THE AUTONOMIC NERVOUS SYSTEM (ANS)

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THE AUTONOMIC NERVOUS SYSTEM (ANS)
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What is the autonomic nervous system?

• The autonomic nervous system (ANS) is the motor
  division of the peripheral nervous system that
  controls visceral activities, with the goal of
  maintaining internal homeostasis.

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What does the autonomic nervous system do?

• It provides motor fibers to smooth and cardiac
  muscles and glands
• It operates subconsciously
• It causes excitation and inhibition
• It makes adjustments to support body activities
• It has an efferent pathway made up of a
  two-neuron chain preganglionic and ganglionic

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Two divisions of the ANS

• 1) Parasympathetic division
• 2) Sympathetic
• Although they have different roles, they have
  effects on many of the same organs of the body

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Organs affected by both divisions
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Roles of the two different divisions

• 1. The parasympathetic division
• _conserves body energy and maintains body
  activities at basal levels
• _is involved in digestion, diuresis, and
  defecation
• _causes heart rate and blood pressure to be low,
  the skin to be warm and the pupils to be
  constricted

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• 2. The sympathetic division
• _activates the body under conditions of emergency
  which is why it is often called the fight or
  flight system
• _is involved in emergency, exercise, and
  excitement
• _causes blood flow to organs to reduce and blood
  flow to muscles to increase
• _also causes skin to be cold, heart rate to
  increase, and rapid breathing

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Parasympathetic division outflow

• Neurons of the cranial and sacral outflows of the
  parasympathetic division are located in the
  following nerves and create an effect on the
  organs mentioned in the table below.

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Table of parasympathetic outflow
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Parasympathetic division outflow
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Sympathetic division outflow

• Sympathetic division outflow is different from
  parasympathetic division outflow because
• It arises from spinal cord segments T1 through L2
• Preganglionic fibers pass through the white rami
  communicantes and synapse in the chain
  (paravertebral) ganglia
• Fibers from T5-L2 form splanchnic nerves and
  synapse with collateral ganglia
• Postganglionic fibers innervate the numerous
  organs of the body
• Sympathetic neurons produce the lateral horns of
  the spinal cord

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Sympathetic division outflow
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ANS PhysiologyNeurotransmitters and Receptors

• The two major neurotransmitter involved in the
  ANS are
• Acetylcholine (ACh)
• Norepinephrine (NE)
• ACh is the same neurotransmitter that is found in
  the somatic motor neurons and is released in the
  ANS
• All ANS preganglionic axons
• All parasympathetic postganglionic axons at
  synapses with their effectors
• ACh releasing fibers are called Cholinergic
  Fibers
• NE is released by most sympathetic postganglionic
  axons
• NE releasing fibers are called Adrenergic Fibers
• The effects of ACh and NE are either excitation
  or inhibition which is dependant on the receptor
  type allowing them to exert these different
  effects at different areas in the body

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Receptors

• ACh binds to two types of receptors
• Nicotinic
• Muscarine
• Nicotinic Receptors are located on
• Motor end plates of skeletal muscles (somatic
  targets)
• All ganglionic neurons (sypathetic and
  parasympathetic)
• The hormone-producing cells of the adrenal
  medulla
• ACh always produces a stimulatory effect when it
  binds with nicotinic receptors
• Muscarine Receptors occur on all effector cells
  stimulated by postganglionic cholinergic fibers
  (parasympathetic targets like the eccrine sweat
  glands and some blood vessels of skeletal muscle)
• ACh binding produces a inhibitory or excitatory
  depending on the receptor type of the target
  organ

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Receptors

• There are also receptors know as Adrenergic
  Receptors (two types)
• Alpha with two subtypes (A1,A2)
• Beta with three subtypes (B1,B2,B3)
• The general effects of NE binding are
• Alpha receptors are mostly stimulatory
• Beta receptors are mostly inhibitory
• However, an exception to this is when NE binds to
  the Beta receptors of cardiac muscle and the
  result is stimulatory

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Overview of the location and effects of receptor
subclasses
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Drugs and the ANS

• Knowing the locations of the cholinergic and
  adrenergic receptors subtypes allows specific
  drugs to be prescribed to obtain the desired
  inhibitory or stimulatory effect on selected
  organs
• An example of this is Atropine (blocks
  parasympathetic effects)
• administered during surgery to prevent salivation
  and dry up respiratory secretions
• Ophthalmologists use it to dilate the pupils for
  an eye exam
• There are several examples of drugs in Table 14.4
  that influence the Activity of the ANS on page
  544

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Interactions of the Autonomic Divisions

• Most visceral organs receive innervation by both
  sympathetic and parasympathtic fibers
• This dual innervation produces a dynamic
  antagonism that allows visceral activity to be
  precisely controlled
• Sympathetic fibers increase heart and respiratory
  rates. They also inhibit digestion and
  elimination
• Parasympathetic fibers allow for digestion and
  elimination and decrease respiratory and heart
  rates

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Parasympathetic Sympathetic

• The parasympathetic division is know as the
  resting and digesting division
• The sympathetic division controls blood pressure
  and keeps the blood vessels in a continual state
  of partial constriction
• Flight or Fright response the sympathetic can
  override the parasympathetic in an emergency to
  increase heart and respiratory rates while
  inhibiting digestion.
• Parasympathetic division restores the heart and
  respiratory rates back to resting levels when the
  emergency is over and then returns back to its
  functions of digestion and elimination

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Sympathetic Tone Parasympathetic Tone

• The Sympathetic Tone
• Constricts blood vessels controls blood
  pressure to rise in response to the bodys needs
• Prompts vessels to dilate if blood pressure needs
  to be decreased
• The Parasympathetic Tone
• Slows the heart
• Controls Digestion and Elimination
• ANS control is best seen in the external
  genitalia
• Parasympathetic fibers cause vasodilation
  resulting in the erection of the penis and
  clitoris
• Sympathetic fibers cause ejaculation of sperm and
  reflex peristalsis in females

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Sympathetic Division

• The adrenal medulla, sweat glands and the
  arrector pili muscles of the skin, the kidneys,
  and most blood vessels receive only sympathetic
  fibers.
• The Sympathetic Division controls
• Thermoregulatory Responses to heat
• Release of Renin from the kidneys
• Metabolic Effects

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Control of Autonomic Functions

• Autonomic functions are controlled by several
  factors
• Reflex Activities mediated by the Spinal Cord and
  Brain Stem (Medullary Center)
• Hypothalamic integration centers interact with
  both higher and lower centers of autonomic,
  somatic, and endocrine response
• Cortical centers influence autonomic functioning
  via connections with the limbic system conscious
  controls are rare but maybe learned via
  biofeedback training