Module 1 Lesson 6 - 4

The Refractory Period

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Throughout this unit, you have discovered that normal communication involves periods of interruption. In this case, while repolarization was occurring, no chance for communication was available. Another sodium ion rush could not occur and no impulse can be generated. This brief period when no stimulus can elicit a response is called the refractory period. Its duration usually is only one to two milliseconds. The membrane cannot be stimulated again until the potential returns to -70 mV.

A Recap of Action Potential

1. At rest, the membrane potential is –70 mV.
2. Stimulus is applied and the threshold potential of –55 mV is reached.
3. Depolarization begins and the membrane voltage rises to +30 mV (Na+ open).
4. Repolarization begins and the membrane voltage returns to negative value (K+ open). 
5. Hyperpolarization occurs because the potassium channels are slow to close.
6. Membrane potential returns to resting state.

All-or-None Principle

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The stimulus that begins the influx of sodium ions and starts depolarization has to reach the threshold potential to open the voltage-gated sodium channels. When this voltage shift is reached, depolarization begins.

• It does not matter if the voltage shift is higher because, when the gates are open, a wave of depolarization occurs.
• If the stimulus is not strong enough to open the gates in the neuron membrane, nothing happens.

This ability of the neuron to respond only to a disturbance in electrical charge of a specific threshold value is called the all-or-none response. The neuron either transmits the wave of depolarization, or it does not. In an all-or-none response, no in-betweens can occur. Similar to falling dominoes, when the wave of depolarization begins, it results in an impulse.

In addition, an action potential is the same for every stimulus that reaches the threshold potential. A stronger stimulus that is past the threshold will not result in a bigger action potential. An individual action potential is not bigger than another is. Instead, multiple action potentials of the same size from multiple neurons yield a greater sensation of pain or a stronger muscle contraction.

Watch and Listen

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 Watch the following segment of Biologix-02 to view how the action potential is propagated.