The structure indicated is a synaptic vesicle located in a presynaptic bouton.
Synaptic vesicles are structures found at synaptic terminals which contain various different types of neurotransmitter, the chemicals which mediate neurotransmission.
Synaptic vesicles are contained within pools within the nerve terminal. There are three types of pools:
- Readily releasable pool
- Recycling pool
- Reserve pool
Release of synaptic vesicles is mediated by voltage-gated calcium channels. When an action potential arrives in the synaptic bouton, voltage-gated calcium channels are opened causing calcium influx into the synaptic terminal. This influx then permits fusion of vesicles to the presynaptic membrane and subsequent release of neurotransmitter across the synaptic junction. The synaptic vesicle cycle consists of the following steps:
- Trafficking of vesicles to the synapse
- Loading of the vesicle with neurotransmitter
- Docking of vesicle near to presynaptic membrane
- Priming of vesicles prior to presynaptic membrane fusion
- Fusion of vesicle to presynaptic membrane – triggered by calcium influx
- Endocytosis – reuptake of synaptic vesicles
Learn more about the anatomy of the synapse in this video tutorial.