Understanding the brain: how are neurotransmitters released in the brain?
The release of neurotransmitters is a very specific type of SNARE mediated exocytosis. The action potential from the influx of sodium propagates along the axon and reaches the axon terminal, containing the calcium channels. This causes the calcium cations (ions) to travel down the electrochemical gradient. The Ca2+ then binds to a protein. The vesicles containing the neurotransmitter called synaptotagmin (Ca sensors), would cause conformational changes leading to the formation of a SNARE complex. Some good animations can be seen here and here.  

Regarding whether there is an energy threshold: there is a threshold on the amount of Calcium needed to cause the activation of synaptotagmin, as with all types of SNAREs. 

See this article: http://www.pnas.org/content/93/23/13327.full.pdf

Past a certain calcium threshold, all the neurotransmitter should in theory be released, but whether they cause a response would depend on the receptor and on the postsynaptic membrane, as each synapse is specific in the neurotransmitters and receptors it expresses.