[3.6.2.2] Synaptic Transmission

Question 1

Describe the structure of a synapse.

Answer 1

Question 2

What are cholinergic synapses?

Answer 2

• Synapses that use the neurotransmitter acetylcholine (ACh).

Question 3

Describe transmission across a cholinergic synapse.

Answer 3

At pre-synaptic neurone

1. Depolarisation of pre-synaptic membrane causes opening of voltage-gated Ca²⁺ channels.
   
   • Ca²⁺ diffuse into pre-synaptic neurone / knob.
2. Causing vesicles containing ACh to move and fuse with pre-synaptic membrane.
   
   • Releasing ACh into the synaptic cleft (by exocytosis).

At post-synaptic neurone

3. ACh diffuses across synaptic cleft to bind to specific receptors on post-synaptic membrane.
4. Causing Na⁺ channels to open.
   • Na⁺ diffuse into post-synaptic knob causing depolarisation.
   • If threshold is met, an action potential is initiated.

Question 4

Explain what happens to acetylcholine after synaptic transmission.

Answer 4

• It is hydrolysed by acetylcholinesterase.
• Products are reabsorbed by the presynatpic neurone.
• To stop overstimulation - if not removed it would keep binding to receptors, causing depolarisation.

Question 5

Explain how synapses result in unidirectional nerve impulses.

Answer 5

• Neurotransmitter only made in / released from pre-synaptic neurone.
• Receptors only on post-synaptic membrane.

Question 6

Describe summation by synapses.

Answer 6

• Addition of a number of impulses converging on a single post-synaptic neurone.
• Causing rapid build-up of neurotransmitter (NT).
• So threshold more likely to be reached to generate an action potential.

Question 7

Describe spatial summation.

Answer 7

• Many pre-synaptic neurones share one synaptic cleft / post-synaptic neurone.
• Collectively release sufficient neurotransmitter to reach threshold to trigger an action potential.

Question 8

Describe temporal summation.

Answer 8

• One pre-synaptic neurone releases neurotransmitter many times over a short time.
• Sufficient neurotransmitter to reach threshold to trigger an action potential.

Question 9

Describe inhibition by inhibitory synapses.

Answer 9

• Inhibitory neurotransmitters hyperpolarise postsynaptic memebrane as:
  
  • Cl⁻ channels open -> Cl⁻ diffuse in.
  • K⁺ channels open -> K⁺ diffuse out.
• This means inside of axon has more negative charge relative to outside / below resting potential.
• So more Na⁺ required to enter for depolorisation,
• Reduces likelihood of threshold being met / action potential formation at post-synaptic membranes.

Question 10

Describe the structure of a neuromuscular junction.

Answer 10

• Very similar to a synapse except:
  
  • Receptors are on muscle fibre sarcolemma instead of postsynaptic membrane and there are more.
  • Muscle fibre forms clefts to store enzyme e.g. acetylcholinesterase to break down neurotransmitter.

Question 11

Compare transmission across cholinergic synapses and neuromuscular junctions.

Answer 11

In both: transmission is unidirectional.
CHOLINERGIC SYNAPSE

• Neurone to neurone (or effectors, glands).
• Neurotransmitters can be excitatory or inhibitory.
• Action potential may be initiated in post-synaptic neurone.

NEUROMUSCULAR JUNCTION

• Motor neurone to muscle.
• Always excitatory.
• Action potential propagates along sarcolemma down T tubules.

Question 12

Use examples to explain the effect of drugs on a synapse.

Answer 12

• Some drugs stimulate the nervous system, leading to more action potentials, e.g.:
  
  • Similar shape to neurotransmitter.
  • Stimulate release of more neurotransmitter.
  • Inhibit enzyme that break downs neurotransmitter -> Na⁺ continues to enter.
• Some drugs inhibit the nervous system, leading to fewer action potentials.
  
  • Inhibit release of neurotransmitter e.g. prevent opening of calcium ion channel.
  • Block receptors by mimicking shape of neurotransmitters.