You have isolated two neurons, joined by a synapse, in a Petri dish. You have st

There are a number of reasons for failure of synaptic transmission. Starting with the presynaptic cell, it is possible that Ca2+ is not able to enter the axon terminal (either the channels are not opening, or there is too little Ca2+ in the extracellular fluid). If Ca2+ cannot enter, then it will not signal the readily releasable pool of neurotransmitter (NT) to be exocytosed. If the Ca2+ is able to enter, perhaps it is not interacting with the storage pool of neurotransmitter. In this case, after the initial release, there is no more NT on the docking proteins. Other problems may occur in the synapse itself. The NT may be degraded by enzymes before it has a chance to bind to receptors on the postsynaptic neuron, or it may diffuse away too quickly. Finally, the receptors on the postsynaptic membrane may be malfunctioning. If they are unable to bind to the NT, or cannot transduce the signal into an electrical one, there will be no detectable response by the postsynaptic cell.