ANS Signal Transmission & Receptors

Question 1

Autonomic Nervous System

Characteristics

Answer 1

• Modulates the visceral organs
• Primarily involved in maintaining homeostasis
• Parasympathetic vs sympathetic divisions
  
  • Each pathway consists of a presynaptic & postsynaptic neuron

Question 2

Parasympathetic Nervous System

(PNS)

Answer 2

• Craniosacral outflow
• Long preganglionic fibers
  
  • Release Ach which act at nicotinic receptors
• Short postganglionic fibers
  
  • Release Ach which act at muscarinic receptors
• Actions terminated by acetylcholinesterases
• Overall function of PNS is to conserve energy
• Enhances digestion

Question 3

Sympathetic Nervous System

(SNS)

Answer 3

• Short preganglionic fibers originate from thoracic & upper lumbar spine
  
  • Synapse on paravertebral & prevertebral ganglia
  • Can synapse with many post-ganglionic fibers = stimultaneous activation of multiple organ systems
  • Release Ach which acts at nicotinic receptors
• Long postganglionic fibers
  
  • Release norepi acting at alpha & beta adrenergic receptors
    
    • Exception is sweat glands where Ach acts at muscarinic receptors
• Actions terminated by reuptake and degradative enzymes
• Overall function is to mobilize body for activity
• Also operates at low basal levels to continueously modulate functions of organs

Question 4

Acetylcholine

Answer 4

• Synthesized from choline and acetate by choline acetyltransferase
• Release from vesicles triggered by Ca⁺⁺
  
  • ^​Inhibited by botulinum toxin
• Acts at nicotinic or muscarinic receptors
• Degraded by acetylcholinesterase
  
  • ​Defect in enzyme results in desensitization

Question 5

Norepinephrine

Answer 5

• Synthesis
  
  • Tyrosine converted to DOPA by tyrosine hydroxylase
  • DOPA converted to dopamine by decarboxylase
  • Dopamine conerted to norepi by dopamine-beta-hydroxylase
  • In adrenal medulla, norepi converted to epi by PNMT
• Termination
  
  • Reuptake primary mech for norepi
  • Degradative enzymes
    
    • Monoamine oxidase (MAO)
    • Catechol-O-methyltransferase

Question 6

Pulmonary Innervation

Answer 6

SNS → Norepi → β₂ → bronchodilation → enhanced air movement

PNS → Ach → M₂ → bronchoconstriction → decreased air movement

Question 7

Cardiac Innervation

Answer 7

SNS → Norepi/Epi → β₁ AdR

SA node: + chronotropic effect → inc HR

AV node: + dromotropic → inc conduction

Myocytes: + inotropic → inc force of contraction

PNS → Ach → M₂ ChR

SA node: - chronotropic effect → dec HR

AV node: - dromotropic → dec conduction

Myocytes: no innervation

Question 8

Impaired Chronotropic Response

Answer 8

Failure to achieve 85% of age predicted maximal HR.

Increases mortality rate.

Question 9

Vascular Innervation

Answer 9

SNS:

• α₁ AdR: skin & viscera - vasoconstriction
  
  • norepi & epi
• β₂ AdR: skeletal muscle - vasodilation
  
  • epi

PNS:

No vascular innervation.

Exogenous muscarinic drugs will cause vasodilation

Question 10

Eye Innervation

Answer 10

1. Pupil
   
   • Dilator radial muscle
     
     • Controlled by SNS
     • Alpha-1 receptors
     • Leads to dilation
   • Constrictor circular muscle
     
     • Controlled by PNS
     • M3 receptors
     • light reflex triggers
     • Leads to contraction
2. Ciliary Muscle: controls lens
   
   • SNS through beta-2 receptors
     
     • Causes relaxation
     • Allows far vision
   • PNS through M3 receptors
     
     • Causes contraction
     • Near vision (accomodation)

Question 11

Liver Innervation

Answer 11

SNS releases norepi

Causes increased glucose release.

Question 12

Fight or flight reponse

Answer 12

1. Release of epi leads to corrdinated response
2. Vasoconstriction of visceral beds
3. Vasodilation of skeletal muscle beds
4. Increases HR, conduction, inotrophy
5. Bronchodilation
6. GI tract & bladder relaxation
7. Eyes to dilate
8. Skin to sweat
9. Glycogenolysis/gluconeogenesis by liver for energy