What Is the All-or-None Law?

Neurons and Muscle Fibers Always Give a Full Response to a Stimulus

All or nothing law
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The all-or-none law is a principle that states that the strength of a response of a nerve cell or muscle fiber is not dependent upon the strength of the stimulus. If a stimulus is above a certain threshold, a nerve or muscle fiber will fire. Essentially, there will either be a full response or there will be no response at all for an individual neuron or muscle fiber.

How Does the All-or-None Law Work?

If a stimulus is strong enough, an action potential occurs and a neuron sends information down an axon away from the cell body and toward the synapse.

Changes in cell polarization result in the signal being propagated down the length of the axon.

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.

Like Firing a Gun

This process works 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. In this analogy, the stimulus represents the force applied to the trigger while the firing of the gun represents the action potential.

How Does the Body Determine the Strength of a Stimulus?

How do you determine the strength or intensity of a stimulus if the strength of the action potential does not relay this information?

Obviously, being able to determine the intensity of a stimulus is important, from detecting how hot a cup of coffee is as you take an initial sip to determining how firmly someone is shaking your hand.

In order to gauge stimulus intensity, the nervous system relies on the rate at which a neuron fires and how many neurons fire at any given time.

A neuron firing at a faster rate indicates a stronger intensity stimulus. Numerous neurons firing simultaneously or in rapid succession would also indicate a stronger stimulus.

If you take a sip of your coffee and it is very hot, the sensory neurons in your mouth will respond at a rapid rate. A very firm handshake from a co-worker might result in both rapid neural firing as well as a response from many sensory neurons in your hand. In both cases, the rate and number of neurons firing provide valuable information about the intensity of the original stimulus.

Discovery of the All-or-None Law

The all-or-none law was first described in 1871 by physiologist Henry Pickering Bowditch. In his descriptions of the contraction of the heart muscle, he explained, "An induction shock produces a contraction or fails to do so according to its strength; if it does so at all, it produces the greatest contraction that can be produced by any strength of stimulus in the condition of the muscle at the time."

While the all-or-none law was initially applied to the muscles of the heart, it was later found that neurons and other muscles also respond to stimuli according to this principle.

Source:

Martini F, Nath JL. Anatomy & Physiology. San Francisco, CA: Benjamin Cummings; 2010.

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