Refractory periods and excitability !
- Concept
- Exam priority: medium
Right after firing, a nerve cannot fire again for a moment. Blood potassium and calcium change how easily nerves and muscles fire.
Updated 2026-11-01
Also called: Absolute refractory period, Relative refractory period, Accommodation
See it in 3D: Median nerve
What it is
After an action potential, Na⁺ channels need time to recover from inactivation. During the absolute refractory period no stimulus can fire the cell; during the relative refractory period a stronger-than-normal stimulus can. Excitability also depends on the distance between resting potential and threshold, which extracellular K⁺ and Ca²⁺ shift in opposite directions.
Key facts
- In large nerve fibers the absolute refractory period lasts about 0.4–1 ms, allowing several hundred spikes per second at most.
- In ventricular muscle the effective refractory period lasts about as long as the plateau (≈ 250 ms), protecting the heart from tetanus.
- Hypercalcemia raises the threshold: lethargy, weakness and hyporeflexia.
Action potential
Rest −62 mVThreshold −55 mVPeak +25 mVDuration ≈ 1–2 ms
Hyperkalemia: with more K⁺ outside the cell the K⁺ gradient falls and the resting potential becomes less negative (closer to threshold). The sustained depolarization inactivates some Na⁺ channels: the upstroke is slower and lower (in the heart, a wide QRS) and the myocyte repolarizes sooner (peaked T wave). Excitability first rises, then falls (weakness, arrhythmias).
Tap a phase (the pills above or its band) or a refractory period (ARP, RRP) at the base of the plot.
- Mainly Na⁺
- Mainly K⁺
- Rest
- Threshold
- ARP: absolute refractory period
- RRP: relative refractory period
- Curve at normal K⁺
Electrolytes and excitability
| Disorder | Resting potential | Threshold | Net effect |
|---|---|---|---|
| Hyperkalemia | Less negative (depolarized) | Unchanged | Early irritability, then Na⁺ channel inactivation: weakness, arrhythmia |
| Hypokalemia | More negative (hyperpolarized) | Unchanged | Harder to fire: weakness, ileus; cardiac repolarization delayed |
| Hypocalcemia | Unchanged | Lower (closer to rest) | Hyperexcitable: paresthesias, tetany, laryngospasm |
| Hypercalcemia | Unchanged | Higher | Less excitable: weakness, hyporeflexia, constipation |
Clinical notes
- Hypocalcemia after thyroid or parathyroid surgery shows first as perioral tingling and carpopedal spasm (Trousseau sign with a blood pressure cuff).
- In severe hyperkalemia, intravenous calcium is used because it raises the threshold and restores the gap between rest and threshold in the heart; it does not lower K⁺.
Test yourself
Think of the answer, then tap the question to see it.
Why does an action potential not travel backward along an axon?
The membrane just behind the impulse is in its absolute refractory period: its Na⁺ channels are still inactivated, so local current can only fire the membrane ahead.
Why can severe hyperkalemia cause flaccid weakness even though it depolarizes the membrane?
Sustained depolarization keeps many Na⁺ channels inactivated. Too few are available to generate a spike, so muscle fibers become inexcitable.
How does hypocalcemia cause tetany when Ca²⁺ is not the ion that carries the nerve upstroke?
Extracellular Ca²⁺ stabilizes Na⁺ channels. When it falls, the channels open at less negative voltages, threshold moves toward rest and nerves fire spontaneously.
What is the difference between the absolute and relative refractory periods in terms of channels?
Absolute: Na⁺ channels mostly inactivated, no response. Relative: some Na⁺ channels recovered while K⁺ channels are still open, so a stronger stimulus can fire the cell.
See it in 3D
The structures of Durgical's 3D anatomy where it happens.
Related topics
- The nerve action potential
- The resting membrane potential
- Potassium balance
- Calcium regulation: PTH, vitamin D and calcitonin
Related conditions
In Biochemistry
Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.
Sources: 12.4 The Action Potential (OpenStax (Rice University), 2022); 19.2 Cardiac Muscle and Electrical Activity (OpenStax (Rice University), 2022); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 5). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).
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Representative model, for educational purposes only.