"All-or-None" Law
This law describes how the tissue response depends on the strength of the applied stimulus (using single rectangular pulses).
- If the stimulus reaches or exceeds the threshold level (suprathreshold), an action potential (AP) is generated. Its amplitude remains constant and does not increase with further increases in stimulus strength (the "all" principle).
- If the stimulus is subthreshold, only a local response (local potential) is produced. Its amplitude increases gradually (graded response) as the stimulus strength increases, but no propagating excitation occurs (the "none" principle).
Strength-Duration Relationship
This relationship links the strength (amplitude) of a stimulus with its duration. The relationship is inverse: the shorter the duration, the stronger the stimulus must be to trigger excitation.
This relationship is represented by the Weiss-Lapicque (or Hoorweg-Weiss) curve. Each point on the curve represents a threshold value for a specific duration. To the left of the curve, there is no excitation (subthreshold zone); to the right, excitation occurs (suprathreshold zone).
Key excitability parameters derived from this curve include:
- Rheobase — the minimum current strength capable of eliciting a response with an infinitely long duration of application.
- Utilization time — the minimum time during which a current of one rheobase must flow to elicit excitation.
- Chronaxie — the time required for a current of twice the rheobase to initiate an AP. It is measured using a chronaximeter because it reflects excitability more accurately than the rheobase alone (measured in milliseconds for human skeletal muscle, down to fractions of a second for smooth muscle).
Law of Accommodation
Discovered by E. du Bois-Reymond, this law states that the rate of rise (slope) of a stimulus is critical for eliciting excitation. Tissue is capable of adapting (accommodating) to a slow increase in current strength.
If the current rises sharply, an action potential is triggered. If it rises slowly, the excitation threshold gradually increases (the critical depolarization level shifts). As a result, voltage-gated sodium channels have time to inactivate, and potassium channels activate. Depolarization fails to reach the critical threshold, producing only a local response, even if the final current magnitude is large.
Pflüger's Electrotonus and Polar Law
Describes the effect of direct current (DC) on tissue excitability based on electrode polarity.
- At current closure, excitation originates under the cathode (negative electrode).
- At current opening, excitation originates under the anode (positive electrode).
The resting excitability of the tissue also changes directly beneath the electrodes. At the start of current flow, excitability increases under the cathode (catelectrotonus) and decreases under the anode (anelectrotonus). However, with prolonged current flow, this situation reverses: cathode depression (decreased excitability) develops under the cathode, while anode exaltation (increased excitability) develops under the anode.