Neuronal communication is a two step process. In the second step, which takes place between two neurons, what does the arrival of an action potential at the axon terminal of the first neuron signal

When an action potential does arrive at the terminal, the neurotransmitter must be quickly and efficiently released from the terminal and into the synaptic cleft. The neurotransmitter must then be recognized by selective receptors on the postsynaptic cell so that it can pass along the signal and initiate another action potential.