Biology – Control and coordination in mammals | e-Consult

The absolute and relative refractory periods are distinct phases following an action potential, each characterized by different levels of excitability.

Absolute Refractory Period: This period is completely unresponsive to any stimulus, no matter how strong. It occurs because the voltage-gated sodium channels are inactivated and cannot be reopened. Therefore, an action potential cannot be initiated during this period. This is a critical mechanism for ensuring unidirectional propagation of the action potential and preventing the signal from travelling backwards.

Relative Refractory Period: During the relative refractory period, the axon *can* be stimulated to fire an action potential, but only with a stronger-than-normal stimulus. This is because some of the sodium channels have recovered from inactivation, but not all. The potassium channels are still open, making it more difficult to depolarize the membrane to the threshold potential. This period limits the frequency of action potentials by making it harder to initiate a new AP, even if the stimulus is strong enough to overcome the potassium efflux.

Contribution to frequency regulation:

• Absolute Refractory Period: Completely prevents action potential generation, effectively setting a lower limit on the frequency of impulses. The frequency cannot exceed zero during this period.
• Relative Refractory Period: Limits the maximum frequency of impulses. The frequency is reduced because a stronger-than-normal stimulus is required to initiate a new action potential. This prevents the axon from firing at an unsustainable rate.