Announcements
• Mid term room assignments posted to
webpage
Lecture 01
A – Ho
S361 (Pavilion)
Hoang – Lischka
S309
Lishingham - Ngui
S143
Nguyen – Seguin
S128
Sek – Zia
H305 (correction)
Lecture 02
S319
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•
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Office hours: today 2-4pm, Wed 11-12
TA Office hours: Wed 12-1
Tutorial Thurs – extra office hours only
Answers to practice questions will be posted
later today
• Bring a calculator to the midterm test
– Scientific OK ; Not programmable
• Midterm schedule conflicts
– Wednesday 5pm deadline – bring your ROSI
timetable
• Material on midterm – end of today’s lecture
• Last Lecture
– Synaptic integration
• Spatial & Temporal Summation
• Today
– Synaptic Plasticity
Synaptic Plasticity
Summation
Synaptic inputs
Soma and
dendrites
Axon Hillock
Passive current
flow
Above threshold?
Yes
Action Potential
No
Passive Current
Decays to zero
Synaptic Plasticity
• Changes in the strength of synaptic
transmission associated with activity or
experience
• Basis for behaviour such as learning and
memory
Types of synaptic plasticity
1. Heterosynaptic Modulation
2. Long-term Potentiation
3. Homosynaptic Modulation
1. Facilitation
2. Post-tetanic Potentiation
Heterosynaptic facilitation
• Aplysia californica (sea slug)
Aplysia (sea slug) gill withdrawal reflex
• Tap the tail and the gill is withdrawn
weakly
• Tap the head and then the tail the gill is
withdrawn strongly
Heterosynaptic facilitation
• Aplysia (sea slug) gill withdrawal reflex
Sensory neuron
Head
tail
Gill Muscle
serotonin
interneuron
Sensory neuron
Motor neuron
Heterosynaptic facilitation
1.
2.
3.
4.
5.
6.
Serotonin released from interneuron
 cAMP in sensory nerve terminal
Closes potassium channels
Delays repolarization of action potentials
Allows more Ca++ to enter the nerve terminal
More transmitter release
Tail Sensory Neuron
Action potential
After Serotonin
Broader AP allows more Ca++ in
Motor Neuron
Synaptic potential
Long-term Potentiation
• Mammalian hippocampus, and other brain
regions
• Hippocampus is a structure that is
important for learning, especially spatial
learning
Long-term Potentiation
EPSP Amplitude
• Long-lasting increase (hours to days) in
synaptic strength following short high
frequency stimulation
200%
100%
100 Hz
Time
2 hours
Long-term potentiation
• Depends on two types of glutamate receptors
1. AMPA
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permeable to Na+, K+
Function under all conditions
2. NMDA
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•
Permeable to Na+, K+, and Ca++
BUT normally blocked by Mg++
Only operate under high frequency stimulation
Normal Stimulation
Na+
Mg++
NMDA
AMPA
Strong Stimulation
Ca++
Mg++
Na+
Ca++
Ca++
NMDA
Depolarization
AMPA
Ca++ activates second messenger
NMDA receptor
• Key Points
1. Normally blocked by Mg++
2. Strong depolarization removes Mg++ and
allows Ca++ to enter postsynaptically
3. Ca++ activates second messenger
pathways that strengthen synaptic
transmission
•
Probably activates more AMPA receptors
Homosynaptic Modulation
Facilitation
• Use-dependent increase in synaptic
transmission
• eg. two stimuli are applied to a motor
nerve in rapid succession, the second
response is bigger than the first
• Not the same as temporal summation
Facilitation
Synaptic potentials
2
1
Stimuli close together
Amplitude of 2 is greater than amplitude of 1
Stimuli farther apart – amplitudes are the same
Why get facilitation?
• If two stimuli are close together there is an
accumulation of Ca++ inside the
presynaptic nerve terminal
– Called residual calcium
• If the two stimuli are far apart the Ca++
from first stimulus dissipates before
second stimulus
• Lasts for seconds
Na+
Ca++
Depolarization
Ca++
Ca++
Na+
Ca++
Depolarization
Post-tetanic potentiation
• Tetanic stimulation  high frequency (50 –
100 times per second)
• Lasts for minutes
Normal Ca++ saline
EPSP Amplitude
Test stimuli
1 / 30 sec
Potentiation
depression
0
50 Hz
for 1 min
10 minutes
Time
Post-tetanic potentiation
Initial test stimuli
establish baseline
During tetanus
High release
 depletion of
vesicles
Post-tetanus
Replenishment
of Vesicles by
recycling
Accumulation of
internal Ca++
Greater release due
to high internal Ca++
Summary
• Synaptic plasticity is a use-dependent
change in synaptic strength
• Heterosynaptic plasticity – one synapse
modulates another
– Aplysia gill withdrawal reflex
• Homosynaptic plasticity includes:
facilitation and potentiation
– Both depend on calcium