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Anatomy and Cell Biology
Anatomy and Cell Biology 2221
Suzanne Schmid

Synaptic Transmission Synapse joint between two neuronsAxon terminal pre synapticSynaptic TransmissionAction potential is traveling down axon and enters pre synaptic terminalOpens voltage gated calcium channels open because of depolarizationCalcium moves into the cell Vesicles containing neurotransmitters will fuse to pre synaptic membrane and will release neurotransmitters into synaptic cleftEventually will bind to neurotransmitter receptors on post synaptic cell membrane Receptors are ligand gated channels and will only open when specific neurotransmitters bind to themOnce opened there is an influx of Na which then depolarizes the post synaptic cell Passive spread through neuron occursCalcium influxCalcium is a very important messenger that triggers many processes ie apoptosis membrane plasticityCalcium is very tightly regulated within the intracellular spaceCalcium is never elevated for a long time within a specific cell compartmentit is immediately removed from the cytosolCalcium voltage gated channels are located right beside the neurotransmitter vesicles distance of 100 nmCalcium concentration is at its highest when near a voltage gated channel but as you move away from the channel 100 nm the concentration drops Residual calcium elevated concentration in the pre synaptic terminal occurs after voltage gated channels close and neurotransmitters stop being releasedVaricosities Not all synapses are at axon terminals Synapses where the pre synaptic terminal is NOT an AXON terminal but a swelling of the axon Swelling at the axon ie varicosity contains neurotransmittersmitochondrion Two ways of releasing neurotransmitters in varicosities1 Varicosity lies very close to the post synaptic membrane tight connection between pre synaptic varicosity and post synaptic receptors 2 Release of neurotransmitter over larger distances increase concentration of neurotransmitter in extra cellular space that will eventually bind extra synaptic receptorsVesicle recycling Once the vesicles are fused to the membrane and release neurotransmitters they are then recycled Mostly recycled as clatherin coated endocytosisThey are then sorted and re loaded with neurotransmitters and put into storage At one point the vesicles will be recruited from storage and dockedanchored onto the pre synaptic membrane The vesicle is then primed and will begin fusing to the membrane upon elevation of calcium levelsStored vesicles are anchored to the cell with microtubulesactinVesicles do not always completely fused Many synaptic vesicles are undergoing process of kiss and runDuring this process the vesicle kisses the pre synaptic membrane and opens up to release a large quantity but not all neurotransmitters It is then retrieved back and goes directly into the direct recycling cycle Advantage vesicles can be recycled faster than through clatherin endocytosisReadily releasable neurotransmitter vesicle pool only vesicles that are primed and ready to release neurotransmitters at the rise of calcium levels For most synapses there are 210 readily releasable vesicles Clatherin mediated endocytosis1 Stored in endosomewhen needed they bud out from endosome and then are refilled with neurotransmitters2 Directly recycled Calyx of Held Synapse seen in the auditory system can sense the difference in timing of the signal reaching your left ear and reaching your right earImportant for sound localization Synaptic transmission is very rapid thousands of vesicles compared to regular synapses 12 ms the time for voltage gated channels to open rise of calcium fusion of vesicles etcIn order for it to be so fastAxon is bigInstant release of neurotransmitters several thousands of vesiclesPre synaptic terminal engulfs the entire soma of post synaptic cell allows for effective depolarization of the entire post synaptic terminalSynaptic proteinsComplicated processesInvolves a huge variety of proteins that anchor the vesiclesSNARE complex Synaptobrevin Syntaxin SNAP25Involved in docking and priming Toxins Botulinum venom from spiders bacteriao Interfere with SNARE complex proteins o Used as a local anesthetic o Vesicles cant be docked and primed anymoreno neurotransmitter release o Acts on neuromuscular junctionso People will die from respiratory paralysis Proteins of the synaptic densityComplex processes
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