introduction.
a. The autonomic nervous system (ANS) regulates the
activities of effector organs which include smooth
muscle, cardiac muscle, and certain glands.
b. The ANS generally operates without conscious control and
was originally thought to be autonomous of the central
nervous system (CNS).
c. The CNS regulates the ANS through CNS centers such as
the hypothalamus and medulla oblongata.
d. The ANS and the endocrine system are the 2 major
controllers of body homeostasis.
2. Review of the nervous system divisions.
a. The nervous system has 2 major divisions.
1) The central nervous system (CNS) consists of:
a) Brain.
b) Spinal cord.
2) The peripheral nervous system (PNS) consists of:
a) Cranial nerves--arise from the brain.
b) Spinal nerves--emerge from the spinal cord.
The PNS is subdivided into:
1) Afferent (sensory) system--nerve cells
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that conduct impulses toward the brain.
2) Efferent (motor) system--nerve cells that
conduct impulses from the brain to muscles
and glands. The afferent and efferent
systems are further subdivided into:
(a)
Somatic nervous system (SNS)—
neurons under conscious control
that conduct excitation impulses
to skeletal muscles only.
(b) Autonomic nervous system (ANS)—
neurons generally not under conscious
control that convey either excitation
or inhibition impulses to:
(1)
Smooth muscle tissue.
(2)
Cardiac muscle tissue.
(3)
Glands.
(c) The autonomic nervous system is
subdivided again into:
(1) Sympathetic division--which
usually increases an organ's
activity.
(2) Parasympathetic division—
which usually decreases an
organ's activity.
b. Organs that receive impulses from both sympathetic and
parasympathetic nerve fibers are said to have dual
innervation.
3. Autonomic motor pathways.
a. Sensory neurons of the ANS receive their afferent input
from the special senses, general visceral senses, and
general somatic senses.
b. Autonomic efferent pathways consist of 2 sets of motor
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neurons that extend from the CNS, as opposed to 1 motor
neuron for the somatic nervous system.
1) The axon of the first ANS motor neuron
(preganglionic neuron) is myelinated.
a) Its cell body (soma) is located within the
gray matter of the brain or the lateral gray
horns of the spinal cord.
NOTE 1:
Gray matter refers to neurons having
nonmyelinated nerve axons, as opposed to white
matter formed by myelinated axons.
NOTE 2:
The gray matter of the spinal cord is composed
of the anterior gray horns, the lateral gray
horns, and the posterior gray horns.
b) The preganglionic axon extends from the CNS
and synapses in a ganglion with a second neuron
(postganglionic neuron). A ganglion is a
collection of neuron cell bodies outside the CNS.
c) Acetylcholine is the neurotransmitter substance
released within the autonomic ganglion formed at
the junction of the preganglionic neuron with
the postganglionic neuron.
2) The axon of the postganglionic ANS neuron is
unmyelinated.
a) Its cell body lies entirely outside the CNS in
an autonomic ganglion, from which an axon
extends peripherally.
b) The axon of a postganglionic neuron, the postganglionic fiber, terminates in a visceral
effector (end organ).
c) Either acetylcholine or epinephrine/norepinephrine
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are released at the end organs. Epinephrine and
norepinephrine are hormones produced by the
adrenal medulla that are collectively termed
catecholamines.
(1) Epinephrine (adrenalin) stimulates
effector organs by causing profound
contraction of vascular smooth muscle
tissue (vasoconstriction), accelerating
heart rate, and relaxation of bronchial
smooth muscles (bronchodilation).
(2) Norepinephrine (noradrenaline) is
similar to epinephrine in causing
vasoconstriction, but it has little
effect on heart rate or bronchodilation.
d) There are 30 times more postganglionic fibers
than preganglionic, thus the effects are very
widespread.
c. Cell bodies (somas).
1) Sympathetic preganglionic somas are located within
the lateral gray horns of two spinal cord areas:
a) The 12 thoracic segments.
b) The first 2 or 3 lumbar segments.
c) The sympathetic neuron arrangement is
sometimes termed thoracolumbar distribution.
2) Parasympathetic preganglionic somas have two
general locations:
a) The first location is found in the brain stem
nuclei (clusters of CNS neurons) of 4 cranial
nerves.
(1) CN III (oculomotor)--movement of the iris
of the eye.
(2) CN VII (facial)--movement of facial
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muscles and secretion of tears.
(3) CN IX (glossopharyngeal)--taste and
saliva secretion.
(4) CN X (vagus)
(a) Located in the medulla of the brain
stem.
(b) Associated with the organs of the
trunk.
(c) Carries about 75-80% of all
parasympathetic fibers.
b) The second location is found in the lateral
gray horns of the second through fourth sacral
segments. These cell bodies are associated
with the sex organs, urinary system, and rectum.
c) The parasympathetic neuron arrangement is
sometimes termed craniosacral distribution.
d. Autonomic ganglia.
1) There are 3 classifications for ANS ganglia.
a) Sympathetic trunk (vertebral chain or
paravertebral ganglia)--extends downward on
both sides of the spine as 2 chains of linked
sympathetic ganglia.
b) Collateral (prevertebral) ganglia--anterior to
the spinal column and associated with both
divisions.
c) Terminal (intramural) ganglia--near or inside
the visceral effectors of both divisions.
2) The ANS fibers may:
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a) Synapse directly with sympathetic trunk ganglia.
b) Ascend or descend the sympathetic trunk to
reach collateral ganglia.
c) Continue directly through the sympathetic
trunk to synapse with specific ganglia.
Important collateral ganglia are:
(1) Sympathetic collateral ganglia--celiac
ganglion (solar plexus), superior
mesenteric ganglion, and inferior
mesenteric ganglion.
(2) Parasympathetic collateral ganglia—
ciliary ganglion, pterygopalatine
ganglion, submandibular ganglion, and
otic ganglion.
d) Tie in as nets or bundles of nerves associated
with specific organ systems.
(1) The greater splanchnic nerves are bundles
of preganglionic nerve fibers that branch
from the 7 lower thoracic segments of the
spinal cord. They pass through the
sympathetic trunk as a net and converge to
synapse on the celiac ganglion.
(2) Plexuses are bundles of postganglionic
nerve fibers associated with major organs
(cardiac plexus, respiratory [pulmonary]
plexus, and hypogastric plexus).
4. Actions of the ANS.
a. Most body systems receive dual innervation from the
sympathetic and parasympathetic divisions.
b. Usually one division causes excitation and one causes
inhibition. The relationship is paradoxical.
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c. The effects of sympathetic stimulation are longer
lasting and more widespread than those of parasympathetic
stimulation.
d. Autonomic neurons are classified on the basis of the
neurotransmitter substances they liberate. There are 2
such classifications.
1) Cholinergic neurons release acetylcholine (ACh).
They innervate most sweat glands and blood vessels
in skeletal muscles. There are 2 types of cholinergic
receptors.
a) Nicotinic receptors.
(1) Found on postganglionic neurons.
(2) Innervated by parasympathetic and
sympathetic ganglion cells.
(3) The effects are short in duration.
(4) Produce excitation only.
(5) They are so named because nicotine mimics
the action of ACh on such receptors.
.
(6) The drug curare blocks the nicotinic
receptors and causes skeletal muscle
relaxation (flaccid paralysis).
(Succinylcholine/pancuronium)
b) Muscarinic receptors.
(1) Present at neuroeffector junctions such
as sphincter muscles and sweat glands.
(2) Innervated by the parasympathetic system
only.
(3) May produce either excitation or
inhibition.
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(4) The effects are of long duration.
(5) They are so named because muscarine mimics
the action of ACh on such receptors.
Muscarine is a neurotoxic poison produced
by a species of mushroom.
(6) The drug atropine blocks the muscarinic
receptors and causes pupil dilation,
reduced glandular secretions, and
relaxation of the lower intestinal tract.
1. Adrenergic neurons release norepinephrine and/or
epinephrine. They are found on visceral effectors
innervated by most sympathetic postganglionic
neurons. There are 2 types of adrenergic receptors.
a) Alpha receptors
(1) Respond to norepinephrine and epinephrine.
2) React by opening channels that depolarize
the cell membrane and cause excitation.
(3) Cause constriction of peripheral blood
vessels through stimulation of the
surrounding smooth muscles.
(4) The drug phenylephrine (Entex, Robitussin
Night Relief) acts upon alpha receptors
by
causing vasoconstriction of the
nasal mucosa to relieve nasal congestion.
b) Beta receptors
(1) Especially sensitive to epinephrine.
(2) Found in many organs--blood vessels of
skeletal muscles, lungs, heart, and liver.
(3) They cause increased BP through
vasoconstriction.
(4) Beta blockers like propranolol (Inderal)
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reduce BP by blocking the beta receptors
in the heart.
f. Sympathomimetic drugs (agonists), promote sympathetic
nerve effects, and sympatholytic drugs (antagonists)
block sympathetic effects.
1) Phenylephrine is an agonist.
2) Propranolol is an antagonist.
g. Parasympathetic and sympathetic responses.
1) The parasympathetic division regulates those
activities that conserve and restore body energy.
It is an energy conservation-restorative system
(rest and digest).
(a) The acronym SLUD stands for the primary
responses driven by the parasympathetic
division.
(1) Salivation
(2) Lacrimation
(3) Urination
(4) Defecation
(b) Fear may cause a massive activation of the
parasympathetic system when there is no
avenue of escape. It is sometimes manifested
by crying and a loss of control over urination
and defecation.
2) The sympathetic division prepares the body for
emergency situations. A series of physiological
activities called the fight-or-flight response
produces the following:
a) Epinephrine is released by the adrenal medulla.
b) Pupils dilate.
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c) Heart rate and BP increase.
d) The blood vessels of nonessential organs
constrict.
e) The blood vessels of essential organs like the
heart, skeletal muscles, lungs, and liver dilate.
f) The liver splits glycogen into glucose for
immediate use by the cells (glycolysis).
g) Bronchioles dilate to allow for deeper
breathing.
5. Visceral autonomic reflexes.
a. A visceral autonomic reflex adjusts the activity of a
visceral effector.
1) Examples would be the contraction and relaxation of
smooth muscles in the GI tract or changes in the
secretions of glands.
2) The homeostasis of heart rate, BP, respiration,
digestion, defecation, and urination are visceral
autonomic reflexes.
b. Visceral sensations do not always reach the cerebral
cortex. Most are unconscious, although hunger, nausea,
and fullness of the urinary bladder and rectum are
conscious expressions of visceral autonomic reflexes.
6. Control by higher centers.
a. The hypothalamus controls and integrates the autonomic
nervous system. It is connected to both the sympathetic
and parasympathetic divisions through centers in the
medulla.
1) However, during emotional stress, the cerebral
cortex
can influence the autonomic nervous system.
can increase BP and heart rate.
Anxiety
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2) Conversely, unpleasant sights can cause vasodilation
with a parasympathetic lowering of BP resulting in
syncope (fainting).
b. Biofeedback is a process in which people learn to monitor
visceral functions and to control them consciously. It
has been used to control heart rate, alleviate migraine
headaches, and facilitate childbirth.
7. Homeostatic Imbalances.
a. Achalasia is a dysfunction of the myenteric plexus which
innervates the smooth muscles at the lower end of the
esophagus. The muscles constrict and prevent food from
entering the stomach. Treatment is with a balloon
device (bougie tube) that stretches the lower esophageal
sphincter (cardiac sphincter). A cardiomyotomy, or
cutting of the sphincter muscles, may be required.
b. In Horner's syndrome, the sympathetic innervation to one
side of the face is lost due to injury or disease (MS).
Sweating stops on the affected side (anhidrosis), the
pupil becomes markedly constricted (miosis), and the
eyelid droops (ptosis). Electrostimulation is sometimes
beneficial. Horner's syndrome should not be confused
with Bell's palsy, which causes unilateral facial
paralysis as a result of somatic motor pathology.
(Ptosis, miosis, and anhidrosis.)
c. Raynaud's disease often affects young women. Sympathetic
stimulation causes excessive peripheral vasoconstriction.
The fingers and toes are deprived of blood, become
painful, and may become necrotic from lack of oxygen and
nutrients. A sympathectomy to cut the sympathetic
innervation may provide some relief.