Q. How does the depolarization of the neuronal membrane start?

The primary cause of the neuronal depolarization is the binding of neurotransmitters released in the synapse by the axon of the neuron that sent the signal to specific receptors in the membrane of the neuron that is receiving the stimulus. The binding of neurotransmitters to those receptors is a reversible phenomenon that alters the membrane permeability of the region since the binding cause sodium channels to open when positive sodium ions enter the cell in favor of their concentration gradient, the membrane voltage increases thus lessening the negative polarization. If this depolarization reaches the excitation threshold (about -50 mV) the depolarization continues the action possible is reached and the impulse is transmitted along the cell membrane.