Lecture 5- Synaptic Transmission

Question 1

Can a cell be both presynaptic and postsynaptic?

Answer 1

Yes, in fact it is rarely only one. Therefore when we are talking about these terms we are referring to a specific synapse.

Question 2

How is an electrical synapse different from a chemical synapse?

Answer 2

• Electrical synapses are where cells exist very close together. Gap junctions act as continuous pores between the cells and ions can flow continuously from one to the other (causing Depolarisation). Due to this this form of transmission is very fast but also lacks regulation. The opening of gap junctions can be from a number of factors including voltage, pH, Ca2+ and receptors
• On the other hand chemical transmission is a lot more controlled and also slower cause it occurs in a series of steps/ complex events. This is where we have neurotransmitter released at the cleft and interacts with post-synaptic receptors on the membrane of the next cell. Synapse strength can also be modified which cannot occur in electrical synapses.

Question 3

What are the three types of chemical synapses?

Answer 3

• Classical neurotransmission (vesicles)
• Gaseous neurotransmission (can pass directly across membrane to act in other cells. These cells can be distant)
• Neuropeptide neurotransmission (peptides diffuse into extracellular space and bind to synaptic and extrasynaptic G-protein- complex receptors)

Question 4

What are the two types of neurotransmitter exocytosis and recycling?

Answer 4

• Classical: full fusion of vesicles membrane and cell membrane in order for exocytosis to occur
• ‘Kiss and run’ : partial fusion. Only fuse a small amount.

In both the vesicle is recycled.

Question 5

How come the membrane of vesicles and the cell membrane can merge during exocytosis of neurotransmitters?

Answer 5

Because they are essentially the same (both lipid bilayers)

Question 6

Describe the sequence of events that occurs in classical neurotransmission….

Answer 6

1. Action potential propagates down the axon – to the presynaptic terminal
2. Presynaptic terminal is depolarised and voltage-gated Ca2+ channels open allowing calcium in
3. Ca2+ ions trigger vesicle fusion with the presynaptic membrane
4. Neurotransmitter is released into the synaptic cleft – vesicle is recycled, “Kiss
   and Run” – usually happens within one second, full fusion is also fast
5. Neurotransmitter diffuses within synaptic cleft
6. Neurotransmitter binds to its specific receptors on the postsynaptic membrane

7. A. Na+ channels open – local postsynaptic cell depolarisation – the excitatory
post synaptic potential (EPSP)
B. K+ or Cl- channels open – local postsynaptic cell hyperpolarisation (going more negative from RMP/ threshold) – the inhibitory post synaptic potential (IPSP)
which happens (A or B) depends on which neurotransmitter receptors bind to 

8. Neurotransmitter degraded or taken up (by glia or presynaptic terminal)
9. Recycling and refilling of the vesicles – clathrin-mediated in ~20 seconds

Question 7

Electrical synapses are faster than chemical synapses BECAUSE
electrical synapses conduct ions directly from cell to cell but chemical
synapses release neurotransmitters to activate postsynaptic receptors.

Answer:-
A. if both statements are true, and the second causes the first.
B. if both statements are true but the second does not cause the first.
C. if the first statement is true and the second is false.
D. if the first statement is false and the second is true.
E. if both statements are false.

Answer 7

A. both true and second causes the first

Question 8

Will all neurotransmitter released at the clef bind to a receptor? What happens when it does?

Answer 8

No, and when it does it only binds very briefly (bind and leave)

Question 9

What is temporal summation?

Answer 9

Summation between EPSPs from the same input that occur close enough together in time

Question 10

What is spatial summation?

Answer 10

Summation between events that occur close enough together in space from different inputs – still requires them to be close in time!

Question 11

Are local potentials enough on their own to generate APs?

Answer 11

No, integration via spatial or temporal summation is required to reach threshold for AP generation (at the axon hillock)

Question 12

What happens with increased distance from the point of depolarization to the axon hillock?

Answer 12

More distance= less depolarization at axon hillock (lower amplitude)… fades

Question 13

What are the two ways synapses can be modulated?

Answer 13

• Facilitation within a single synapse. Lasts seconds to minutes and is due to an increased presynaptic calcium and/or neurotransmitter availability
• Long-term potentiation (and depression). Lasts hours to days. As a result of multiple postsynaptic mechanisms.