Updated: 10/16/2017

Autonomic Pathways

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Topic
Noradrenergic nerve terminal
  • Norepinephrine production 
    • adrenergic neurons transport tyrosine by a transport protein to synthesize norepinephrine
      • adrenergic and dopaminergic neurons use the same pathway
        • adrenergic neurons modify NE to Epi
        • dopaminergic neurons terminates at dopamine
    • tyrosine → dopa → dopamine → norepinephrine → epinephrine
    • the rate limiting step is the conversion of tyrosine to dopa by tyrosine hydroxylase
      • can be inhibited by metyrosine 
  • NE pathway feedback
    • NE: negative feedback by acting on presynaptic α2 autoreceptors to inhibit further NE release
    • ACh: acts on M2 receptors also to inhibit further NE release
    • angiotensin II: activates NE release
  • Pathway Inhibitors
    • reserpine: inhibits the vesicular monoamine transporter (VMAT) preventing the storage of neurotransmitters
    • guanethidine: replaces the NE inside the neurotransmitter vesicles and depletes the vesicle of NE
    • cocaine and tricyclic antidepressants: inhibits the NET (reuptake)
Cholinergic Nerve Terminal
  • Acetylcholine (ACh) production
    • synthesized from choline and acetyl-CoA by choline acetyltransferase
      • acetyl-CoA is made in the mitochondria
      • choline is transported into the nerve by choline transporter (CHT)
  • Pathway Inhibitors
    • hemicholinium: blocks the uptake of choline by inhibiting CHT 
    • vesamicol: inhibits vesicular acetylcholine transporter (VAT) preventing the storage of acetylcholine
    • botulinum toxin: cleaves the SNAP proteins thereby preventing the presynaptic vesicles from docking at the terminal and releasing acetylcholine

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Questions (3)
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(M1.PH.12.40) A 32-year-old farmer is brought to the emergency department by his wife. The patient was reportedly anxious, sweaty, and complaining of a headache and chest tightness before losing consciousness en route to the hospital. Which of the following is mechanistically responsible for this patient's symptoms?

QID: 101704
1

Competitive inhibition of acetylcholine at post-junctional effector sites

10%

(25/261)

2

Binding of acetylcholine agonists to post-junctional receptors

6%

(15/261)

3

Inhibition of presynaptic exocytosis of acetylcholine vesicles

4%

(10/261)

4

Irreversible inhibition of acetylcholinesterase

56%

(147/261)

5

Reversible inhibition of acetylcholinesterase

21%

(56/261)

M 1 E

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