Each cell is covered by a protective membrane called the plasma membrane that protects the cell from the external environment. It is made up of lipid bilayer (phospholipids and glycolipids). This bi-layered membrane is semipermeable and involves in the regulation of ions being transported in and out of the cell. By this transport mechanism, the membrane potential of the cell is maintained.The charge of the cell is determined by the concentration of ions that are available within the cell. The membrane potential of the cell at a resting stage (that is, when there is no impulse or any stimuli) is -60mV. When an external stimulus is applied, an action potential is generated in the cell. The sodium channels are opened and sodium ions enter into the cell leads to the depolarization of the cell (the negativity of the cell increases) and attains zero. Once the membrane reaches the threshold, sodium channels are closed and the potassium channel opens through which more potassium ions are discharged and the cell reaches its resting potential and hyperpolarization.
Resting Membrane potential: The membrane potential is the difference in the ionic charges between the intracellular and extracellular environments. The potential is maintained by the permeability of ions through the cell membrane. The membrane potential of the cell when there is no impulse is the resting membrane potential and it lies between -50mV and -75mV. This potential is maintained by the ion channels namely potassium channel, sodium channel, and chloride channel. When a cell is at rest, their concentration of potassium ions within the cell is higher than in the extracellular region and this mainly confers to the negative membrane potential. Moreover, the level of sodium ions and chloride ions in the intracellular region is low than the extracellular. In addition, all the ion channels will be closed at the resting stage and there is no transport of ions. Sodium-Potassium ATPase pump regulates the resting membrane potential at equilibrium by the efflux of three sodium ions from the cell while two potassium ions are being transported into the cell. This pump does not have a role during repolarization.
Depolarization: Upon signaling, the resting membrane potential gets altered. The voltage-gated sodium channel opens and this results in the influx of sodium ions into the cell through passive transport. This will result in the rising of the resting membrane potential from -70 mV to 0 mV. This causes the depolarization of the cell and the cell attains a positive charge. The influx of sodium ions continues until the membrane potential reaches a threshold of +30 or +40 mV. An action potential is generated once the threshold is achieved.
Repolarization: After the attainment of the threshold, the voltage-gated sodium channel closes, and therefore, the sodium ions no longer enter into the cell. At this point, the electrochemical gradient within the cell will result in the opening of the potassium channels and this involves the efflux of potassium ions out of the cell. This results in a decrease in the membrane potential and the cell start approaching towards the resting membrane potential
Hyperpolarization: Once the membrane reaches its resting membrane potential of -70 mV, the closing of the potassium pump is slow and thus more potassium ions is pumped out and the cell reaches a potential lower than its resting membrane potential. This state is called hyperpolarization. The hyperpolarization of the cell is brought back to normal resting membrane potential by the sodium-potassium ATPase pump.
