Tissue Classification and the Concept of Polarization
All body tissues are divided into two large groups based on their response to stimulation:
- Non-excitable tissues: bone, connective, and epithelial tissue.
- Excitable tissues: nervous and muscle tissue. Their main property is that upon stimulation, they generate excitation capable of propagating away from the origin.
For a cell to respond to a stimulus, its membrane must be polarized at rest. This means there is a strict charge distribution: the inner surface carries a negative charge (–), and the outer surface carries a positive charge (+).
Ionic Asymmetry and Active Transport
Polarization does not occur on its own. Its foundation is the uneven distribution of ions on both sides of the cell membrane, established by the membrane enzyme $Na^+, K^+$-ATPase (sodium-potassium pump).
Using the energy of ATP hydrolysis, this pump performs active transport of ions strictly against their concentration gradient:
- Potassium ions ($K^+$) are pumped inside the cytoplasm. As a result, their concentration becomes 20–30 times higher inside, where they bind to negatively charged cytoplasmic proteins.
- Sodium ions ($Na^+$) are pumped outside. Their concentration in the extracellular fluid becomes 10–15 times higher than the intracellular concentration.
In addition, due to the action of other membrane enzymes, a high concentration of calcium ($Ca^{2+}$) and magnesium ($Mg^{2+}$) is maintained outside—20 to 30 times higher than inside. Chlorine ($Cl^-$) is 15–25 times more abundant outside, which is necessary to balance the positive charge of cations accumulated there.
Stages of Membrane Potential Formation
The primary role in forming the membrane's electrical charge belongs to $Na^+$ and $K^+$ ions. The process of spontaneous polarization can be divided into logical stages:
- Sodium Blockade. The $Na^+$ ions accumulated outside tend to enter the cell down their concentration gradient. However, at rest, the membrane is completely impermeable to them, so the high sodium concentration is reliably maintained in the extracellular environment.
- Potassium Diffusion. Unlike sodium, the membrane is partially permeable to $K^+$ ions. Through membrane proteins forming leakage channels, potassium begins to passively exit the cell outward down its concentration gradient.
- Electrostatic Interaction. The exited $K^+$ ions cannot go far. Large protein anions ($A^-$) remain inside the cell. They are negatively charged and, due to their large size, cannot pass through the membrane. These anions electrostatically attract the positive potassium, holding it near the outer surface. As a result, less than 1% of potassium ions leave the cell.
- Potassium Equilibrium. A dynamic equilibrium of two forces is formed: the concentration gradient pushes $K^+$ outward, while electrostatic attraction pulls it inward. This gives rise to the potassium equilibrium potential—the main cause of polarization.
- Final Correction. Chloride ions ($Cl^-$) passively redistribute following the electric field. At the same time, minor leakage currents exist—a weak passive influx of $Na^+$ and $Cl^-$ into the cell. This process slightly shifts the final value of the membrane potential away from the pure potassium value, making it slightly less negative.
Recording the Resting Potential
To measure the membrane potential, the glass microelectrode technique is used. These are ultra-thin micropipettes filled with an electrolyte, whose tip is so small that it is inserted directly inside the cell without disrupting its membrane.
The device records the electrical potential difference between two points:
- A microelectrode inside the cytoplasm (where the charge is negative).
- A macroelectrode on the outer surface of the cell (where the charge is positive).
In physiology, it is standard to analyze all changes in membrane potential relative specifically to the inner surface. The magnitude of the potential varies across different tissues, ranging from −50 to −100 mV. This difference is explained by two factors: varying cellular ATPase activity and differing numbers of $K^+$ ion leakage pores in different types of excitable cells.