Synaptic Transmission II

Question 1

Physiological evidence intracellular recording techniques uncovered ?

Answer 1

The basics of presynaptic operation

Question 2

Morphological evidence revealed ?

Answer 2

The structure of a synapse and offered clues to a possible mechanism of transmission

Question 3

Who uncovered the fundamental properties of synaptic transmission ?

Answer 3

Bernard Katz

Question 4

What did Katz use ?

Answer 4

Katz used the squid giant axon - a preparation where pre and postsynaptic structures can be directly accessed and recorded at the same time

Question 5

What did blockers of voltage-gated Na+ (TTX) and K+ channels (TEA) reveal ?

Answer 5

It revealed that neither Na+ or K+ channels are required for effective release

Question 6

What did Katz and co establish ?

Answer 6

They established that depolarisation (artificial or by an action potential) opens special voltage-gated Ca2+ channels at the presynaptic terminal

Question 7

What drives the transmission process ?

Answer 7

It is the influx of Ca2+ down its concentration gradient and triggered by depolarisation that drives the transmission process

Question 8

What happens when Ca2+ channels are not open ?

Answer 8

Internal [Ca2+] is kept low in the cell by:

• active removal from the neuron
• internalisation of Ca2+ into organelles (e.g. mitochondria and endoplasmic reticulum)

Question 9

Work from Neher and Sakmann working in the giant auditory synapse, the calyx of Held, found that ?

Answer 9

The short delay between pre and postsynaptic depolarisation is accounted for mainly by slow channel opening

Question 10

What is needed to evoke full transmission ?

Answer 10

A [Ca2+] rise of ~10-30μm

Question 11

What is another finding Katz made ?

Answer 11

Katz made another major finding: release is QUANTAL

Question 12

In frog NMJ, Fatt and Katz showed that ?

Answer 12

Even without stimulation, small postsynaptic responses could be seen - called miniature endplate potentials (mEPPs)

Question 13

These spontaneous responses were always ?

Answer 13

Singles or multiples of a ‘unit’ response

Question 14

An increase in external Ca2+ concentration does not enhance ?

Answer 14

The size of a quantum of transmitter but increases the average number of quanta released in response to a presynaptic action potential

Question 15

The greater the Ca2+ influx into the terminal, the larger ?

Answer 15

The number of transmitter quanta get released

Question 16

The quantal nature of release and the observation that small ‘packets’ or vesicles were present at presynaptic terminals, were brought together to suggest?

Answer 16

That vesicles were the organelles of transmitter storage and release

Question 17

Explain the ‘vesicular hypothesis of synaptic transmission’ ?

Answer 17

Neurotransmitter is packaged in synaptic vesicles and released in ‘quanta’ of roughly equal size

Question 18

We think of synapses as being faithful communicators. Explain this statement ?

Answer 18

• At some synapses - for example the NMJ or the calyx of Held - this is generally true but mainly because of deviations from the general design: for example, by having multiple release sites for one connection
• However, many central synapses only have a single release site and a release probability much less than 1 - in some cases as low as 0.1 (10%)
• Such a feature is not a design fault but instead allows substantial flexibility

Question 19

Synaptic vesicles are clustered around a specialised area of the presynaptic membrane called ?

Answer 19

• The active zone which contains the protein ‘machinery’ necessary for vesicle fusion
• They are organised into different pools

Question 20

The influx of Ca2+ is thought to drive two main processes related to release:

Answer 20

1. Activation of vesicle fusion with the active zone membrane
2. Mobilisation of reserve vesicles towards the active zone

Question 21

Fusion is orchestrated by?

Answer 21

A series of special synaptic proteins lying at or near the active site

Question 22

The key elements are a group of proteins called?

Answer 22

Soluble N-Ethylmaleimide-Sensitive Factor Attachment Protein Receptors or SNAREs

Question 23

Where are SNAREs located ?

Answer 23

One type located on the vesicle membrane (synaptobrevin) and two types on the cell membrane (SNAP-25 and syntaxin)

Question 24

What do the SNAREs act as ?

Answer 24

A molecular machine, forming a super-stable complex that pulls the vesicle membrane and plasma membrane together

Question 25

This fusion process is driven by a calcium sensing vesicle protein called ?

Answer 25

Synaptotagmin

Question 26

Ca2+-bound synaptotagmin catalyses ?

Answer 26

Membrane fusion, binding to SNAREs and the plasma membrane

Question 27

What is the Synapsin used for?

Answer 27

Reserve vesicles are not freely mobile, being fused to cytoskeletal elements via a specialised protein type

Question 28

Without a process to compensate for vesicle fusion, the plasma membrane would swell as new vesicle membrane is added and the pool of synaptic vesicles would decline. What prevents this ?

Answer 28

1. Once vesicles have fused they are reclaimed for reuse- they are ‘recycled’
2. This appears to be necessary to allow fast sustained transmission (up to 200 Hz)

Question 29

Clathrin concentrates ?

Answer 29

Clathrin concentrates membrane elements into small packages

Question 30

What are the two vesicle cycles ?

Answer 30

1. Full-Collapse Fusion
   - Docked, fusion, collapse and retrieval
2. Kiss-and-Run
   - Docked, Kiss-and-Run

Question 31

What imaging method is used to visualise individual synapses and monitor the process of vesicle recycling ?

Answer 31

FM-dyes
e.g. FM4-64labelling of vesicles (only living cells)

Steps are:
Apply, Stimulate, Wash, Stimulate and Destain

Question 32

What is another imaging approach used for studying vesicle recycling ?

Answer 32

pHluorins
- Take advantage of the pH difference between the lumen and the extracellular solution

Steps are:
Pre, Stimulation, Endocytosis and Re-acidification