15.6 Transmission across a synapse

Question 1

What is a cholinergic synapse

Answer 1

one where the neurotransmitter is acetylcholine

Question 2

What is acetylcholine made of

Answer 2

Made of two parts:

• Acetyl
• choline

Question 3

Where do cholinergic synapses occur

Answer 3

Common in vertebrates, between neurones and muscles
Neuromuscular junctions

Question 4

Describe the process of acetylcholine across cholinergic synapse

Answer 4

• Action potential arrives at end of presynaptic neurone causing calcium channels to open
• Ca2+ ions enter synaptic knob
• This influx of calcium ions causes the synaptic vesicles to fuse with presynaptic membrane, releasing acetylcholine to synaptic cleft
• Acetylcholine molecules diffuse across narrow synaptic cleft very quickly because the diffusion pathway is short.
  It then binds to receptors on sodium ion protein channels in postsynaptic membrane
• So sodium ion channels open allowing sodium ions to rapidly diffuse in along concentration gradient
• This influx of sodium ions generates a new action potential
• Acetylcholinesterase hydrolyses acetylcholine into choline and ethanoic acid so they diffuse back across synaptic cleft into presynaptic neurone.
  This recycles it.
  The rapid breakdown of acetylcholine also prevents it from continuously generating a new action potential in postsynaptic neurone
• Rapid breakdown of acetylcholine also prevents it from continuously generating a new action potential in the post synaptic neurone.
  So leads to discrete transfer of information across synapse
• ATP released by mitochondria is used to recombine choline and ethanoic acid into acetylcholine. This is stored in synaptic vesicles for future use

Question 5

How do drugs act on synapses

Two ways

Answer 5

• They stimulate nervous system by creating more action potentials in postsynaptic neurones. Drug can mimic a neurotransmitter so can stimulate release of more neurotransmitter or inhibits enzyme that breaks down neurotransmitter.
• They inhibit nervous system by creating fewer action potentials in postsynaptic neurones. Drug can inhibit release of neurotransmitters or can block receptors for sodium/potassium ions on postsynaptic neurone.