Transcript INTRODUCTION - Faculty & Staff Webpages

INTRODUCTION
• The autonomic nervous system (ANS) operates via
reflex arcs.
• Operation of the ANS to maintain homeostasis, however,
depends on a continual flow of sensory afferent input,
from receptors in organs, and efferent motor output to
the same effector organs.
• Structurally, the ANS includes autonomic sensory
neurons, integrating centers in the CNS, and autonomic
motor neurons.
• Functionally, the ANS usually operates without conscious
control.
• The ANS is regulated by the hypothalamus and brain
stem.
Chapter 15
The Autonomic Nervous System
• Regulate activity of smooth muscle,
cardiac muscle & certain glands
• Structures involved
– general visceral afferent neurons
– general visceral efferent neurons
– integration center within the brain
• Receives input from limbic system and
other regions of the cerebrum
SOMATIC AND AUTONOMIC
NERVOUS SYSTEMS
• The somatic nervous system contains both
sensory and motor neurons.
• The somatic sensory neurons receive input from
receptors of the special and somatic senses.
• These sensations are consciously perceived.
• Somatic motor neurons innervate skeletal
muscle to produce conscious, voluntary
movements.
• The effect of a motor neuron is always
excitation.
SOMATIC AND AUTONOMIC NERVOUS SYSTEMS
• The autonomic nervous system contains both autonomic
sensory and motor neurons.
– Autonomic sensory neurons are associated with
interoceptors.
– Autonomic sensory input is not consciously perceived.
• The ANS also receives sensory input from somatic senses
and special sensory neurons.
• The autonomic motor neurons regulate visceral activities by
either increasing (exciting) or decreasing (inhibiting)
ongoing activities of cardiac muscle, smooth muscle, and
glands.
– Most autonomic responses can not be consciously
altered or suppressed.
SOMATIC vs AUTONOMIC NERVOUS SYSTEMS
• All somatic motor pathways consist of a
single motor neuron
• Autonomic motor pathways consists of
two motor neurons in series
– The first autonomic neuron motor has its cell
body in the CNS and its myelinated axon
extends to an autonomic ganglion.
• It may extend to the adrenal medullae rather than
an autonomic ganglion
– The second autonomic motor neuron has its
cell body in an autonomic ganglion; its
nonmyelinated axon extends to an effector.
AUTONOMIC NERVOUS
SYSTEM
• The output (efferent) part of the ANS is
divided into two principal parts:
– the sympathetic division
– the parasympathetic division
– Organs that receive impulses from both
sympathetic and parasympathetic fibers are
said to have dual innervation.
• Table 15.1 summarizes the similarities and
differences between the somatic and
autonomic nervous systems.
Sympathetic Ganglia
• These ganglia include the sympathetic trunk or
vertebral chain or paravertebral ganglia that lie
in a vertical row on either side of the vertebral
column (Figures 15.2).
• Other sympathetic ganglia are the prevertebral
or collateral ganglia that lie anterior to the spinal
column and close to large abdominal arteries.
– celiac
– superior mesenteric
– inferior mesenteric ganglia
– (Figures 15.2 and 15.4).
Dual Innervation, Autonomic Ganglia
• Sympathetic
(thoracolumbar) division
– preganglionic cell
bodies in thoracic and
first 2 lumbar segments
of spinal cord
• Ganglia
– trunk (chain) ganglia
near vertebral bodies
– prevertebral ganglia near
large blood vessel in gut
(celiac, superior
mesenteric, inferior
mesenteric)
• Parasympathetic
(craniosacral)
division
– preganglionic cell
bodies in nuclei of
4 cranial nerves
and the sacral
spinal cord
• Ganglia
– terminal ganglia in
wall of organ
Autonomic Plexuses
• These are tangled networks of sympathetic
and parasympathetic neurons (Figure 15.4)
which lie along major arteries.
• Major autonomic plexuses include
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–
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–
–
cardiac,
pulmonary,
celiac,
superior mesenteric,
inferior mesenteric,
renal and
hypogastric
Structures of Sympathetic NS
• Preganglionic cell bodies at T1 to L2
• Rami communicantes
– white ramus = myelinated = preganglionic fibers
– gray ramus = unmyelinated = postganglionic fibers
• Postganglionic cell bodies
– sympathetic chain ganglia along the spinal column
– prevertebral ganglia at a distance from spinal
cord
• celiac ganglion
• superior mesenteric ganglion
• inferior mesenteric ganglion
Postganglionic Neurons:
Sympathetic vs. Parasympathetic
• Sympathetic preganglionic neurons pass to the
sympathetic trunk. They may connect to
postganglionic neurons in the following ways. (Figure
17.5).
– May synapse with postganglionic neurons in the ganglion it
first reaches.
– May ascend or descend to a higher of lower ganglion before
synapsing with postganglionic neurons.
– May continue, without synapsing, through the sympathetic
trunk ganglion to a prevertebral ganglion where it synapses
with the postganglionic neuron.
• Parasympathetic preganglionic neurons synapse with
postganglionic neurons in terminal ganglia (Figure 17.3).
Organs Innervated by Sympathetic NS
• Structures innervated by each spinal nerve
– sweat glands, arrector pili mm., blood vessels to
skin & skeletal mm.
• Thoracic & cranial plexuses supply:
– heart, lungs, esophagus & thoracic blood vessels
– plexus around carotid artery to head structures
• Splanchnic nerves to prevertebral ganglia
supply:
– GI tract from stomach to rectum, urinary &
reproductive organs
Circuitry of Sympathetic NS
• Divergence = each preganglionic cell
synapses on many postganglionic cells
• Mass activation due to divergence
– multiple target organs
– fight or flight response explained
• Adrenal gland
– modified cluster of postganglionic cell
bodies that release epinephrine &
norepinephrine into blood
Structure of the Parasympathetic Division
• The cranial outflow consists of preganglionic
axons that extend from the brain stem in four
cranial nerves. (Figure 15.3).
– The cranial outflow consists of four pairs of ganglia
and the plexuses associated with the vagus (X)
nerve.
• The sacral parasympathetic outflow consists of
preganglionic axons in the anterior roots of the
second through fourth sacral nerves and they
form the pelvic splanchnic nerve. (Figure15.3)
Parasympathetic Cranial Nerves
• Oculomotor nerve
– ciliary ganglion in orbit
– ciliary muscle & pupillary constrictor muscle inside
eyeball
• Facial nerve
– pterygopalatine and submandibular ganglions
– supply tears, salivary & nasal secretions
• Glossopharyngeal
– otic ganglion supplies parotid salivary gland
• Vagus nerve
– many brs supply heart, pulmonary and GI tract as
far as the midpoint of the colon
Cholinergic Neurons and Receptors
• Cholinergic receptors are integral membrane proteins
in the postsynaptic plasma membrane.
• The two types of cholinergic receptors are nicotinic
and muscarinic receptors (Figure 15.6 a , b).
– Activation of nicotinic receptors causes excitation of the
postsynaptic cell.
• Nicotinic receptors are found on dendrites & cell bodies of autonomic
NS cells (and at NMJ.)
– Activation of muscarinic receptors can cause either
excitation or inhibition depending on the cell that bears the
receptors.
• Muscarinic receptors are found on plasma membranes of all
parasympathetic effectors
Adrenergic Neurons and Receptors
• The main types of adrenergic receptors are alpha
and beta receptors. These receptors are further
classified into subtypes.
– Alpha1 and Beta1 receptors produce excitation
– Alpha2 and Beta2 receptors cause inhibition
– Beta3 receptors (brown fat) increase thermogenesis
• Effects triggered by adrenergic neurons typically
are longer lasting than those triggered by
cholinergic neurons.
• Table 15.2 describes the location of the subtypes
of cholinergic and adrenergic receptors and
summarizes the responses that occur when each
type of receptor is activated.
Receptor Agonists and Antagonists
• An agonist is a substance that binds to
and activates a receptor, mimicking the
effect of a natural neurotransmitter or
hormone.
• An antagonist is a substance that binds to
and blocks a receptor, preventing a natural
neurotransmitter or hormone from exerting
its effect.
• Drugs can serve as agonists or
antagonists to selectively activate or block
ANS receptors.
Physiological Effects of the ANS
• Most body organs receive dual innervation
– innervation by both sympathetic &
parasympathetic
• Hypothalamus regulates balance (tone)
between sympathetic and parasympathetic
activity levels
• Some organs have only sympathetic
innervation
– sweat glands, adrenal medulla, arrector pili mm &
many blood vessels
– controlled by regulation of the “tone” of the
sympathetic system
Sympathetic Responses
• Dominance by the sympathetic system is caused by
physical or emotional stress -- “E situations”
– emergency, embarrassment, excitement, exercise
• Alarm reaction = flight or fight response
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dilation of pupils
increase of heart rate, force of contraction & BP
decrease in blood flow to nonessential organs
increase in blood flow to skeletal & cardiac muscle
airways dilate & respiratory rate increases
blood glucose level increase
• Long lasting due to lingering of NE in synaptic gap
and release of norepinephrine by the adrenal gland
Parasympathetic Responses
• Enhance “rest-and-digest” activities
• Mechanisms that help conserve and restore body
energy during times of rest
• Normally dominate over sympathetic impulses
• SLUDD type responses = salivation, lacrimation,
urination, digestion & defecation and 3 “decreases”--decreased HR, diameter of airways and diameter of pupil
• Paradoxical fear when there is no escape route or no
way to win
– causes massive activation of parasympathetic division
– loss of control over urination and defecation
PHYSIOLOGICAL EFFECTS OF THE ANS
- Summary
• The sympathetic responses prepare the
body for emergency situations (the fightor-flight responses).
• The parasympathetic division regulates
activities that conserve and restore body
energy (energy conservation-restorative
system).
– Table 15.4 summarizes the responses of
glands, cardiac muscle, and smooth muscle
to stimulation by the ANS.
Autonomic or Visceral Reflexes
• A visceral autonomic reflex adjusts the activity
of a visceral effector, often unconsciously.
– changes in blood pressure, digestive functions etc
– filling & emptying of bladder or defecation
• Autonomic reflexes occur over autonomic reflex
arcs. Components of that reflex arc:
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sensory receptor
sensory neuron
integrating center
pre & postganglionic motor neurons
visceral effectors
Control of Autonomic NS
• Not aware of autonomic responses because
control center is in lower regions of the brain
• Hypothalamus is major control center
– input: emotions and visceral sensory information
• smell, taste, temperature, osmolarity of blood, etc
– output: to nuclei in brainstem and spinal cord
– posterior & lateral portions control sympathetic NS
• increase heart rate, inhibition GI tract, increase temperature
– anterior & medial portions control parasympathetic NS
• decrease in heart rate, lower blood pressure, increased GI
tract secretion and mobility
Autonomic versus Somatic NS - Review
• Somatic nervous system
– consciously perceived sensations
– excitation of skeletal muscle
– one neuron connects CNS to organ
• Autonomic nervous system
– unconsciously perceived visceral sensations
– involuntary inhibition or excitation of smooth
muscle, cardiac muscle or glandular secretion
– two neurons needed to connect CNS to organ
• preganglionic and postganglionic neurons
Autonomic Dysreflexia
• Exaggerated response of sympathetic NS in
cases of spinal cord injury above T6
• Certain sensory impulses trigger mass
stimulation of sympathetic nerves below the
injury
• Result
– vasoconstriction which elevates blood pressure
– parasympathetic NS tries to compensate by
slowing heart rate & dilating blood vessels above
the injury
– pounding headaches, sweating warm skin above
the injury and cool dry skin below
– can cause seizures, strokes & heart attacks