Synapses

Question 1

What is the aim of synapses?

Answer 1

Transmit signal from one neuron to another

Question 2

What are the two types of synapses?

Answer 2

• Electrical synapses
• Chemical synapses

Question 3

What is a post-synaptic potential?

Answer 3

• EPSP (excitatory postsynaptic potential)
• IPSP (inhibitory postsynaptic potential)

Question 4

What characterizes one-way transmission in synapses?

Answer 4

Rectifying

Question 5

What characterizes two-way transmission in synapses?

Answer 5

Non-rectifying

Question 6

What is the primary mechanism of electrical synapses?

Answer 6

Direct transfer of ions

Question 7

What is the synaptic delay in chemical synapses?

Answer 7

0.5 - 2 ms

Question 8

Who discovered ‘Vagusstoff’ and when?

Answer 8

Otto Loewi in 1921

Question 9

What are SNARE proteins?

Answer 9

• Synaptobrevin (present on vesicles)
• SNAP-25 (present on nerve terminal membrane)
• Syntaxin (present on nerve terminal membrane)

Question 10

What triggers vesicular release in chemical synapses?

Answer 10

Rise in intracellular calcium concentrations

Question 11

What is meant by ‘quantal release’?

Answer 11

Release of neurotransmitters in discrete packets (quanta)

1 quantum = amount of transmitter per vesicle

Question 12

How many quanta are typically released per action potential?

Answer 12

Up to 200 quanta

Question 13

What does MEPP stand for?

Answer 13

Miniature End Plate Potential

Question 14

What is the role of clathrin in vesicle recycling?

Answer 14

Forms a coated pit for vesicle retrieval

Question 15

What toxin blocks P/Q-type voltage-gated calcium channels?

Answer 15

ω-agatoxin IVA

Question 16

What is the LD50 of botulinum toxin in humans?

Answer 16

Around 2 ng/kg

Question 17

What is the primary action of botulinum toxin?

Answer 17

Cleaves SNARE proteins to stop neurotransmitter release

Question 18

What does α-bungarotoxin block?

Answer 18

Nicotinic acetylcholine receptors

Question 19

What is the effect of physostigmine?

Answer 19

Inhibits acetylcholinesterase, increasing acetylcholine levels

Question 20

Fill in the blank: The vesicle cycle is maintained by _______.

Answer 20

Clathrin

Question 21

True or False: Chemical synapses have a faster transmission speed than electrical synapses.

Answer 21

False

Question 22

What is a characteristic of ‘mini’s’ in neurotransmitter release?

Answer 22

Occur spontaneously, even in zero extracellular Ca2+
Have amplitudes that are multiples of a quantal unit
Due to release of one or a few quanta (vesicles) = never more than this

Question 23

What is the effect of lowering extracellular Ca2+ on EPSP amplitude?

Answer 23

Decreases in a step-wise manner

Question 24

What are the two types of release in quantal release?

Answer 24

• Spontaneous release = small response (MEP) —> no action potential
• Evoked release = large response —> evoked by action potential triggers opening

Question 25

Key features of electrical synapses

Answer 25

- Gap junctions - Direct transfer of ions - Non-rectifying - Fast transmission - Signal often attenuated Made up of 2 connexons (in each of the neurons) Each connexon = 6 connexins

Question 26

Chemical synapses

Answer 26

More common than electrical Very adaptable and allows for flexibility

Question 27

Mechanism of chemical synapses

Answer 27

1. Action potential triggers opening of Ca2+ channels 2. Ca2+ triggers vesicle fusion and neurotransmitter release 3. Neurotransmitters bind to receptors on the PSN, causing channels to open or close 4. Excitatory or inhibitory postsynaptic potential is generated 5. Neurotransmitter is removed by glial uptake or enzymatic degradation 6. Vesicular membrane is retrieved from the plasma membrane

Question 28

Mechanism of SNARE proteins

Answer 28

1. SNARE proteins bind to each other, drawing the vesicle close to the membrane 2. When intracellular Ca2+ concentrations rise, vesicle fuses with the membrane and the contents are released into the synapse 3. Ca+ is sensed by a vesicle protein called synaptotagmin

Question 29

How is quantal content measured?

Answer 29

Quantal content = EPP current (from evoked release) ————————————————— 1 quantum current quantal content is the average size of a full end plate potential

Question 30

Mechanism of evoked release (EPP)

Answer 30

As intracellular Ca2+ is lowered, EPSP amplitude decreases in a step-wise manner Involve release of up to 200 quanta per action potential Each quantum (vesicle) contains several thousand molecules of ACh

Question 31

Role of clathrin coat

Answer 31

Drags vesicle away from membrane and then leaves the vesicle