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Presentation on theme: "Announcements Slides used at tutorial posted to webpage."— Presentation transcript:

1 Announcements Slides used at tutorial posted to webpage

2 Last lecture: –Finished Action potentials Today –Start synaptic transmission

3 Within neurons Between neurons electrical chemical & electrical Neural Signalling

4 Definitions 1.Synapse : the functional contacts between neurons and other cells 2.Synaptic Transmission: the process used by neurons to relay information from one cell to the next

5 Definitions Presynaptic cell Postsynaptic cell Synapse

6 Types of Synapses 1.Electrical  Direct flow of electrical current from one cell to the next 2.Chemical  Secrete neurotransmitter molecules that activate receptors

7 Electrical synapses Physical link between two neurons called a gap junction Protein is called connexon

8 Electrical Synapses Action potential in one cell can cause ions to flow through the gap junction into the next cell

9 Inject current Record Reduced depolarization

10 Electrical Synapses Electrical synapses : 1.Can be unidirectional or bidirectional 2.Are very fast 3.Uses A.Electrical synapses first discovered in crayfish neurons involved in escape response B.Synchronizing neural activity, e.g. hormone secreting neurons in the mammalian hypothalamus

11 Presynaptic nerve terminal Postsynaptic nerve terminal Synaptic vesicles Neurotransmitter receptors Synaptic cleft Chemical synapse

12 Chemical Synapses Features: 1.Use chemical neurotransmitters 2.There is a space between the pre- and postsynaptic neuron, called the synaptic cleft 3.Neurotransmitter is stored in synaptic vesicles

13 Chemical Synapses Features: 4.Can be excitatory or inhibitory Depends on neurotransmitter and receptor 5.Can activate ion-channels or second messenger pathways 6.The amount of transmitter released is variable and can be modulated

14 Real synapse Drosophila neuromuscular junction Synaptic contacts Active Zone Synaptic vesicles

15 Ca++ Na+ Depolarization

16 Sequence of Events at a Chemical Synapse 1.Action potential depolarizes nerve terminal 2.Voltage-gated Ca++ channels open & Ca++ flows into the nerve terminal 3.Ca++ causes synaptic vesicles to fuse with the plasma membrane 4.Neurotransmitters are released into synaptic cleft 5.Neurotransmitter binds to receptors 6.Opens ion channels and positive current flows into postsynaptic cell 7.Current flow gives postsynaptic potential 8.If postsynaptic potential = threshold  Action Potential

17 Ca++ Na+ Depolarization

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

19 Chemical Synaptic Transmission some key questions 1.What is the nature if vesicular release? 2.What is the role of Ca++? 3.What happens to the neurotransmitter after it is secreted? 4.What are the electrical effects on the postsynaptic cell?

20 The Synaptic Vesicle Cycle Docking Priming Fusion Endosome Budding ExocytosisEndocytosis

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

22 Evidence for Vesicle Cycle 1.Stimulate synaptic transmission in the presence of extracellular marker (HRP) 2.Wash marker away 3.Examine the intracellular distribution of the marker by electron microscope 4.Marker first seen in coated vesicles 5.After 5-20 minutes, found in endosome 6.After 1 hour found in synaptic vesicles

23 Summary & Concepts 1.Two types of synaptic communication 1.Electrical through gap junctions 2.Chemical  neurotransmitters 2.Depolarization, Ca++ influx, and vesicle fusion release neurotransmitter 3.Neurotransmitter opens ion channels changing the electrical potential of the postsynaptic cell 4.Synaptic vesicles go through a cycle of exocytosis and endocytosis

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