Specifics of the local response
For local excitation to occur in a tissue, it must be affected by an insufficient stimulus — that is, a subthreshold stimulus. The main feature of this response lies in its name: it is strictly local.
The excitation appears exclusively at the point where the stimulus was applied. An attempt by this signal to spread to neighboring membrane areas fails because it fades very rapidly. Meanwhile, the amplitude of the resulting response is not constant — it directly depends on how strong the applied stimulus was. Due to this direct dependence, the local response does not obey the classical "all-or-none" law.
The most important property of local excitation is the ability to undergo summation. If subthreshold stimuli act repeatedly, their effects can add up. At the same time, membrane excitability always increases in the area of the local response. If, as a result of summation, the excitation reaches a critical level, the local process develops into a full-scale propagating one.
Features of propagating excitation
Unlike the local response, propagating excitation manifests as a full-fledged action potential (AP). Its triggering requires threshold or suprathreshold stimuli. It can also be the result of successful summation of a series of local excitations.
The action potential differs radically in its physiological characteristics:
- Propagation without decrement. The signal is capable of being transmitted over huge distances (on the scale of the cell), while its initial amplitude does not decrease at all.
- Absence of summation. Unlike the local response, action potentials cannot add up with each other.
- The "all-or-none" law. This excitation strictly obeys this rule. The tissue either produces the maximum possible response for its state (if the stimulus has reached the threshold) or does not respond at all (if the stimulus turned out to be too weak).
Brief comparison
| Characteristic | Local excitation | Propagating excitation |
|---|---|---|
| Cause | Subthreshold stimuli | Threshold and suprathreshold stimuli |
| Propagation | Fades in adjacent areas | Travels long distances without loss of strength |
| Summation | Capable of summation | Incandapable of summation |
| The "All-or-None" Law | Does not obey (depends on stimulus strength) | Strictly obeys |
Briefly: answers to topical questions
What ionic mechanism underlies the generation of local excitation?
The generation of local excitation (local response) is based on the opening of a small number of voltage-gated sodium channels. As a result, sodium ions penetrate into the cytoplasm, which leads to local membrane depolarization and a decrease in membrane potential.
What is the critical level of depolarization?
The critical level of depolarization (CLD) is the threshold level of the membrane potential (excitation threshold) upon reaching which voltage-gated channels open and an action potential is triggered.
What is the propagation of excitation with decrement?
Propagation of excitation with decrement means that the amplitude of excitation (membrane potential) gradually decreases to zero as it moves away from the site of stimulus action. This property is characteristic of the local response and electrotonus.
Why is the action potential incapable of summation?
The action potential is incapable of summation. It obeys the "all-or-none" law: the amplitude does not depend on the strength of the stimulus, and a maximal response occurs or does not occur at all. During the spike depolarization phase, the membrane is in a state of absolute refractory period — excitation is impossible, and the threshold is infinitely high.
Imagine a stone thrown into water. A local response is the small ripples from a dropped grain of sand: it is visible only at the point of impact, disappears quickly, and its size depends on the size of the grain of sand (does not obey the "all-or-none" law). An action potential is a tsunami: it occurs only from a strong push (threshold), travels all the way to the shore without losing strength (without decrement), and cannot add up with another tsunami.
Class and exam questions
Why does local excitation not obey the "all-or-none" law?
Because its amplitude changes: the stronger the applied subthreshold stimulus, the higher the amplitude of the local response will be.
Can a local response turn into an action potential?
Yes. Due to the property of summation, repeated local excitations can accumulate, reach a critical level, and trigger a propagating action potential.
Does the amplitude of the action potential decrease during its propagation?
No, propagating excitation is transmitted without decrement, that is, without a decrease in its initial magnitude along the entire path.
What else to study on this topic
- Mechanisms of summation of subthreshold stimuli
- Physiological significance of the "all-or-none" law
- Critical level required for the transition of a local response into an AP
- The concept of decrement and the reasons for its absence in AP
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