Intro | Presynaptic Neuron | Postsynaptic Neuron | Terminal Button | Axon | Neural Impulse | Synaptic Vesicles | Neurotransmitter Molecules | Cell Membrane | Transmitter does not fit at receptor | Transmitter fits receptor | Receptor Sites | Synaptic Cleft
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6. Synaptic Vesicles are small membrane sacs that carry neurotransmitters from the soma where they are produced to the terminal button or presynaptic membrane where they are released.
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The majority of synaptic vesicles (vesicle meaning "little bladder") are found in the region close to the presynaptic membrane, where they are released upon stimulation. This region is aptly called the release zone. There are both small and large vesicles. Small vesicles are produced by the Golgi apparatus located in the soma and transported down the axon via current flow in the cytoplasm (fluid inside the cell). Small vesicles store neurotransmitters. In some neurons, larger vesicles are also found in smaller quantity. These large vesicles store neuropeptides or neuromodulators such as the endorphins.
The positively charged ion, calcium (Ca+2), is essential to synaptic transmission. The release zone of the presynaptic membrane contains voltage-dependent calcium channels. These channels open in the presence of an action potential, and calcium flows into the cell where this ion is normally found in smaller concentration (propelled by electrostatic pressure). This calcium influx is essential to the synaptic process; it has been demonstrated that no neurotransmitters are released in response to action potentials when calcium is absent from this region. Calcium ions are also essential for the release of neurotransmitters into the synaptic cleft. Calcium binds with clusters of proteins within the membrane structure of both the vesicles and presynaptic membrane to induce the formation of fusion pores, a hole through both membranes that enables them to fuse together temporarily for neurotransmitter release. A former synaptic vesicle that has fused with the presynaptic membrane is called an omega figure because the appearance of this membrane fusion is very similar to the appearance of the omega letter of the Greek alphabet.