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Announcements
• Slides used at tutorial posted to webpage
• Last lecture:– Finished Action potentials
• Today– Start synaptic transmission
Within neurons
Between neurons
electrical chemical & electrical
Neural Signalling
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
Definitions
Presynaptic cell Postsynaptic cell
Synapse
Types of Synapses
1. Electrical Direct flow of electrical current from one cell
to the next
2. Chemical Secrete neurotransmitter molecules that
activate receptors
Electrical synapses
• Physical link between two neurons called a gap junction
Protein is called connexon
Electrical Synapses
Action potential in one cell can cause ions to flow through the gap junction into the next cell
Inject current
Record
Reduced depolarization
Electrical Synapses
Electrical synapses :
1. Can be unidirectional or
bidirectional
2. Are very fast
3. UsesA. Electrical synapses first discovered in crayfish
neurons involved in escape response
B. Synchronizing neural activity, e.g. hormone secreting neurons in the mammalian hypothalamus
Presynaptic nerveterminal
Postsynaptic nerveterminal
Synaptic vesicles
Neurotransmitterreceptors
Synaptic cleft
Chemical synapse
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
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
Real synapseDrosophila neuromuscular junction
Synaptic contacts
Active Zone
Synaptic vesicles
Ca++Na+
Depolarization
Sequence of Events at a Chemical Synapse
1. Action potential depolarizes nerve terminal2. Voltage-gated Ca++ channels open & Ca++ flows into
the nerve terminal3. Ca++ causes synaptic vesicles to fuse with the
plasma membrane4. Neurotransmitters are released into synaptic cleft5. Neurotransmitter binds to receptors6. Opens ion channels and positive current flows into
postsynaptic cell7. Current flow gives postsynaptic potential8. If postsynaptic potential = threshold Action Potential
Ca++Na+
Depolarization
Measuring Synaptic Transmission• Frog Neuromuscular Junction
mV
Time (ms)
Postsynaptic potential
Stimulus
Stimulate nerveRecord from muscle
Muscle cell
Chemical Synaptic Transmissionsome 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?
The Synaptic Vesicle Cycle
Docking
Priming Fusion
Endosome
Budding
Exocytosis Endocytosis
Evidence for Vesicle Cycle
Extracellular Marker(HRP)
Marker found in coated vesicles
Marker found in endosome
Marker found in vesicles
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
Summary & Concepts
1. Two types of synaptic communication1. 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