How is the depolarization of the neuronal plasma membrane generated? How does the cell return to its original rest?

When the neuron receives a stimulus by the binding of neurotransmitters to exact receptors sodium channels open and the permeability of the plasma membrane in the postsynaptic region is changed. Sodium ions then go into the cell causing lowering (less negative) of the membrane potential. If this decreasing of the membrane potential reaches a level known as the excitation threshold, or threshold potential, about -50 mV, the action potential is generated, i.e., the depolarization intensifies until reaching its maximum level and the depolarization current is transmitted with the remaining length of the neuronal membrane.

If the excitation threshold is reached voltage-dependent sodium channels in the membrane open permitting more sodium ions to enter the cell in favor of the concentration gradient and an approximate -35 mV level of positive polarization of the membrane is achieved. The voltage-dependent sodium channels then close and more voltage-dependent potassium channels open. Potassium ions then exit the cell in favor of the concentration gradient and the potential difference of the membrane decreases, a process known as repolarization.