Synaptic Transmission Flashcards
What is the function of the synapse?
Transmit information over short distances
What are the features of an electrical synapse?
Capacitive current flows directly from presynaptic axon to postsynaptic cell via gap junctions
Very small synaptic cleft
Postsynaptic response can only be excitatory (depolarizing)
What are the features of a chemical synapse?
Capacitive current ends at axon terminal
Axon terminal releases neurotransmitters that can produce either an excitatory or inhibitory response
Effect not determined by size
What are the two key organelles located at the axon terminal?
Mitochondria
Synaptic vesicles – storage for neurotransmitters
How are synaptic vesicles loaded with neurotransmitters?
ATP transporter pumps protons into the vesicle
Proton transporter exchanges protons for neurotransmitter molecules into the vesicle
What is the active zone?
Protein dense area that is the site of transmitter release
Describe the extracellular matrix at a synapse.
Structural alignment of pre and postsynaptic elements
Presence of AChE enzyme for ACh breakdown (only at cholinergic synapses)
What types of proteins are located at the postsynaptic density?
Receptors and cytoskeletal proteins
What does an action potential reaching the axon terminal cause?
Open of voltage-gated Ca channels in the active zone
Describe the driving force for calcium at the synaptic terminal.
Will be inward at any potential between Ek and Ena
What is synaptobrevin?
v-SNARE protein that is associated with the vesicle membrane
What are SNAP-25 and syntaxin?
t-SNARE proteins that are associated with the membrane at the active zone
What is docking?
The connection of the v- and t- SNARE proteins that holds the vesicle at the active zone
What is priming?
The zipping of the v- and t-SNAREs pulls the vesicle extermely close to the plasma membrane so that it is ready to fuse
What is synaptotagmin?
Calcium sensor protein that triggers the fusion process
Located on the vesicle
What happens when synaptotagmin binds calcium?
Increases lipid solubility and moves into the lipid bilayer
This drags the vesicle even closer to the plasma membrane, which causes spontaneous fusion
What are the two mechanisms of exocytosis?
Kiss and Run - membranes do not completely fuse
Complete fusion - fusion pore dilates and the membranes completely fuse
What is mobilization?
Allows vesicles to be released from the reserved pool and be docked and primed
What is synapsin?
Tethers reserved vesicles to the actin cytoskeleton
How are reserved vesicles mobilized?
Influx of calcium causes the phosphorylation of synapsin, which causes it to dissociate from the actin and release the vesicle
What are three ways calcium is regulated in the axon terminal?
Plasma membrane transporters - pump it out
Sequestration into organelles - primarily mitochondria
Calcium binding proteins in cytoplasm
How are calcium influx and phosphorylation of synapsin related?
Phosphorylation is proportional to the amount of free calcium in the cytoplasm
How is free calcium and action potential frequency related?
Free calcium is proportional to action potential frequency
More action potentials –> more calcium –> more synapsin phosph. –> more vesicle mobilization
What is a Quantal?
One quantum = amount of transmitter in one vesicle
