In neuroscience we learn that when the membrane potential of a neuron reaches a threshold (typically around ) it "spikes": That is, it actively propagates a signal. I have two related questions with this respect:
The spike initiation zone is typically (e.g. for mammals) at the axon hillock - from there the action potential is actively (opening of ion channels..) propagated through the axon. But what happens then at and after the (chemical) synapse? Is the propagation after the synapse to the postsynaptic cell passive?
Newer imaging techniques (e.g. calcium imaging) can capture sub-threshold changes in the membrane potential. How are these sub-threshold potentials relevant for information processing? Are they propagated to postsynaptic cells, albeit again only in a passive manner?