As you can see, the SNARE proteins have twisting ability so they can fuse the vesicle to the target membrane. The tethering proteins help them to get in contact with each other. RAB-GTP is one of these tethering proteins. There is an additional SNARE protein involved in order to allow for twisting of these proteins together. Interestingly, the tetanus toxin protein called tetanospasmin cleaves a specific SNARE protein so that neurotransmitters cannot fuse with the cell membrane. The same is true of botulinum toxin, which prevents neurotransmitter release. These toxins do basically the same thing but have opposite effects on the muscle cell. One blocks inhibitory neurotransmitters, while others block excitatory neurotransmitters.
SECRETORY PATHWAYS IN NERVE CELLS There is a regulated release of neurotransmitters through exocytosis in the nerve cell. The process by which it does this is similar to all vesicular trafficking. Neurotransmitters are released from small vesicles that take part in many rounds of recycling and fusion at the presynaptic terminals. These are not made in the Golgi apparatus but are made in presynaptic endosomal compartments. The vesicles are filled after being formed, which is different than normal vesicles. Loaded neurotransmitter vesicles are sequestered in a reserve pool in the cytoplasm or cluster at “active zones” near the presynaptic membrane. When calcium levels rise enough in the cell, the vesicles will discharge into the synaptic cleft. In fact, this is the major trigger for neurotransmitter vesicle release. Synaptic vesicles need to find the right acceptor membrane in order to discharge. This relies on SNARE proteins that are specific to the vesicle and synaptic cleft, similar to other vesicles. The SNARE proteins are called synaptobrevin, syntaxin, and SNAP-25. There is also a v-SNARE and a t-SNARE protein involved as in other vesicular attachment. VAMP is another protein linked to this type of vesicular transport and attachment. At rest, neurotransmitters inside vesicles are stored either in the active zone near the synaptic cleft, where the actual neurotransmitters are released. Most neurotransmitters
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