Functional Lability, Optimum, and Pessimum
According to N.E. Vvedensky's theory, tissues respond differently to the frequency of incoming stimuli. The ability of a structure to reproduce a given rhythm without transformation is called functional lability. The tissue's response directly depends on how the stimulation frequency relates to its intrinsic measure of lability:
- Optimum of stimulation occurs when a high stimulation frequency does not exceed the tissue's lability limit. In this case, the cell responds to every impulse with a maximal reaction. This happens because each new signal arrives during the supernormal excitability phase remaining from the previous cycle.
- Pessimum of stimulation develops when the signal frequency exceeds the lability limit. The tissue ceases to respond adequately to stimulation, and the response drops sharply or disappears completely. The underlying mechanism is that successive impulses arrive during the refractory period (inexcitability), making a new response impossible to generate.
The Theory of Parabiosis
The concept of parabiosis (from Greek para — near, bios — life) was also introduced by N.E. Vvedensky. It is a specific state in which tissue excitability decreases and eventually transitions into deep inhibition due to the formation of a stagnant excitation focus.
Various factors can trigger parabiosis:
- Chemical: exposure to poisons, alcohol, anesthetic agents.
- Physical: rapid temperature fluctuations (heating or cooling), electromagnetic radiation.
In clinical practice, typical examples of parabiotic states include the induction stages of anesthesia, various shock states, and severe poisonings. The primary feature of this process is a distinct progression from hyperexcitation to complete inhibition.
Stages of Parabiosis
As the parabiotic state deepens, the tissue goes through three consecutive phases, altering its response to stimuli of varying intensities:
- Equalization phase. The cell loses the ability to differentiate stimulus strength: both weak and strong stimuli elicit responses of identical amplitude.
- Paradoxical phase. The response becomes inverted. Weak stimuli elicit a strong response, while strong stimuli elicit very weak responses.
- Inhibitory phase. Complete inexcitability sets in, during which the structure fails to respond to any stimuli whatsoever.
Excitability Dynamics During an Action Potential
During the generation of an action potential (AP), membrane excitability changes constantly. To study this dynamics in physiology, the paired-stimulus method is used. By delivering a series of repeated stimuli at different phases of the AP, researchers calculate the stimulation threshold—the minimum current strength required to evoke a new response.
Each stage of the electrical charge change (AP) strictly corresponds to a specific phase of excitability:
| AP Phase | Membrane Change | Excitability State | Threshold Characteristic |
|---|---|---|---|
| Pre-spike | Local response | Supernormal excitability | Decreased (excitability increased) |
| Spike (depolarization) | Avalanche-like sodium influx | Absolute refractory period | Infinitely high (response impossible) |
| Spike (repolarization) | Inactivation of sodium pores | Relative refractory period | Increased (supra-threshold stimulus needed) |
| Negative after-potential | After-depolarization | Exaltation | Decreased (excitability above normal) |
| Positive after-potential | After-hyperpolarization | Subnormal excitability | Increased (excitability below normal) |