Synapses

Question 1

Advantages of Electrical Synapses

Answer 1

Fast and almost instantaneous transmission

Bidirectional transmission

Question 2

Disadvantages of Electrical Synapses

Answer 2

Not as versatile as chemical synapses

No inhibitory signalling

Question 3

Advantages of Chemical Synapses

Answer 3

Multiple neurostransmitters can create different responses at one postsynaptic neuron
Generate IPSP
Modify signals

Question 4

Disadvantages of Chemical Synapses

Answer 4

Delay in transmission

Question 5

Evidence that chemical synapses exist

Answer 5

Otto Loewi

Electrical stimulation of the vagus nerve slows the heartbeat by releasing a chemical signal (ACh)

Question 6

Speed of action by small-molecule neurotransmitters vs neuropeptides?

Answer 6

Small-molecule neurotransmitters mediate rapid synaptic action but neuropeptides modulate slower ongoing functions

Question 7

What are the criteria for neurotransmitters?

Answer 7

1) The substance must be present within the pre-synaptic neuron
2) The substance must be released in reponse to presynaptic depolarization and must be Ca2+ dependent
3) Specific recpetors for the substance must be present on the postsynaptic cell.

Question 8

Chemical neurotransmission sequence

Answer 8

1) Transmitter is synthesized and packed into vesicle
2) Action potential arrives at presynaptic terminal
3) Depolarization of presynaptic membrane triggers voltage gated Ca2+ ion channels
4) Influx of Ca2+ into cell
5) Ca2+ causes the vesicles to fuse with presynaptic membrane in the action zone
6) Exocytosis of vesicles: releasing neurotransmitters
7) Transmitter travels across cleft and bind to specific receptors on the postsynaptic terminal
8) Opening or closing of postsynaptic channels
9) Either inhibitory or excitatory potential that canges the excitability of postsynaptic cell
10) Neurotransmitter removed from celft by glia, or enzyme or diffusion
11) Recyclying of membrane via endocytosis

Question 9

What is the evidence that EPPs are made of MEPPs?

Answer 9

Katz experiment:
Lowered the Ca2+ concentration to reduce the EPPs below the threshold for examination using electrodes
1)The smallest EPP is very similar in amplitude and shape of MEPP
2) Increments of EPP response occur in units about the size of MEPP

Question 10

Quantal nature of MEPPs

Answer 10

MEPPs are the units that summed to make up an EPP h positive enough to reach the treshold

Question 11

Evidence that neurotransmitters are released from vesicles

Answer 11

1) 100mM in a 50nm vesicle = 10,000 molecules which corresponds to the amount needed to mimic a MEPP
2) Used electron micorscopy to examine # of vesicles fused and varied it with a drug = strong correlation between number of vesicles fusing and number of quanta released

Question 12

How can there be the discharge of a quanta into the synaptic cleft?

Answer 12

Synaptic vesicles hold bundles of neurotransmitters!

A single vesicles = one quanta = 1 MEPP

Question 13

Evidence for synaptic vesicle recycling

Answer 13

Heuser and Reese in frog neuromuscular junctions

• Fill synaptic cleft with horseradish peroxidase (HRP) that creates visible product in electron microscopy
• Activate endocytosis by action potential
• HRP found in coated vesicles
• HRP later found in endosome after coated vesicle disappear
• HRP later in synaptice vesicle

Question 14

Evidence for synaptic vesicle recycling

Answer 14

Heuser and Reese in frog neuromuscular junctions

• Fill synaptic cleft with horseradish peroxidase (HRP) that creates visible product in electron microscopy
• Activate endocytosis by action potential
• HRP found in coated vesicles
• HRP later found in endosome after coated vesicle disappear
• HRP later in synaptice vesicle

Question 15

Evidence that Ca2+ is necessary for neurotransmitter release

Answer 15

When calcium cheltors (chemicals that bind to Ca2+ and keep the conc low) are injected, there is no postsynaptic response/change in potential showing that no neurotransmitters were released
Katz and Mideli

Question 16

Evidence that Ca2+ is sufficient for neurotransmitter release

Answer 16

Even in the absence of a presynaptic action potential, the micorinjection of Ca2+ into the presynatic terminal can trigger change in potential of the postsynaptic cell, showing that neurotransmitters are released because of Ca2+ only.