The nature and importance of the refractory period in producing discrete…

The idea

Immediately after an action potential the membrane cannot produce another one. During the absolute refractory period the voltage-gated sodium ion channels are inactivated, so no stimulus, however strong, can depolarise that patch of membrane. During the relative refractory period that follows, the channels have reset but the membrane is hyperpolarised because potassium ion channels close slowly, so only a stronger than normal stimulus reaches threshold. Two consequences are examinable. First, action potentials are discrete: each is a separate event that cannot merge with or overlap the one before, and because the membrane behind the wave is refractory the impulse can travel only forwards. Second, the refractory period sets a ceiling on frequency, so a shorter refractory period allows a higher maximum frequency of impulses.

What earns the marks

• State that during the absolute refractory period the membrane cannot be depolarised again because the voltage-gated sodium ion channels are inactivated.
• State that during the relative refractory period a stronger than normal stimulus can trigger an action potential, because the membrane is hyperpolarised.
• Explain that the impulse travels in one direction only, because the membrane behind the action potential is refractory.
• Explain that action potentials are discrete, so they are separated and cannot overlap or merge.
• Explain that the refractory period limits the frequency of impulses, giving a maximum number per second.
• Calculate a maximum frequency from a given refractory period, for example a 2 ms refractory period gives a maximum of about 500 impulses per second.

Common mistakes and how to put them right

• Saying the refractory period stops the impulse travelling backwards because of the myelin sheath, rather than because the membrane behind is refractory.

• Describing the refractory period as the time taken to reach threshold rather than the time during which threshold cannot be reached.

• Claiming that a sufficiently strong stimulus can override the absolute refractory period.

  Put it right: It cannot, because the sodium ion channels are inactivated.

• Confusing a limit on the frequency of action potentials with a limit on their size, which is fixed by the all-or-nothing principle.

• Using the word discrete without saying what it means here, namely that each action potential is a separate event that cannot merge with the next.

Exam tips

• Three separate ideas live here: one direction only, discrete impulses, and limited frequency. Write all three even for a two-mark question.
• If a refractory period is given in milliseconds, convert it into a maximum frequency; that calculation is a common application mark.
• Link the frequency ceiling back to how stimulus intensity is coded, which ties this statement to the all-or-nothing principle.