What are the sources of stimulation that start a nerve impulse?

Stimulations that start nerve impulse (action potential within the neuron) are different:
1. Chemical synapses
The synapse between two neurons consists of presynaptic and postsynaptic cells. Presynaptic neuron (axon) cell release the neurotransmitter which binds to the receptor on the membrane of the postsynaptic neuron (dendrites) inducing the change in voltage (usually via ion channels) and thus, an action potential.
2.Electrical synapses
Some neurons are coupled through channels that allow ions to pass between them, therefore, depolarization in one cell can be transmitted by ions flowing through that gap junction and begin to depolarize the other cell.
3. Receptor potentials
It happens when a stimulus directly opens an ion channel (high temperature for example or sound waves), or it can be mediated through G-protein coupled receptors.
4. Intrinsic conductances
There are intrinsic conductances that can bring a cell to a threshold and trigger an action potential like in the pacemaker cells of the sinoatrial node which initiate cardiac contractions by themselves.

The answer is overmatching