1. resting potential and its generation principle
Resting potential refers to the potential difference existing inside and outside the membrane when the cell is quiet. The principle of bioelectricity can be explained by ion theory. According to this theory, the membrane potential is caused by the uneven distribution of various ions inside and outside the membrane and the different permeability of the membrane to various ions under different conditions. At rest, the permeability of cell membrane to K+ is high, but the K+ in the membrane is higher than that outside the membrane, and K+ diffuses outside the membrane with the concentration difference. The cell membrane has no permeability to protein negative ion (A-), and the macromolecule A- in the membrane is blocked in the membrane, thus forming a negative potential inside the membrane and a positive potential difference outside the membrane. After this potential difference is generated, the further outward diffusion of K+ can be prevented, and the potential difference inside and outside the membrane can reach a stable value, that is, the resting potential. Therefore, the resting potential is mainly the electrochemical equilibrium potential formed by K+ outflow.
2. Action potential and its generation principle
When the cell membrane is stimulated and excited, a diffuse potential change occurs on the basis of resting potential, which is called action potential. Action potential is a continuous process of membrane potential change, and its waveform is divided into rising period and falling period. When the cell membrane is stimulated and excited, the Na+ channel on the membrane opens rapidly. Because the concentration of Na+ outside the membrane is higher than that inside the membrane, and the potential is more positive than that inside the membrane, Na+ flows in with the concentration difference and potential difference, so that the negative potential inside the membrane quickly disappears and then turns into positive potential. This potential gradient is positive inside the membrane and negative outside the membrane, which prevents Na+ from flowing in. When the concentration gradient promoting Na+ inflow is equal to the potential gradient preventing Na+ inflow, Na+ inflow stops. Therefore, the peak of the rising stage of action potential is the electrochemical equilibrium potential formed by the internal flow of Na+.
When the rising phase of the action potential reaches the value, the Na+ channel on the membrane closes rapidly, the permeability of the membrane to Na+ decreases rapidly, and the Na+ inflow stops. At this time, the permeability of the membrane to K+ increases, and the outflow of K+ makes the potential in the membrane drop rapidly until the resting potential level is restored, forming a falling phase of action potential.
Every time an excitable cell has an action potential, the ratio of Na+ to K+ inside and outside the membrane will change, so Na+ entering the membrane will be pumped out by the sodium-potassium pump, and K+ escaping from the membrane will be pumped in, thus restoring the ion distribution inside and outside the membrane at rest and maintaining the excitability of the cell.