The release of neurotransmitter is accomplished by the fusion of neurotransmitter vesicles to the presynaptic membrane.  Although the details of this mechanism are still being studied there is a consensus on some details of the process.  Synaptic vesicle fusion with the presynaptic membrane is known to require a local increase of calcium from as few as a single, closely associated calcium channels  and the formation of highly stable SNARE complexes.  One prevailing model of synaptic vesicle fusion is that SNARE complex formation is catalyzed by the proteins of the active zone such as Munc18, Munc13, and RIM.  The formation of this complex is thought to "prime" the vesicle to be ready for vesicle fusion and release of neurotransmitter (see below: releasable pool).  After the vesicle is primed then complexin binds to the SNARE complex this is called "superprimed".  The vesicles that are superprimed are within the readily releasable pool (see below) and are ready to be rapidly released.  The arrival of an action potential opens voltage gated calcium channels near the SNARE/complexin complex.  Calcium then binds to change the conformation of synaptotagmin.  This change in conformation of allows synaptotagmin to then dislodge complexin, bind to the SNARE complex, and bind to the target membrane.  When synaptotagmin binds to both the SNARE complex and the membrane this induces a mechanical force on the membrane so that it causes the vesicle membrane and presynaptic membrane to fuse.  This fusion opens a membrane pore that releases the neurotransmitter.  The pore increases in size until the entire vesicle membrane is indistinguishable from the presynaptic membrane.