The Autonomic Nervous System

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Chapter 15

• The Autonomic Nervous System

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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.

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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

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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.

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SOMATIC AND AUTONOMIC NERVOUS SYSTEMS

• The autonomic nervous system contains both
  autonomic sensory and motor neurons.
• Autonomic sensory input is not consciously
  perceived.
• 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.

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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.

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Somatic versus Autonomic NS
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Basic Anatomy of ANS

• Preganglionic neuron
• cell body in brain or spinal cord
• axon is myelinated fiber that extends to
  autonomic ganglion
• Postganglionic neuron
• cell body lies outside the CNS in an autonomic
  ganglion
• axon is unmyelinated fiber that terminates in a
  visceral effector

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Sympathetic vs. Parasympathetic NS
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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.

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Sympathetic ANS vs. Parasympathetic ANS
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Divisions of the ANS

• 2 major divisions
• parasympathetic
• sympathetic
• Dual innervation
• one speeds up organ
• one slows down organ
• Sympathetic NS increases heart rate
• Parasympathetic NS decreases heart rate

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Divisions of the ANS

• 2 major divisions
• parasympathetic
• sympathetic
• Dual innervation
• one speeds up organ
• one slows down organ
• Sympathetic NS increases heart rate
• Parasympathetic NS decreases heart rate

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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).

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Parasympathetic Ganglia

• Parasympathetic ganglia are the terminal or
  intramural ganglia that are located very close to
  or actually within the wall of a visceral organ.
• Examples of terminal ganglia include (Figure
  15.3)
• ciliary,
• submandibular,
• otic ganglia

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Sympathetic ANS vs. Parasympathetic ANS
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Dual Innervation, Autonomic Ganglia

• Parasympathetic (craniosacral) division
• preganglionic cell bodies in nuclei of 4 cranial
  nerves and the sacral spinal cord
• Ganglia
• terminal ganglia in wall of organ

• 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)

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Structures of Sympathetic NS

• Preganglionic cell bodies at T1 to L2
• 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

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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).

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Pathways of Sympathetic Fibers

• Spinal nerve route
• out same level
• Sympathetic chain route
• up chain out spinal nerve
• Collateral ganglion route
• out splanchnic nerve to collateral ganglion

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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

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Ganglia Plexuses of Sympathetic NS
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Structure of the Parasympathetic Division

• Preganglionic axons 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)

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Anatomy of Parasympathetic NS

• Preganglionic cell bodies found in
• 4 cranial nerve nuclei in brainstem
• S2 to S4 spinal cord
• Postganglionic cell bodies very near or in the
  wall of the target organ in a terminal ganglia

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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
• supply heart, pulmonary and GI tract as far as
  the midpoint of the colon

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Parasympathetic Sacral Nerve Fibers

• Form pelvic splanchnic nerves
• Preganglionic fibers end on terminal ganglia in
  walls of target organs
• Innervate smooth muscle and glands in colon,
  ureters, bladder reproductive organs

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ANS Neurotransmitters

• Classified as either cholinergic or adrenergic
  neurons based upon the neurotransmitter released
• Adrenergic
• Cholinergic

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Cholinergic Neurons and Receptors

• Cholinergic neurons release acetylcholine
• all preganglionic neurons
• all parasympathetic postganglionic neurons
• few sympathetic postganglionic neurons (to most
  sweat glands)
• Excitation or inhibition depending upon receptor
  subtype and organ involved.

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Cholinergic Neurons and Receptors

• 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

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Adrenergic Neurons and Receptors

• Adrenergic neurons release norepinephrine (NE)
• from postganglionicsympathetic neurons only
• Excites or inhibits organs depending on receptors
• NE lingers at the synapse until enzymatically
  inactivated by monoamine oxidase (MAO) or
  catechol-O-methyltransferase (COMT)

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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.

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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.

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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

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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
• 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

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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

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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
• sensory receptor
• sensory neuron
• integrating center
• pre postganglionic motor neurons
• visceral effectors

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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

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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