If a stimulus is above a certain threshold, a nerve or muscle fiber will fire.
How Does the All-or-None Law Work?
Changes in cell polarization result in the signal being propagated down the length of the axon.
KTSDESIGN / Science Photo Library / Getty Images
The action potential is always a full response.
There is no such thing as a “strong” or “weak” action potential.
Instead, it is an all-or-nothing process.
This minimizes the possibility that information will be lost along the way.
This process is similar to the action of pressing the trigger of a gun.
A very slight pressure on the trigger will not be sufficient and the gun will not fire.
When adequate pressure is applied to the trigger, however, it will fire.
The speed and force of the bullet are not affected by how hard you pull the trigger.
The gun either fires or it does not.
When activated by the stimulus, the membrane depolarizes, causing ion channels to open.
As a result, sodium ions enter the action and change the polarization of the axon.
Once the cell depolarizes to the required threshold, the action potential will fire.
As the all-or-nothing law states, this action is not gradedit either happens, or it doesn’t.
A stimulus might cause sodium to enter the cell, but too few ions might enter the cell.
This means that the cell won’t reach the required threshold and it will not fire.
Determining Stimulus Strength
The body still needs to determine the strength or intensity of a stimulus.
According to the rate law, the more intense a stimulus is, the faster the neuron will fire.
In other words, a strong stimulus will cause the neuron to fire much faster than a weak one.
That is an example of the all-or-nothing law in action.
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Worth Pub; 2007.
American Psychological Association.Refractory period.
Martini F, Nath JL.Anatomy & Physiology.
Benjamin Cummings; 2010.
