Synaptic Transmission II Flashcards
Physiological evidence intracellular recording techniques uncovered ?
The basics of presynaptic operation
Morphological evidence revealed ?
The structure of a synapse and offered clues to a possible mechanism of transmission
Who uncovered the fundamental properties of synaptic transmission ?
Bernard Katz
What did Katz use ?
Katz used the squid giant axon - a preparation where pre and postsynaptic structures can be directly accessed and recorded at the same time
What did blockers of voltage-gated Na+ (TTX) and K+ channels (TEA) reveal ?
It revealed that neither Na+ or K+ channels are required for effective release
What did Katz and co establish ?
They established that depolarisation (artificial or by an action potential) opens special voltage-gated Ca2+ channels at the presynaptic terminal
What drives the transmission process ?
It is the influx of Ca2+ down its concentration gradient and triggered by depolarisation that drives the transmission process
What happens when Ca2+ channels are not open ?
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)
Work from Neher and Sakmann working in the giant auditory synapse, the calyx of Held, found that ?
The short delay between pre and postsynaptic depolarisation is accounted for mainly by slow channel opening
What is needed to evoke full transmission ?
A [Ca2+] rise of ~10-30μm
What is another finding Katz made ?
Katz made another major finding: release is QUANTAL
In frog NMJ, Fatt and Katz showed that ?
Even without stimulation, small postsynaptic responses could be seen - called miniature endplate potentials (mEPPs)
These spontaneous responses were always ?
Singles or multiples of a ‘unit’ response
An increase in external Ca2+ concentration does not enhance ?
The size of a quantum of transmitter but increases the average number of quanta released in response to a presynaptic action potential
The greater the Ca2+ influx into the terminal, the larger ?
The number of transmitter quanta get released
The quantal nature of release and the observation that small ‘packets’ or vesicles were present at presynaptic terminals, were brought together to suggest?
That vesicles were the organelles of transmitter storage and release
Explain the ‘vesicular hypothesis of synaptic transmission’ ?
Neurotransmitter is packaged in synaptic vesicles and released in ‘quanta’ of roughly equal size
We think of synapses as being faithful communicators. Explain this statement ?
- 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
Synaptic vesicles are clustered around a specialised area of the presynaptic membrane called ?
- The active zone which contains the protein ‘machinery’ necessary for vesicle fusion
- They are organised into different pools
The influx of Ca2+ is thought to drive two main processes related to release:
- Activation of vesicle fusion with the active zone membrane
- Mobilisation of reserve vesicles towards the active zone
Fusion is orchestrated by?
A series of special synaptic proteins lying at or near the active site
The key elements are a group of proteins called?
Soluble N-Ethylmaleimide-Sensitive Factor Attachment Protein Receptors or SNAREs
Where are SNAREs located ?
One type located on the vesicle membrane (synaptobrevin) and two types on the cell membrane (SNAP-25 and syntaxin)
What do the SNAREs act as ?
A molecular machine, forming a super-stable complex that pulls the vesicle membrane and plasma membrane together
This fusion process is driven by a calcium sensing vesicle protein called ?
Synaptotagmin
Ca2+-bound synaptotagmin catalyses ?
Membrane fusion, binding to SNAREs and the plasma membrane
What is the Synapsin used for?
Reserve vesicles are not freely mobile, being fused to cytoskeletal elements via a specialised protein type
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 ?
- Once vesicles have fused they are reclaimed for reuse- they are ‘recycled’
- This appears to be necessary to allow fast sustained transmission (up to 200 Hz)
Clathrin concentrates ?
Clathrin concentrates membrane elements into small packages
What are the two vesicle cycles ?
- Full-Collapse Fusion
- Docked, fusion, collapse and retrieval - Kiss-and-Run
- Docked, Kiss-and-Run
What imaging method is used to visualise individual synapses and monitor the process of vesicle recycling ?
FM-dyes
e.g. FM4-64labelling of vesicles (only living cells)
Steps are:
Apply, Stimulate, Wash, Stimulate and Destain
What is another imaging approach used for studying vesicle recycling ?
pHluorins
- Take advantage of the pH difference between the lumen and the extracellular solution
Steps are:
Pre, Stimulation, Endocytosis and Re-acidification
