When a post synaptic neuron is stimulated by a single presynaptic neuron, such that the post synaptic neuron's voltage increases closer towards the activation threshold.

When a post synapic neuron is simultaneously stimulated by a number of presynaptic neurons.

When a post synapic neuron is stimulated repeatedly by one of many presynaptic neurons, without returning to resting potential between stimulations.

When a post synaptic neuron is stimulated by a single presynaptic neuron, such that the post synaptic neuron's voltage decreases further away from the activation threshold.