The Autonomic Nervous System

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The Autonomic Nervous System

• Lecture 5
• Dr. Shawna Heber

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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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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 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.

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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 motor neuron 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 type B fiber that extends to
  autonomic ganglion
• Postganglionic neuron
• cell body lies outside the CNS in an autonomic
  ganglion
• axon is unmyelinated type C 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 ANS vs. Parasympathetic ANS
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ANS Neurotransmitters

•   Classified as either cholinergic or adrenergic
  neurons based upon the neurotransmitter released
• Adrenergic (NE)
• Cholinergic (ACh)

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Cholinergic Neurons and Receptors

• Cholinergic neurons release acetylcholine
  from preganglionic neurons from parasympathetic
  postganglionic neurons
• Excites or inhibits depending upon receptor type
  and organ involved
• Nicotinic receptors are found on dendrites cell
  bodies of autonomic NS cells and at NMJ
• 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 postganglionic sympathetic neurons only
• Excites or inhibits organs depending on receptors
  
• Alpha1 and Beta1 receptors produce excitation
• Alpha2 and Beta2 receptors cause inhibition
• Beta3 receptors (brown fat) increase
  thermogenesis
• NE lingers at the synapse until enzymatically
  inactivated by monoamine oxidase (MAO) or
  catechol-O-methyltransferase (COMT)

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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, adipocytes, kidneys, post
  pituitary, pineal gland, radial muscle of iris
  (dilation of pupil), spleen
• controlled by regulation of the tone of the
  sympathetic system
• The only organ with parasympathetic innervation
  ONLY is the lacrimal gland

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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 levels 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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Clinical Application

• Horners Syndrome
• Classic triad Ptosis, Miosis, Anhydrosis
• Caused by damage to SNS
• Brainstem stroke, tumor, cluster headache, injury
  to carotid artery
• Raynauds Disease
• Caused by an overactivity of the SNS
• Arterioles of fingers and toes go into spasm,
  causing cyanosis and/or paraesthesia
• Cold triggers sympathetic response, causing
  constriction of distal blood vessels in order to
  reserve blood for muscles and brain