The releasable pool is located in the active zone and is bound directly to the presynaptic membrane.  It is stabilized by proteins within the active zone and bound to the presynaptic membrane by SNARE proteins.  These vesicles are ready to release by a single action potential and are replenished by vesicles from the reserve pool.  The releasable pool is sometimes subdivided into the readily releasable pool and the releasable pool.