Absolute Refractory Period: Understanding Nerve Impulse Impossibility

The absolute refractory period (ARP) is the brief interval (~1–2 ms) after an action potential when a second action potential cannot be initiated, regardless of stimulus strength.

Mechanism

• Voltage-gated Na⁺ channels: Activation (m gate) → inactivation (h gate) within ~0.5 ms.
• During ARP, h gates are closed (inactivated) → Na⁺ cannot enter → no depolarization possible.
• K⁺ channels (delayed rectifier) open → repolarization/hyperpolarization.

Relative Refractory Period (RRP)

• Follows ARP; Na⁺ channels recovering, K⁺ channels still open.
• Stronger-than-normal stimulus can elicit AP.
• Ends when excitability returns to baseline.

Physiological Importance

• Unidirectional propagation: AP moves forward; behind it, membrane is refractory.
• Frequency coding: Max firing rate limited by ARP+RRP (~500–1000 Hz theoretical).
• Prevents tetanus: In muscle, refractory period limits contraction summation.

Clinical

• Local anesthetics (lidocaine) bind inactivated Na⁺ channels → use-dependent block → prolong ARP.
• Channelopathies (Nav1.7 mutations) → erythromelalgia (gain) or congenital insensitivity to pain (loss).