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 specific receptors sodium channels open and the permeability of the plasma membrane in the postsynaptic region is altered. Sodium ions then go into the cell causing lowering (less negative) of the membrane potential. If this reduction of the membrane potential reaches a level called 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 along the remaining length of the neuronal membrane. If the excitation threshold is reached voltage-dependent sodium channels in the membrane open allowing 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 called repolarization. The action potential triggers the same electrical phenomenon in neighboring regions of the plasma membrane and the impulse is thus transmitted from the dendrites to the terminal region of the axon.
