Laws of excitation of excitable tissues

All-or-none law

This law describes how the tissue response depends on the strength of the applied stimulus (when using single rectangular pulses).

• If the stimulation reaches or exceeds the threshold level (suprathreshold), an action potential (AP) is generated. At the same time, its amplitude always remains constant and does not increase with a further rise in stimulus strength (the "all" principle).
• If the stimulation is subthreshold, only a local response (local potential) occurs. Its amplitude increases gradually as the stimulus is intensified, but no propagating excitation takes place (the "none" principle).

Strength-duration law

The law demonstrates the relationship between the strength (amplitude) of the stimulus and its duration. This relationship is inverse: the shorter the exposure, the more powerful the stimulus must be to provoke excitation.

This relationship is represented by the Hoorweg-Weiss-Lapicque curve. Each point on it is a threshold value for a specific duration. To the left of the curve, there is no excitation (subthreshold zone); to the right, there is (suprathreshold zone).

Basic excitability parameters are determined using this curve:

1. Rheobase — the minimum current strength capable of eliciting a response during indefinitely long exposure.
2. Minimal useful time — the time during which a current of one rheobase induces excitation.
3. Chronaxie — the time required for a current of two rheobases to initiate an AP. It is measured with a chronaximeter, as it reflects excitability more accurately than the rheobase (in human skeletal muscles, this is milliseconds; in smooth muscles, up to tenths of a second).

Law of accommodation

Discovered by E. du Bois-Reymond, it states that the rate (steepness) of rise of the stimulus is crucial for eliciting excitation. The tissue is capable of adapting (accommodating) to a slow increase in current strength.

If the current rises sharply, an action potential occurs. If slowly, the excitation threshold gradually increases (the critical level of depolarization shifts). As a result, sodium channels have time to inactivate, and potassium channels to activate. Depolarization does not reach the critical level, and only a local response is formed, even if the final current strength is large.

Pflüger's polar law

Describes the effect of direct current on excitability depending on the polarity of the electrodes.

• Upon closure of the circuit, excitation originates under the cathode (negative electrode).
• Upon opening of the circuit, it originates under the anode (positive electrode).

The initial excitability of the tissue also changes under the electrodes. At the onset of current flow, it increases beneath the cathode (catelectrotonus) and decreases beneath the anode (anelectrotonus). However, with prolonged current passage, the situation reverses: depression (decreased excitability) occurs under the cathode, while exaltation (increase) occurs under the anode.

Class and exam questions

What else to study on this topic

• Phases of membrane potential changes during slow current rise (curve analysis)
• Differences in accommodation rates among various types of nerve fibers
• Local response and its properties compared to the action potential
• Mechanisms of development of cathodic depression and anodic exaltation

Other topics in the section "General properties, membrane potentials, laws of stimulation, excitability, and inhibition"

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.