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Announcements

Announcements. Today Vesicle cycle Two types of synaptic potentials vesicles and quantal transmitter release depolarization and calcium. Na+. Ca++. Depolarization. Summary & Concepts. Two types of synaptic communication Electrical through gap junctions Chemical  neurotransmitters

phelan-cortez
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Announcements

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  1. Announcements

  2. Today • Vesicle cycle • Two types of synaptic potentials • vesicles and quantal transmitter release • depolarization and calcium

  3. Na+ Ca++ Depolarization

  4. Summary & Concepts • Two types of synaptic communication • Electrical through gap junctions • Chemical  neurotransmitters • Depolarization, Ca++ influx, and vesicle fusion release neurotransmitter • Neurotransmitter opens ion channels changing the electrical potential of the postsynaptic cell

  5. The Synaptic Vesicle Cycle Endosome Budding Exocytosis Endocytosis Budding Docking Fusion Priming

  6. Evidence for Vesicle Cycle Marker found in endosome Marker found in coated vesicles Marker found in vesicles Extracellular Marker (HRP)

  7. Evidence for Vesicle Cycle • Stimulate synaptic transmission in the presence of extracellular marker (HRP) • Wash marker away • Examine the intracellular distribution of the marker by electron microscope • Marker first seen in coated vesicles • After 5-20 minutes, found in endosome • After 1 hour found in synaptic vesicles

  8. Synaptic Potentials • Two types • Spontaneous – occur without nerve Action Potentials • Evoked – require action potentials

  9. Frog Neuromuscular Junction Measuring Synaptic Transmission Record from muscle Stimulate nerve Postsynaptic potential mV Muscle cell Time (ms) Stimulus

  10. Spontaneous No nerve stimulation Muscle cell

  11. 3 2 Number of Occurrences 1 0 0.5 1.0 1.5 Amplitude Spontaneous 1.3 mV 1.1 mV 0.7 mV 0.8 mV 1 mV

  12. Nerve Evoked Low Ca++ saline 1 mV Stimulate nerve 1 2 3 4 5 6 7

  13. 3 2 Number of Occurrences 1 0 0.5 1.0 1.5 2.0 2.5 3.0 Amplitude 2.0 mV 3.0 mV 1 mV 1.0 mV 1.9 mV 1.0 mV Failure 1.0 mV

  14. Failure  to secrete neurotransmitter i.e. no vesicle fused because not enough Ca++ went into nerve terminal

  15. 3 Spontaneous 2 Number of Occurrences 1 0 0.5 1.0 1.5 Evoked 3 2 Number of Occurrences 1 0 0.5 1.0 1.5 2.0 2.5 3.0 Amplitude

  16. Synaptic Potentials • There are discrete, basic “units” of synaptic release • Discrete units are called ‘quanta’ • An Action Potential causes simultaneous release of a large number of “quanta” • At frog NMJ, each quanta = 10,000 molecules of Acetylcholine

  17. Relationship among synaptic potentials +60 mV 0 mV threshold -80 mV • Spontaneous • mepp • One vesicle • Stimulated • epp • Subthreshold • several vesicles • Stimulated • epp • suprathresholdAP • Many vesicles

  18. SEM of frog NMJ Before During • Stimulate high rate of transmitter release • Use quick-freeze fixation to capture release events for electron microscopy • Simultaneous electro-physiological recordings • Heuser et al. 1979 Cross-section

  19. Maybe Quantal units are Synaptic Vesicles? • Vesicles contain neurotransmitter Measured by electrophysiology Exact correspondence Number of quanta recorded 1000 2000 3000 1000 2000 3000 Measured by Electron microscopy Number of vesicles released

  20. Neurotransmitter is Released from Synaptic Vesicles1 MEPP = 1 quanta = 1 vesicle

  21. What is the role of nerve terminal depolarization?

  22. Squid Giant Synapse with tetrodotoxin blocks Na+ channelsno AP Presynaptic neuron Depolarizing current Presynaptic response Postsynaptic response Postsynaptic neuron Increasing Postsynaptic response with increasing Presynaptic depolarization

  23. Effectiveness of Presynaptic Depolarization depends on Ca++ 22 mM Ca++ Postsynaptic response (mV) 11 mM Ca++ 10 20 30 2 mM Ca++ 50 100 200 Presynaptic Response (mV)

  24. Role of Ca++ Presynaptic neuron Filled with aequorin Depolarizing current Presynaptic response Photo-tube Postsynaptic response Postsynaptic neuron Calcium signal corresponds to postsynaptic response

  25. Relationship between Ca++ and synaptic transmission Ca++ concentration zero low Synaptic Potential amplitude (mV) med high Ca++ concentration (mM)

  26. Summary • Neurotransmitter release can cause synaptic potentials • Spontaneous events correspond to smallest nerve stimulated events • The quantal unit • AP depolarization necessary to admit Ca++ to presynaptic nerve terminal

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