• Announcements
– Midterm room assignments Thursday
– Midterm conflict policy posted later tonight
– Some practice questions from previous
midterms will be posted
– TA office hours Mon 10-12, Wed 12-1
• S161 or S526
• Last lecture
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Vesicle cycle
Spontaneous and evoked release
Discrete units – quanta
Evidence for vesicles = quanta
• Today
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Role of depolarization
What does Calcium do in the nerve terminal?
Some molecules involved in Vesicle fusion
Excitation and Inhibition
• What is the role of nerve terminal
depolarization?
Squid Giant Synapse
with tetrodotoxin  blocks Na+ channelsno AP
Presynaptic neuron
Depolarizing current
Presynaptic response
Postsynaptic response
Postsynaptic neuron
These are
postsynaptic
potentials
Increasing Postsynaptic response with increasing
Presynaptic depolarization
30
22 mM Ca++
20
11 mM Ca++
2 mM Ca++
10
Postsynaptic response (mV)
Effectiveness of Presynaptic
Depolarization depends on Ca++
50
100
200
Presynaptic Response (mV)
Presynaptic neuron
Filled with aequorin
Role of Ca++
Depolarizing current
Presynaptic response
Photo-tube
Postsynaptic response
Postsynaptic neuron
Calcium signal corresponds to postsynaptic
response
Relationship between Ca++ and
synaptic transmission
Ca++ concentration
low
med
Synaptic Potential
amplitude (mV)
zero
high
Ca++ concentration (mM)
Summary
1. Neurotransmitter release can cause
synaptic potentials
2. Spontaneous events correspond to
smallest nerve stimulated events
3. The quantal unit
4. AP depolarization necessary to admit
Ca++ to presynaptic nerve terminal
What does Ca++ do
to cause fusion?
• Triggers the final fusion of the vesicle
membrane with the presynaptic membrane
– Likely that Ca++ binds a calcium sensing
protein called synaptotagmin
• Protein-protein interactions are required
– SNARE proteins likely the minimal proteins
required for fusion
Use Fruit Fly mutations to reduce the amount of SNARE proteins
Measure synaptic transmission at different [Ca]
Wild-type
Syntaxin mutant
Synaptobrevin mutant
• Botulinum toxin (BoTox)
• Tetanus toxin
– Enzymes that cleave SNARE proteins
 Blocks neurotransmitter release
Excitation & Inhibition
• Excitatory synapse makes it more likely a
cell will fire AP
• Inhibitory synapse makes it less likely a
cell will fire AP
• ‘Likely’ depends on:
1. Neurotransmitter receptor
2. The permeable ions
A. Excitor
B. Inhibitor
Record voltage
Simple case:
Threshold
A
B
Vm
Depolarizing  excitatory
EPSP
Threshold
A+B=smaller
Vm
hyperpolarizing  inhibitory
IPSP
How to get hyperpolarizing
potential?
• Neurotransmitter receptor is permeable to
an ion whose Eion is more negative than
resting membrane potential
• usually Cl- or K+
Hyperpolarizing Synaptic Potential
+60 mV
0 mV
+
+
K+
-80 mV
More complex case:
Threshold
A
B
Vm
Why???
Depolarizing  excitatory
Threshold
A+B=smaller
Vm
Depolarizing  inhibitory
Reversal Potential
• Membrane potential at which there is no
net synaptic current
Measuring Reversal Potential
eg. Frog NMJ
Current source
+25
stimulus
Control resting
membrane potential
0
Reversal potential
-50
-100
Stimulate nerve
Record membrane potential
• Many neurotransmitter receptors are
permeable to more than one ion
– Non-selective
• The reversal potential depends on the
equilibrium potential and permeability of
each ion
– It will usually be between the equilibrium
potential of the permeable ions
eg. Acetylcholine channel
• Permeable to both K+ and Na+
• For Frog muscle:
• EK = -90 mV
• ENa = +60 mV
Neurotransmitter
receptor
ENa = +60 mV
K+
VmENa
+25
0
Reversal potential
Vm=Erev
-50
Erev>Vm>EK
-90
EK = -90 mV
VmEK
Na+
How can depolarizing potential be
inhibitory?
• Excitatory synapses have a reversal
potential more positive than threshold
• Inhibitory synapses have a reversal
potential more negative than threshold
How can depolarizing potential be
inhibitory?
Erev
Threshold
Erev
A
B
Vm
Example: Cl- permeable receptor
in a cell whose Vthresh >ECl- > Vm
Inhibition
• Channels of inhibitory synapses ‘shortcircuit’ excitatory synapses
• Because neurotransmitter channels will
drive the membrane potential toward their
reversal potential
Summary
• Evidence that depolarization is important
• Evidence that depolarization regulates
presynaptic intracellular [Ca]
• SNARE proteins and Synaptotagmin
• Excitation and Inhibition
– Importance of synaptic reversal potential