How Electrically Excitable Membrane Changes Its Electric Potential

2. Considering the nervous system, please briefly explain how the electrically excitable membrane changes its electric potential. In other words, how is the cell membrane excited?

At the surface of the cell membrane, there are three classes of gated channels exist, chemically gated or ligand-gated channels, voltage-gated channels, and mechanically gated channels. Originally, at the resting membrane potential, most of the gated channels are closed. However, when these gated channels open, the movement of ions across the membrane will changes the membrane potential. In the nervous system, the voltage-gated channels are the mainly responsible for the generation of action potential on the axon.

When the stimulation in form of graded potential is larger than the threshold of the membrane potential, the voltage-gated sodium channels open up. The opening of sodium channel makes the plasma membrane more permeable to sodium ions. Due to the difference in electrochemical gradient, sodium ions from the outside enter the cytoplasm rapidly, rapid repolarization occurs. The polarity of the membrane changes from -70mV (resting potential) to a positive value.
This allow positively charged potassium ions to be released from the cytosol, returning the membrane potential back to the resting state. Repolarization of membrane begins.

When the membrane potential reaches the normal resting potential, the potassium channels start to close. The process of hyperpolarization occurs before all the potassium channels are closed. The membrane return to its resting potential and prepares for another stimulation.

The generation of cell membrane action potential follows the all-or-none phenomenon with a refractory period of