Essence and quantitative assessment of lability
The concept of lability should not be confused with terms such as resting and action potential difference, minimum stimulus strength, or excitation conduction velocity. Functional mobility describes exclusively the temporal framework of the signal generation process itself: how quickly a tissue can "flare up" and how quickly it returns to its initial state.
Quantitatively, this indicator is assessed as the value inversely proportional to the duration of excitation. More specifically, calculations are based on the time of the absolute refractory period — the time interval during which the tissue is fundamentally incapable of responding to a new, even strongest stimulus.
The measure of functional mobility is generally considered to be the maximum frequency of rhythmic stimulation that a specific structure can reproduce without rhythm transformation (i.e., each applied stimulus elicits a full response without dropouts or fusions).
Comparative characteristics of excitable structures
Various elements of the nervous and muscular systems possess completely different capacities for reproducing high rhythm frequencies. This directly depends on the duration of their non-excitable period.
| Excitable structure | Absolute refractory period (ms) | Measure of lability (imp./s) |
|---|---|---|
| Myelinated nerve | 1 | 1000 |
| Unmyelinated nerve | 2 | 500 |
| Striated muscle | 5 | 200 |
| Neuromuscular synapse | 10 | 100 |
The main conclusion to be drawn from the data presented is that the neuromuscular junction acts as the "slowest" link in the signal transmission chain, as it possesses the lowest functional mobility. At the same time, myelinated nerve is the "fastest" structure, capable of conducting up to a thousand impulses every second.
Features of nerve-muscle interaction
When studying signal transmission pathways, it is important to consider the ratio of parameters of various tissues. According to classical data, the lability of nerve fibers directly heading to muscles is usually 10 times lower than the value of functional mobility of the muscle tissue itself. This fact plays an important role in understanding the mechanisms of neuromuscular interaction and protecting effector structures from over-excitation.
Briefly: answers to topical questions
Which scientist introduced the concept of lability into physiology?
The concept of lability into physiology was introduced by N.Ye. Vvedensky. According to N.Ye. Vvedensky, lability is "the greater or lesser speed of those elementary reactions that accompany the physiological activity of a given apparatus." The measure of lability is the maximum possible number of elementary excitation cycles that an excitable tissue can reproduce per unit of time in accordance with the frequency of the presented stimulus.
How does tissue lability change during the development of parabiosis?
During the development of parabiosis, the lability of excitable tissue progressively decreases. In the equalization phase of parabiosis, an initial decrease in lability is observed. Frequent impulses fall into the relative refractory phase and partially fade out, due to which the tissue responds with tetanus of the same amplitude to both frequent and rare stimuli. In the paradoxical phase, an even greater decrease in lability occurs: frequent impulses do not pass through the parabiotic region at all and do not elicit a response, while rare stimuli elicit only contractions of small amplitude.
What is the phenomenon of stimulation frequency pessimum?
The phenomenon of pessimum consists of a sharp decrease in amplitude or disappearance of the tissue response despite stimulation; for muscle, this may manifest as relaxation during rhythmic stimulation. Pessimum develops when the stimulation frequency exceeds the measure of tissue lability. In sources, this is described as inhibition; for muscle, the condition for pessimum is indicated as the stimulus falling into the latent period and the refractory period of the previous excitation.
To easily memorize the table of indicators, use the "halving" rule: the myelinated nerve produces 1000 impulses, the unmyelinated one produces half as many (500), the striated muscle produces even fewer (about 200), and the slowest synapse produces exactly 100.
Class and exam questions
What serves as a measure of lability?
Its measure is the maximum stimulation frequency to which the tissue responds by generating impulses without transforming the initial rhythm.
Which physiological period is lability associated with?
It is inversely proportional to the duration of the absolute refractory period. The shorter the non-excitable period, the higher the lability.
Which excitable structure has the lowest lability?
Neuromuscular synapse. Due to the prolonged absolute refractory period (10 ms), its lability is only 100 imp./s.
How does lability differ from excitation conduction velocity?
Lability characterizes the duration of the excitation process itself at a single point (the rate of appearance and disappearance), rather than the speed of signal propagation along the fiber.
What else to study on this topic
- Mechanisms of excitation rhythm transformation
- Refractory phases: absolute and relative
- Physiological role of low synaptic lability
- Differences in impulse conduction along myelinated and unmyelinated nerves
- Resting potential and action potential
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