Functional labileness, optimum, and pessimum
According to N.Ye. Vведенский's doctrine, tissues are capable of reacting differently to the frequency of incoming stimuli. The ability of a structure to reproduce a given rhythm without transformation is called functional labileness. The tissue response directly depends on how the stimulus frequency correlates with its own measure of labileness:
- Optimum of stimulation occurs when a high frequency of stimuli does not exceed the tissue's measure of labileness. In this case, the cell responds to each impulse with a maximum reaction. This happens because each new signal falls into the phase of supernormal excitability left over from the previous cycle.
- Pessimum of stimulation develops if the signal frequency becomes higher than the measure of labileness. The tissue ceases to respond adequately to stimulation, and the response drops sharply or disappears completely. The mechanism lies in the fact that subsequent impulses arrive during the refractory period (unexcitable period), so a new response simply cannot form.
The doctrine of parabiosis
The concept of parabiosis (from Greek para — near, bios — life) was also introduced by N.Ye. Vведенский. This is a specific state in which tissue excitability decreases and then transitions into deep inhibition due to the formation of a stagnant excitation focus.
Parabiosis can be triggered by a wide variety of factors:
- Chemical: exposure to poisons, alcohol, narcotic substances.
- Physical: sharp temperature drops (heating or cooling), electromagnetic radiation.
In clinical practice, typical examples of developing parabiotic states include the stages of a patient entering anesthesia, various shock states, and severe poisonings. The main feature of this process is a clear staging of the transition from overexcitation to complete inhibition.
Stages of parabiosis development
As the parabiotic state deepens, the tissue goes through three consecutive phases, changing its response to stimuli of varying intensity:
- Equalizing stage. The cell loses the ability to distinguish the strength of impact: both weak and strong stimuli elicit responses of completely identical amplitude.
- Paradoxical stage. The reaction becomes inverted. The tissue gives a strong response to weak stimuli, and conversely, a very weak response to strong ones.
- Inhibitory stage. Complete inexcitability sets in, during which the structure does not respond to any stimuli at all.
Excitation dynamics during the action potential
During the generation of an action potential (AP), the level of membrane excitability constantly changes. To study this dynamic in physiology, the paired-stimuli method is used. By applying a series of repeated stimuli at different phases of the AP, researchers calculate the stimulation threshold — the minimum current strength required to elicit a new response.
Each stage of electrical charge change (AP) strictly corresponds to its own excitability phase:
| AP Phase | Membrane Change | Excitability State | Threshold Characteristics |
|---|---|---|---|
| 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) |
Briefly: answers to topical questions
What is the ionic mechanism behind the development of after-hyperpolarization (subnormal excitability)?
The ionic mechanism of after-hyperpolarization is associated with increased membrane polarization: the membrane potential drops below the resting level.
Main mechanisms:
- increased outward efflux of K⁺ ions;
- sometimes — influx of Cl⁻ ions into the cell.
The positive after-potential phase corresponds to subnormal excitability: the membrane is hyperpolarized, the difference between the membrane charge level and the critical firing level increases, the stimulation threshold rises, and excitability decreases. The restoration of ionic balance following the action potential is ensured by the work of the Na⁺,K⁺-ATPase.
For quick memorization of the stages of parabiosis, use the abbreviation E-P-I (Equalizing: reactions equal, Paradoxical: everything is opposite, Inhibitory: no reactions).
Class and exam questions
What is the main criterion for assessing tissue excitability?
The primary measure is the stimulation threshold — the minimum impact strength capable of eliciting a propagating action potential. The relationship is inversely proportional: the lower the threshold, the higher the cell's excitability.
During which phase of the action potential is the tissue completely inexcitable?
During the peak of the action potential (spike), the absolute refractory period sets in. At this moment, all sodium channels are maximally open, and no stimulus strength can additionally enhance the avalanche-like influx of ions.
Why does the response reaction disappear during stimulation pessimum?
The frequency of incoming signals exceeds the tissue's measure of labileness. As a result, new impulses reach the membrane at a time when it is in the refractory period from the previous excitation, so a new cycle is not triggered.
What else to study on this topic
- The paired-stimuli method for plotting excitability
- Ionic mechanisms of membrane after-potentials
- Comparison of excitability parameters of nerve and muscle tissues
- Chronaxie and latent period as additional measures of excitability
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