synaptic t ransmission and its modulation synaptic transmission 1 ... slides/synaptictrans... ·...
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Synaptic transmission
1) Communication within the nervous system
2) A site of plasticity for learning and memory
3) A site of action for disease,
psychoactive drugs
Synaptic Transmission and its Modulation!
Whimpy axon!
Synaptic input
“Minis”
Quantal nature of transmitter release
Record spontaneous transmitter release
Spontaneous “mini” synaptic potentials
Quantal hypothesis of
transmitter release
Post synaptic depolarization
results from the coordinated release of transmitter packets
(quanta) from the vesicles as the AP enters the terminal
Reduce [Ca]o
Record evoked transmitter release
Normally around 300 quanta at NMJ/AP
(could be only a few at central synapses)
Spontaneous “mini” and evoked potentials- quantal steps
Quantal content: m = mean EPP/mean MEPP
Physical evidence for vesicle fusion for transmitter release
4-AP broadens presynaptic AP
Fusion of vesicles
to membrane
Ca++ channels
Steps in transmitter
release
What is the direct cause of
transmitter release?
Squid giant synapse as a
model system
EPSP
giant axon
~1 mm
Presynaptic axon
George Augustine, Marine Biological Laboratory, 2008
Importance of calcium for
transmitter release
Block transmitter release with presynaptic Ca buffer
Presynaptic Ca injection causes
transmitter release and Ca indicator dye shows presynaptic Ca entry
Any depolarization will cause transmitter release, even high Kout-
Triggers opening of voltage-gated Ca channels
What is the role of the AP in
transmitter release?
provides trigger and timing
for calcium entry
Steps in transmitter
release
What is the cause of
synaptic delay?
EPSP
Synaptic delay due to Ca entry at end of AP
ICa
EPSC
200 microS!
Plasticity of synaptic transmission
Intrinsic Synaptic Plasticity (short term)
Change in synaptic strength due to activity of synapses Important for:
coincidence detection gain control
oscillatory networks (phase onset and offset, cycle period)
Habituation, sound localization Learning and Memory
Extrinsic Synaptic Plasticity
Change in synaptic strength due to neuromodulatory substances
Important for: Network reconfiguration
Plasticity of synaptic transmission
Experimental protocol
Intrinsic Synaptic Plasticity- transforms time interval into voltage amplitude
Facilitation, Post-tetanic potentiation, Depression
Intrinsic synaptic modulation:
Facilitation
Facilitation:!! a) MEPP sizes do not change.!! b) MEPP frequency increases.!! Quanta content (m = PSP/mini) is increased!Thus, it is a presynaptic effect.!
Intrinsic synaptic modulation:
Post-tetanic potentiation
Intrinsic synaptic modulation:
Depression
Intrinsic synaptic modulation in vertebrate brains: Depends on the synapse
Dietmann et al 2000
Depression
Facilitation
Mixed