The role of depolarization in the action potential
The generation of an action potential (AP)—a brief change in potential difference—is impossible without depolarization. This process occurs in several stages:
- Slow depolarization. When a stimulus is applied, slow sodium channels open. Sodium ions begin to gradually enter the cell, reducing its negative charge.
- Rapid depolarization. As soon as the charge reaches the critical level of depolarization (CLD), the all-or-none law comes into effect. All voltage-gated fast sodium channels open in an avalanche-like manner. Sodium rushes into the cell in a massive influx, bringing positive charges.
- Recharge (overshoot). The membrane charge crosses zero (complete depolarization). The cell's interior becomes positively charged relative to the exterior. This is the peak of excitation.
Subsequently, to restore the initial ion balance after all AP phases are complete, the sodium-potassium pump actively engages, pumping the internalized sodium back into the extracellular environment.
Local response and synaptic transmission
Not every stimulus leads to a full-fledged spike. If the stimulus is weak (subthreshold), only a partial, reversible decrease in the membrane potential occurs—the local response. At the same time, the excitability of the tissue locally increases, and these responses are capable of summation.
In interneuronal contacts, this process manifests as an excitatory postsynaptic potential (EPSP):
- The released neurotransmitter opens polyselective channels.
- Due to the high gradient between the resting potential and the sodium equilibrium potential, the inward sodium current significantly outweighs the outward potassium current.
- A local depolarization with an amplitude of only 2–10 mV occurs.
A single EPSP never reaches the CFL. The physiological significance of this phenomenon is that the neuron acts as an analyzer: it gathers signals from numerous other cells. Only if the spatial or temporal summation of EPSPs proves sufficiently strong will the cell generate a nerve impulse.
After-depolarization and excitability
After the cell restores its charge (passes through the repolarization phase), after-depolarization (negative after-potential) may be observed. During this inconstant phase, membrane permeability to potassium decreases slightly, and the return to the baseline state slows down.
The level of depolarization strictly determines the current excitability of the tissue:
| Phase of the process | Excitability period | Characteristic |
|---|---|---|
| Rapid depolarization and peak | Absolute refractory period | Tissue is completely unexcitable, new stimuli are ignored |
| After-depolarization | Supernormal period | Excitability is increased, a reaction is possible even to weak stimuli |
Depolarizing inhibition
Paradoxically, depolarization can lead not only to excitation, but also to its suppression. Persistent depolarizing inhibition occurs when incoming signals arrive at too high a frequency.
The mechanism of this active biological process is associated with the accumulation of the excitatory neurotransmitter. As a result, the membrane cannot return to its resting state. The difference between the current membrane charge and the critical firing level increases. Ultimately, a state similar to accommodation or parabiosis develops—the cell loses the ability to generate new action potentials.
It is easy to remember the connection between phases and ions using this association: Depolarization — Drift of sodium Down inside (into the cell). The cell thereby "wakes up" and becomes excited. Meanwhile, Repolarization — Relationship ends with potassium (it flows out), and the cell returns to rest.
Class and exam questions
What is the critical firing level (CFL)?
This is the threshold value of the membrane potential (usually a 20–30% shift from the resting level) upon reaching which sodium channels open in an avalanche and a full action potential is generated.
How does local depolarization differ from an action potential?
Local depolarization (local response) occurs in response to subthreshold stimuli; its amplitude depends on the strength of the stimulus, and it is capable of summation. The action potential obeys the "all-or-none" law and always propagates along the membrane.
Can depolarization cause inhibition?
Yes, persistent depolarizing inhibition exists. It develops during high-frequency stimulation when an excess of neurotransmitter prevents the membrane from repolarizing, blocking the generation of new impulses.
What else to study on this topic
- Repolarization and after-hyperpolarization phases
- The operation of the sodium-potassium pump
- Inhibitory postsynaptic potential (IPSP)
- Mechanisms of stabilization and hyperpolarization inhibition
- The "all-or-none" law in excitable tissues
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