Resting Membrane Potential (RMP)
At rest, the inner side of the cell membrane is negatively charged relative to the external environment. This potential difference is called the resting potential (RP). It is measured using a glass microelectrode inserted inside the cell and a reference electrode placed in the extracellular environment (whose charge is conventionally taken as zero). The magnitude of the RP ranges from –70 to –95 mV.
Any decrease in the absolute value of this charge (shifting toward zero) is called depolarization, and an increase (shifting further down the scale) is called hyperpolarization. The stability of the resting potential is maintained by normal cellular metabolism.
Action potential and its phases
Action potential (AP) is a rapid fluctuation of membrane charge, the physiological basis of propagating excitation. It occurs when depolarization reaches critical level of depolarization (CLD) — typically a shift of 20–30% from the resting membrane potential. Until the CLD is reached, only a local local response is observed.
The process of AP generation (spike) is divided into several stages:
- Slow depolarization: slow sodium channels open, and the charge begins to drop.
- Rapid depolarization: upon reaching the CLD, fast sodium channels open in an avalanche-like manner. Sodium ions rush into the cell.
- Overshoot: the charge crosses zero, and the inside of the cell becomes positive. This is the main reason for impulse propagation.
- Repolarization: sodium channels close, and potassium channels open. Potassium flows outward, restoring the negative charge to the membrane.
The AP amplitude reaches 100–110 mV in nerve cells and 110–120 mV in muscle cells. The spike may be followed by inconstant phases: after-depolarization and after-hyperpolarization.
Ion channels and restoration of balance
Voltage-gated ion channels play a key role in the generation of biopotentials. Unlike ligand-gated structures that respond to ligands, they open in response to changes in membrane charge. For example, calcium channels selectively pass Ca2+ ions upon depolarization. This is critically important for muscle contraction, impulse generation in the cardiac sinus node, and neurotransmitter release.
After the passage of the excitation wave, the sodium-potassium pump actively comes into play. It pumps out the internalized sodium and returns the lost potassium, fully restoring the initial ion concentrations and membrane potential.
Changes in excitability during AP generation
During the development of an action potential, the cell's ability to respond to new stimuli (excitability) changes systematically. The stimulation threshold serves as a measure of excitability.
| Period | Corresponding phase of AP | Excitability State |
|---|---|---|
| Absolute refractory period | Depolarization, peak, and onset of repolarization | Reduced down to complete absence |
| Relative refractory period | Remaining part of repolarization | Gradually recovers to normal |
| Supernormal period | After-depolarization | Increased |
| Subnormal period | After-hyperpolarization | Reduced |
History of studying biopotentials
The study of bioelectrical activity has a rich history. In 1791, Luigi Galvani, in experiments with frog legs, proved the existence of “animal electricity”. Soon Alessandro Volta put forward the concept of “metallic electricity”. In 1840, Matteucci used a galvanometer to discover the injury potential, showing that the traumatized tissue area is electronegative.
In 1848, Emil du Bois-Reymond established the electronegativity of an excited nerve. In the late 19th century, L. Hermann and J. Bernstein proposed the first membrane hypotheses. The modern membrane theory of excitation was developed starting in 1949 by researchers Hodgkin, Huxley, and Katz thanks to the introduction of microelectrode technique.
Class and exam questions
What is a local response?
This is a local, non-propagating process of excitation. It occurs under the action of subthreshold stimuli, when depolarization has not yet reached the critical level, while the cell's excitability increases.
What is the difference between depolarization and hyperpolarization?
Depolarization is a decrease in membrane potential (shifting toward zero), while hyperpolarization is an increase in its absolute value (enhancement of membrane polarization).
Which cells are capable of generating an action potential?
The action potential is generated exclusively by excitable cells. Meanwhile, the resting potential is inherent to absolutely all living cells.
Why is overshoot needed?
The transition of the charge past the zero mark, when the inner side of the membrane temporarily becomes positive, is the main reason for the further propagation of excitation along the tissue.
What else to study on this topic
- Mechanisms of the sodium-potassium pump
- Summation of cell responses to subthreshold stimuli
- The role of calcium channels in platelet aggregation
- Galvani's experiments: “contraction without metals”
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