The ANS and Visceral Sensory Neurons

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The ANS and Visceral Sensory Neurons

• Chapter 15

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The ANS and Visceral Sensory Neurons

• The ANS (autonomic self-governing) a system
  of motor neurons
• - general visceral motor division of the PNS
• - distinct from the somatic motor division that
  innervates skeletal muscles
• Innervates - smooth muscle, cardiac muscle, and
  glands
• - essential for maintaining the stability of
  the bodys internal environment
• Regulates visceral functions heart rate (HR),
  blood pressure (BP), digestion, urination

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The ANS and Visceral Sensory Neurons
Figure 15.1
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Terms

• Synapse junction between 2 neurons that
  communicates the message from the presynaptic
  neuron to the postsynaptic neuron
• Ganglion (pl. ganglia) a cluster of neuronal
  cell bodies in the PNS
• Preganglionic neuron cell body lies within the
  CNS
• - its axon, the preganglionic fiber synapses
  with the 2nd motor neuron, the ganglionic neuron,
  in a peripheral autonomic ganglion
• Postganglionic fiber (axon) of the ganglionic
  neuron extends to the visceral organs

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Comparison of Autonomic and Somatic Motor Systems

• Somatic motor system
• - one motor neuron extends from the CNS to the
  skeletal muscle being innervated
• - each motor unit consists of a single neuron
  plus the skeletal muscle cells it innervates
• - axons are well myelinated, conduct impulses
  rapidly

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ANS

• Chain of 2 motor neurons - preganglionic neuron
  and ganglionic neuron
• - preganglionic neuron signals the ganglionic
  neuron, in a peripherial autonomic ganglion
• - postganglionic fiber extends to the visceral
  organ
• - stimulates the effector organ
• Conduction is slower than somatic nervous system
  due to
• - thinly myelinated or unmyelinated axons
• - motor neuron synapses in a ganglion

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Autonomic and Somatic Motor Systems
Figure 15.2
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Divisions of the ANS

• Sympathetic and parasympathetic divisions
• - chains of 2 motor neurons
• - innervate mostly the same visceral organs
• - cause opposite effects
• Balance between the 2 divisions keep body systems
  running smoothly

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Sympathetic and Parasympathetic Divisions

• Sympathetic fight, flight, or fright
• - activated during extreme situations (3 Es)
• - exercise, excitement, emergencies
• Parasympathetic rest and digest
• - concerned with conserving energy

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Anatomical Differences

• Issue from different regions of the CNS
• Sympathetic - aka the thoracolumbar division
• Parasympathetic aka the craniosacral division

Figure 15.3
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Anatomical Differences

• Length of postganglionic fibers sympathetic are
  long and parasympathetic are short
• Branching of axons sympathetic axons are highly
  branched to influence many organs while
  parasympathetic axons have few branches so have a
  localized effect
• Sympathetic release norepinephrine (adrenergic)
• Parasympathetic release acetylcholine
  (cholinergic)

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Anatomical Differences
Figure 15.4a
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Anatomical Differences
Figure 15.4b
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Table 15.1
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The Parasympathetic Division

• Cranial outflow comes from the brain
• - innervates organs of the head, neck, thorax,
  and abdomen
• Sacral outflow innervation supplies remaining
  abdominal and pelvic organs

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The Parasympathetic Division
Figure 15.5
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Cranial Outflow (Parasympathetic)

• Is contained in several cranial nerves
• Preganglionic fibers run via the oculomotor nerve
  (III), facial nerve (VII), glossopharyngeal nerve
  (IX) and vagus nerve (X)
• Cell bodies of CNs located in cranial nerve
  nuclei in the brain stem

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Outflow via CN III

• Parasympathetic fibers innervate smooth muscles
  in the eye cause pupil constriction
• Preganglionic cell bodies located in the
  oculomotor nucleus in the midbrain
• Ganglionic cell bodies lie in the ciliary
  ganglion

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Outflow via the CN VII

• Parasympathetic fibers stimulate secretion of
  glands in the head
• Lacrimal nucleus located in the pons
• - synapse in the pterygopalatine ganglion
• Superior salivatory nucleus located in the pons
• - synapse in the submandibular ganglion

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Outflow via CN IX

• Parasympathetic fibers stimulate secretion of
  glands in the head
• - lacrimal nucleus located in the pons
• Synapse in the pterygopalatine ganglion
• - superior salivatory nucleus located in the
  pons
• Synapse in the submandibular ganglion

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Outflow via the Vagus Nerve (X)

• Fibers innervate visceral organs of the thorax
  and most of the abdomen
• Stimulates digestion, reduction in HR and
  reduction in BP
• Preganglionic cell bodies located in dorsal
  motor nucleus in the medulla
• Ganglionic neurons confined within the walls of
  organs being innervated

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Path of the Vagus Nerve

• Sends branches through autonomic nerve plexuses
• - from the cardiac plexus to the heart
• - from the pulmonary plexus to the lungs
• - from the esophageal plexus to the esophagus
  and into the stomach wall
• - from the celiac plexus and the superior
  mesenteric plexus to the other abdominal organs
  (intestines, liver, pancreas, etc.)

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Figure 15.6
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Sacral Outflow

• Emerges from S2 S4
• Innervates organs of the pelvis and lower abdomen
• Preganglionic cell bodies located in visceral
  motor region of spinal gray matter
• Axons run in ventral roots to ventral rami form
  splanchnic nerves
• - run through the inferior hypogastric plexus

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The Sympathetic Division

• Basic organization issues from T1 L2
• Preganglionic cell bodies lie in the visceral
  motor region of the SC - form the lateral gray
  horn
• More complex - supplies visceral organs and
  structures of superficial body regions
• Contains more ganglia than the parasympathetic
  division that fall into 2 classes
• 1) sympathetic trunk ganglia and 2) collateral
  ganglia

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Sympathetic Trunk Ganglia

• Located on both sides of the vertebral column
• Linked by short nerves into sympathetic trunks
• Joined to ventral rami by white and gray rami
  communicantes
• About one sympathetic trunk ganglion for each
  spinal nerve
• - fusion of ganglia ? fewer ganglia (22-24) than
  spinal nerves

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Sympathetic Trunk Ganglia
Figure 15.8
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Collateral Ganglia

• Or prevertebral ganglia differ in 3 ways
• 1) unpaired, and not segmentally arranged
• 2) occur only in abdomen and pelvis
• 3) lie anterior to the vertebral column, mostly
  on the abdominal aorta
• Main ganglia celiac, superior mesenteric,
  inferior mesenteric, and inferior hypogastric
  ganglia

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Sympathetic Division
Figure 15.7
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Sympathetic Pathways

• Preganglionic neurons in the thoracolumbar SC
  send their motor axons through the adjacent
  ventral root into the spinal nerve, white ramus
  communicans, and associated sympathetic trunk
  ganglion
• Synapse with the ganglionic neurons, either
  within a sympathetic trunk ganglion or in a
  collateral ganglion, and the postganglionic axon
  extends to the visceral organ
• In many cases preganglionic fiber ascend or
  descend in the sympathetic trunk before synapsing
  
• allows sympathetic outputs, which come only
  from the thoracolumbar region to supply the
  superior and inferior body regions

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Sympathetic Pathways to the Body Periphery

• Innervate sweat glands, arrector pili muscles,
  and peripheral blood vessel
• Preganglionic fibers enter the sympathetic trunk
  ganglia nd synapse there with ganglionic cell
  bodies

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To the Body Periphery
Figure 15.9
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Sympathetic Pathways to the Head

• Preganglionic fibers originate in spinal cord at
  T1 T4
• Fibers ascend in the sympathetic trunk
• Synapse in superior cervical ganglion
• Postganglionic fibers associate with large
  arteries
• - carried by these structures to glands, smooth
  muscle, and vessels throughout the head

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To the Head
Figure 15.10
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Sympathetic Pathways to Thoracic Organs

• Preganglionic fibers originate at spinal levels
  T1 T6
• Some fibers synapse in nearest sympathetic trunk
  ganglion
• Postganglionic fibers run directly to the organ
  supplied
• Sympathetic fibers to heart have a less direct
  route
• - increase HR dilate bronchioles dilate BVs to
  the heart wall inhibit muscles and glands in the
  esophagus and digestive system

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To Thoracic Organs
Figure 15.11
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Sympathetic Pathways to Abdominal Organs

• Preganglionic fibers originate in spinal cord T5
  L2
• Pass through adjacent sympathetic trunk ganglia
• - then travel in thoracic splanchnic nerves
• - synapse in prevertebral ganglia on the
  abdominal aorta and celiac and superior
  mesenteric ganglia
• Inhibit activity of muscles and glands in
  visceral organs

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To the Abdominal Organs
Figure 15.12
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Sympathetic Pathways to the Pelvic Organs

• Preganglionic fibers originate in spinal cord T10
  L2
• Some fibers synapse in sympathetic trunk
• Other preganglionic fibers synapse in
  prevertebral ganglia
• Postganglionic fibers proceed from plexuses to
  pelvic organs

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To the Pelvic Organs
Figure 15.13
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Role of Adrenal Medulla in the Sympathetic
Division

• Major organ of the sympathetic nervous system
• Constitutes largest sympathetic ganglia
• Secretes great quantities of catecholamines
  norepinephrine and epinephrine
• Stimulated to secrete by preganglionic
  sympathetic fibers

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The Adrenal Medulla
Figure 15.14
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Table 15.2 (1 of 3)
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Table 15.2 (2 of 3)
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Table 15.2 (3 of 3)
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Visceral Sensory Neurons

• General visceral sensory neurons monitor stretch,
  temperature, chemical changes, and irritation
• Cell bodies are located in the dorsal root
  ganglion
• Viscera pain perceived to be somatic in origin
  ? referred pain

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A Map of Referred Pain
Figure 15.15
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Visceral Reflexes

• Visceral sensory and autonomic neurons
  participate in visceral reflex arcs
• - defecation and micturition reflexes
• Some are simple spinal reflexes
• Others do not involve the CNS strictly
  peripheral reflexes

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Figure 15.16
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Central Control of the ANS

• Control by the brain stem and spinal cord
• Reticular formation exerts most direct influence
• - medulla oblongata and periaqueductal gray
  matter
• Control by the hypothalamus and amygdala
• - hypothalamus main integration center of the
  ANS
• - amygdala main limbic region for emotions
• Control by the cerebral cortex

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Central Control of the ANS
Figure 15.17
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Disorders of the ANS

• Raynauds disease characterized by constriction
  of blood vessels
• - provoked by exposure to cold or by emotional
  stress
• Hypertension high blood pressure
• - can result from overactive sympathetic
  vasoconstriction
• Mass reflex reaction uncontrolled activation of
  autonomic and somatic motor neurons
• - affects quadriplegics and paraplegics
• Achalasia of the cardia defect in the autonomic
  innervation of the esophagus

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The ANS Throughout Life

• Preganglionic neurons of the ANS develop from the
  neural tube
• Ganglionic neurons develop from the neural crest
• Development of the sympathetic division some
  cells migrate ventrally to form the sympathetic
  trunk ganglia
• - from there other cells migrate ventrally to
  form the prevertebral ganglia

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Figure 15.18
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The ANS Throughout Life

• Efficiency of the ANS declines with advancing age
• - constipation due to reduced mobility of GI
  tract
• - dry eyes due to reduced tear formation