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					          Chapter 15




The Autonomic Nervous System
• Autonomic nervous system (ANS) or visceral
  nervous system can be defined as a motor
  nervous system that controls glands, cardiac
  muscle and smooth muscle
• Target organs of the ANS are the viscera of the
  thoracic and abdominal cavities, cutaneous
  blood vessels, sweat glands, and pilorector
  muscles
• ANS functions involuntarily as opposed to the
  voluntary nature of the somatic muscle
  system
• Visceral effectors do not depend on the ANS
  to function – the ANS adjusts their activities to
  meet the bodies changing needs
• If the somatic nerve to a skeletal muscle is
  severed the muscle exhibits flaccid paralysis
• If autonomic nerves to cardiac or smooth
  muscle are severed the muscle exhibits
  exaggerated responses (denervation
  hypersensitivity)
• Visceral Reflexes – unconscious, automatic and
  stereotyped responses to stimulation
• Visceral reflex arc includes receptors that detect
  stretch, tissue damage, blood chemicals, body
  temperature and other internal stimuli – also
  includes afferent neurons leading to the CNS,
  interneurons in the CNS, efferent neurons
  carrying motor signals away from the CNS and
  effectors
• Typical example of an autonomic reflex arc
  involves the regulation of blood pressure
• Involves baroreceptors , glossopharyngeal nerve,
  vagus nerve and the sinus node of the heart
• Divisions of the Autonomic Nervous System
• Sympathetic and parasympathetic divisions –
  often innervate the same target organs and
  may have cooperative or contrasting effects
  on them
• Sympathetic division increases alertness,
  heart rate, blood pressure, pulmonary airflow,
  blood glucose concentration, and blood flow
  to cardiac and skeletal muscle – reduces blood
  flow to the skin and digestive tract
• Parasympathetic division – has calming effect
  on many body functions – associated with
  reduced energy expenditure and normal body
  maintenance – associated with digestion and
  waste elimination
• Both systems are active simultaneously
• Produce a background rate of activity called
  autonomic tone – balance between
  sympathetic tone and parasympathetic tone
  shifts with body’s changing needs
• Sympathetic division excites the heart and
  inhibits digestive and urinary function and the
  parasympathetic division does just the
  opposite
• Neural Pathways – ANS has components in the
  central and peripheral nervous systems – it
  has control nuclei in the hypothalamus and
  other regions of the brainstem, motor
  neurons in the spinal cord and peripheral
  ganglia and nerve fibers that travel through
  the cranial nerves and spinal nerves
• Somatic pathways – motor neuron in the
  brainstem or spinal cord issues a myelinated
  axon that reaches all the way to the skeletal
  muscle
• Autonomic pathways – signal must travel
  across two neurons to get to the target organ
  – signal must cross a synapse where these two
  neurons meet in the autonomic ganglion
• First neuron, called the preganglionic neuron
  has a soma in the brainstem or spinal cord and
  its axon terminates in the ganglion – synapses
  with a postganglionic neuron whose axon
  extends all the way to the to the target cells
• Anatomy of the Sympathetic division – called
  the thoracolumbar division because it arises
  from the thoracic and lumbar regions of the
  spinal cord
• Has relatively short preganglionic fibers and
  long postganglionic fibers
• Preganglionic somas are in the lateral horns of
  the spinal cord
• Axons from these somas exit by way of spinal
  nerves from T1 – L2 and lead to the
  sympathetic chain of ganglia along each side
  of the vertebral column
• These chains receive input from only the
  thoracolumbar region of the cord and they
  extend to the cervical and sacrococcygeal
  regions as well
• Usually there are 3 cervical, 11 thoracic, 4
  lumbar, 4 sacral and 1 coccygeal ganglia in
  each chain
• In the thoracolumbar region each ganglion is
  connected to a spinal nerve by two branches
  called communicating rami
• Preganglionic fibers are small myelinated
  fibers that travel from the ventral root of the
  spinal nerve to the ganglion by way of the
  white communicating ramus
• Unmyelinated postganglionic fibers leave the
  ganglion by way of the gray communicating
  ramus and these long fibers extend all the way
  to the target organ
• Not a one to one relationship between the
  preganglionic neurons and the post ganglionic
  neurons in the sympathetic division
• Each postganglionic cell may receive multiple
  synapses from preganglionic neurons – called
  neuronal convergence
• Each preganglionic fiber branches and
  synapses with multiple postganglionic neurons
  – called neuronal divergence
• There are about 17 postganglionic neurons for
  every preganglionic neuron in the sympathetic
  division
• Nerve fibers leave the sympathetic chain by
  three routes – spinal, sympathetic or
  splanchnic nerves
• Spinal nerve route – some postganglionic
  fibers exit a ganglion by way of the gray
  ramus, return to the spinal nerve and travel
  the rest of the way to the target organ – seen
  in sweat glands, pilorector muscles and blood
  vessels of the skin and skeletal muscles
• Some post ganglionic fibers leave by way of
  sympathetic nerves that go to the heart,
  lungs, esophagus and thoracic blood vessels
• Some fibers that arise from spinal nerves T5 –
  T12 pass through the sympathetic ganglia
  without synapsing and continue as splanchnic
  nerves to a second set of ganglia referred to as
  collateral ganglia – these are the celiac,
  superior mesenteric and the inferior
  mesenteric ganglia and these nerves
  contribute to a network called the abdominal
  aortic plexus
• These postganglionic fibers accompany these
  arteries to the target organs
• Adrenal Glands – actually two glands
• Adrenal cortex – secretes steroids
• Adrenal medulla – essentially a ganglion – has
  modified postganglionic neurons without
  dendrites or axons – sympathetic
  preganglionic fibers penetrate through the
  cortex and terminate on these cells
• Adrenal medulla secretes hormones into the
  blood stream – 85% epinephrine, 15%
  norepinephrine, and a trace of dopamine
• The Parasympathetic Division – called the
  craniosacral division because it arises from the
  brain and the sacral region of the spinal cord –
  its fibers travel in certain cranial and sacral
  nerves
• Somas of the preganglionic neurons are
  located in the midbrain, pons, medulla
  oblongata, and segments of S2 – S4 of the
  spinal cord
• Preganglionic neurons issue long preganglionic
  fibers which end in terminal ganglia in or near
  the target organ
• If the terminal ganglion is embedded in the wall
  of the target organ it is called an intramural
  ganglion
• The parasympathetic division has long
  preganglionic fibers which reach almost all the
  way to the target organ and short postganglionic
  fibers that cover the rest of the distance
• There is not as much neuronal diversion in the
  parasympathetic division when compared to the
  sympathetic division – parasympathetic division
  has two postganglionic to every one
  preganglionic fiber
• Parasympathetic fibers leave four cranial
  nerves – the first three supply all the
  parasympathetic innervation to the head and
  the last one supplies the viscera of the
  thoracic and abdominal cavities
• Oculomotor nerve (III) – carries
  parasympathetic fibers that control the pupil
  and the lens of the eye – preganglionic fibers
  enter the orbit and terminate in the ciliary
  ganglion behind the eyeball – postganglionic
  fibers enter the eyeball and innervate the
  ciliary muscle which thickens the lens and the
  pupillary constrictor which narrows the pupil
• Facial nerve (VII) – carries parasympathetic
  fibers that regulate the tear glands, salivary
  glands, and nasal glands – emerges from the
  pons and the parasympathetic fibers split
  away and form two smaller branches
• The upper branch ends at the sphenopalatine
  ganglion and postganglionic fibers continue to
  the tear glands and glands of the nasal cavity,
  palate and areas of the mouth
• The lower branch ends at the submandibular
  ganglion where postganglionic fibers continue
  to the salivary glands and floor of the mouth
• Glossopharyngeal nerve (IX) – carries
  parasympathetic fibers concerned with
  salivation – preganglionic fibers leave this
  nerve soon after its origin and form the
  tympanic nerve – this fiber ends in the otic
  ganglion near the foramen ovale –
  postganglionic fibers continue to the parotid
  salivary gland
• Vagus nerve (X) – carries about 90% of all
  parasympathetic preganglionic fibers – travels
  down the neck and forms three networks in
  the mediastinum of the chest
• Cardiac plexus – supplies fibers to the heart
• Pulmonary plexus – supplies fibers to the
  bronchi and vessels in the lungs
• Esophageal plexus – fibers regulate
  swallowing
• At the lower end of the esophagus these
  fibers give off anterior and posterior vagal
  trunks which contain fibers from the right and
  left vagus – they pierce the diaphragm into
  the abdominal cavity and contribute to the
  aortic plexus – sympathetic fibers synapse
  here but the parasympathetic fibers pass
  through the plexus without synapsing and go
  to the liver, pancreas, stomach, small
  intestine, kidney, ureter and proximal half of
  the colon
• The remaining parasympathetic fibers arise
  from levels S2 – S4 of the spinal cord – they
  travel in the ventral rami of the spinal nerves
  and form the pelvic splanchnic nerves which
  lead to the hypogastric plexus – most fibers
  pass through here and travel by way of the
  pelvic nerves to terminal ganglia in the distal
  half of the colon, the rectum, urinary bladder,
  and the reproductive organs
• The parasympathetic system does not
  innervate body wall structures such as the
  sweat glands, pilorector muscle, or
  subcutaneous blood vessels
• Neurotransmitters in the ANS

• Cholinergic fibers – secrete acetylcholine ACh
• Adrenergic fibers – secrete norepinephrine
  NE
• Sympathetic nervous system tends to have
  longer-lasting effects than the
  parasympathetic system – ACh after being
  secreted is quickly broken down by
  acetylcholinesterase – NE secreted by the
  sympathetic nerve fibers lasts longer
• Receptors – Cholinergic receptors and
  adrenergic receptors – both the sympathetic
  and the parasympathetic divisions have
  excitatory effects on some organs and
  inhibitory effects on others
• Cholinergic receptors – two kinds are nicotinic
  receptors and muscarinic receptors
• Nicotinic receptors occur on the postsynaptic
  cells in all ganglia of the ANS, in the adrenal
  medulla, and in neuromuscular junctions
• Muscarinic receptors occur on all gland ,
  smooth muscle and cardiac cells that receive
  cholinergic innervation
• All cells with nicotinic receptors are excited by
  ACh - some cells with muscarinic receptors
  are excited and others are inhibited by it –
  nicotinic receptors are ligand regulated while
  muscarinic receptors work through a second
  messenger system
• Adrenergic receptors – two types are alpha-
  adrenergic and beta-adrenergic
• Alpha- adrenergic – binding of norepinephrine
  to alpha-adrenergic receptors is usually
  excitatory
• Beta-adrenergic receptors – binding of
  norepinephrine to beta-adrenergic receptors
  is usually inhibitory
• There are exceptions to both of the above
• Effects of the Sympathetic and
  Parasympathetic Nervous System
• Eye – sympathetic effect – pupillary dilatation
  – alpha-adrenergic receptors –
  parasympathetic effect – pupillary constriction

• Adrenal medulla – sympathetic effect –
  hormone secretion – nicotinic receptors –
  parasympathetic effect – no effect
• Heart rate and force – sympathetic effect –
  increased – beta-adrenergic receptors –
  parasympathetic effect – decreased

• Deep coronary arteries – sympathetic effect –
  vasodilatation with beta-adrenergic receptors
  and vasoconstriction with alpha-adrenergic
  receptors – parasympathetic effect – slight
  dilatation
• Blood vessels of most viscera – sympathetic
  effect – vasoconstriction with alpha-
  adrenergic receptors – parasympathetic effect
  – vasodilatation

• Blood vessels of skeletal muscles –
  sympathetic effect – vasodilatation with beta-
  adrenergic receptors – parasympathetic effect
  – no effect
• Blood vessels of the skin – sympathetic effect
  – vasoconstriction with alpha-adrenergic
  receptors – parasympathetic effect –
  vasodilatation

• Bronchi and bronchioles – sympathetic effect
  – bronchodilatation with beta-adrenergic
  receptors – parasympathetic effect –
  bronchoconstriction
• Bladder wall – sympathetic effect – no effect –
  parasympathetic effect – contraction
• Internal urethral sphincter – sympathetic
  effect – contraction, urine retention with
  alpha-adrenergic receptors – parasympathetic
  effect – relaxation with urine release

• Gastrointestinal motility – sympathetic effect
  – decreased with alpha and beta-adrenergic
  receptors – parasympathetic effects –
  increased
• Dual Innervation – most viscera receive fibers
  from both the sympathetic and the
  parasympathetic divisions – the two divisions
  may have either antagonistic or cooperative
  effects on the same organ
• Antagonistic effects oppose each other – the
  sympathetic division speeds up the heart and
  the parasympathetic division slows it down –
  the sympathetic division inhibits digestion and
  the parasympathetic division stimulates it –
  the sympathetic division dilates the pupil and
  the parasympathetic division constricts it
• Antagonistic effects can be exerted by dual
  innervation of the same effector cells such as
  in the heart where nerve fibers from both
  divisions terminate on the same muscle cells
• In other cases antagonistic effects can arise
  because each division innervates two different
  effector cells with opposite effects on organ
  function – iris of the eye is an example –
  sympathetic fibers innervate the pupillary
  dilator and parasympathetic fibers innervate
  the pupillary constrictor
• Cooperative effects – the two divisions act on
  two separate effectors to produce a unified
  overall effect
• Parasympathetic division stimulates serous
  cells of the salivary glands to secrete a watery
  secretion and the sympathetic division
  stimulates mucous cells of the same gland to
  secrete mucus – these are both necessary
  components of saliva
• Even when both divisions innervate a single
  organ they do not always do so equally
• Control Without Dual Innervation
•   Central Control of Autonomic Function
•   Cerebral cortex
•   Hypothalamus
•   Midbrain, Pons and Medulla Oblongata
•   Spinal cord reflexes

				
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