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Announcements

Announcements. Mid term room assignments posted to webpage. Lecture 01. 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

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Announcements

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  1. Announcements • Mid term room assignments posted to webpage Lecture 01

  2. 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

  3. Last Lecture • Synaptic integration • Spatial & Temporal Summation • Today • Synaptic Plasticity

  4. Synaptic Plasticity Summation Soma and dendrites Axon Hillock Synaptic inputs Passive current flow Above threshold? No Yes Passive Current Decays to zero Action Potential

  5. Synaptic Plasticity • Changes in the strength of synaptic transmission associated with activity or experience • Basis for behaviour such as learning and memory

  6. Types of synaptic plasticity • Heterosynaptic Modulation • Long-term Potentiation • Homosynaptic Modulation • Facilitation • Post-tetanic Potentiation

  7. Heterosynaptic facilitation • Aplysia californica (sea slug)

  8. Tap the tail and the gill is withdrawn weakly • Tap the head and then the tail the gill is withdrawn strongly Aplysia (sea slug) gill withdrawal reflex

  9. Heterosynaptic facilitation • Aplysia (sea slug) gill withdrawal reflex Head Sensory neuron tail Gill Muscle serotonin interneuron Sensory neuron Motor neuron

  10. Heterosynaptic facilitation • 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

  11. Tail Sensory Neuron Action potential After Serotonin Broader AP allows more Ca++ in Motor Neuron Synaptic potential

  12. Long-term Potentiation • Mammalian hippocampus, and other brain regions • Hippocampus is a structure that is important for learning, especially spatial learning

  13. Long-term Potentiation • Long-lasting increase (hours to days) in synaptic strength following short high frequency stimulation 200% EPSP Amplitude 100% 100 Hz 2 hours Time

  14. Long-term potentiation • Depends on two types of glutamate receptors • AMPA • permeable to Na+, K+ • Function under all conditions • NMDA • Permeable to Na+, K+, and Ca++ • BUT normally blocked by Mg++ • Only operate under high frequency stimulation

  15. Normal Stimulation Na+ Mg++ NMDA AMPA

  16. Strong Stimulation Ca++ Ca++ Mg++ Na+ Ca++ NMDA AMPA Depolarization Ca++ activates second messenger

  17. NMDA receptor • Key Points • Normally blocked by Mg++ • Strong depolarization removes Mg++ and allows Ca++ to enter postsynaptically • Ca++ activates second messenger pathways that strengthen synaptic transmission • Probably activates more AMPA receptors

  18. Homosynaptic ModulationFacilitation • 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

  19. Facilitation Synaptic potentials 2 1 Stimuli close together Amplitude of 2 is greater than amplitude of 1 Stimuli farther apart – amplitudes are the same

  20. 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

  21. Na+ Ca++ Depolarization

  22. Ca++ Ca++ Na+ Ca++ Depolarization

  23. Post-tetanic potentiation • Tetanic stimulation  high frequency (50 – 100 times per second) • Lasts for minutes

  24. Normal Ca++ saline Test stimuli 1 / 30 sec Potentiation EPSP Amplitude depression 0 10 minutes 50 Hz for 1 min Time

  25. Post-tetanic potentiation Initial test stimuli establish baseline Accumulation of internal Ca++ During tetanus High release  depletion of vesicles Greater release due to high internal Ca++ Replenishment of Vesicles by recycling Post-tetanus

  26. 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

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