איחוי של שלפוחיות הפרשה עם ממברנת הפלזמה

Secretory vesicles first dock, then are primed for fusion, and finally fuse with the plasma membrane to release their contents. Vesicle fusion and exocytosis are aided by SNARE complexes and triggered by increased cytosolic calcium concentration.

In neuronal cells, synaptic vesicles, specialized secretory vesicles, fuse with the  membrane. As the vesicles arrive at the membrane, the SNARE complex enables docking.

The SNARE complex consists of two sets of tethering proteins—vesicular or v-SNAREs and target or t-SNAREs.  v-SNAREs interact and connect with t-SNAREs, bringing the vesicle closer to the plasma membrane.

In the priming step, complexins clamp the SNAREs together, locking them in a partially connected state. Complexins are in turn regulated by calcium and synaptotagmins, calcium-binding vesicular proteins.

When an action potential arrives at a neuron, voltage-gated calcium channels open, causing the influx of calcium ions.

Calcium binds to synaptotagmin, which causes the protein to bind to the membrane, displacing the complexin clamp.

The clamp release enables the SNAREs to fully connect, opening a pore to release the neurotransmitter.