[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Physiology](https://durgical.com/en/physiology/) › [Nerve and muscle](https://durgical.com/en/physiology/#s-nerve_muscle) › Refractory periods and excitability

[Physiology](https://durgical.com/en/physiology/) · [Nerve and muscle](https://durgical.com/en/physiology/#s-nerve_muscle)

# 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](https://durgical.com/en/anatomy/median-nerve/)

## Key points

- Absolute refractory period: most Na⁺ channels are inactivated; no stimulus works. It caps the firing rate and makes conduction one-way.
- Relative refractory period: some Na⁺ channels have recovered and K⁺ conductance is still high; a larger stimulus is needed.
- Hyperkalemia depolarizes the resting membrane: mildly more excitable at first, but sustained depolarization inactivates Na⁺ channels and excitability falls (weakness, cardiac arrest).
- Hypocalcemia lowers the threshold (less Ca²⁺ screening of Na⁺ channels): nerves fire spontaneously → tetany, Chvostek and Trousseau signs.
- Accommodation: a slowly rising depolarization inactivates Na⁺ channels before threshold is reached, so the cell may not fire.

## 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 mV **Threshold** −55 mV **Peak** +25 mV **Duration** ≈ 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⁺

High extracellular K⁺: see how rest moves toward threshold; switch back to normal K⁺ to compare.

## Electrolytes and excitability

| Disorder | Resting potential | Threshold | Net effect |
| --- | --- | --- | --- |
| [Hyperkalemia](https://durgical.com/en/conditions/hyperkalaemia/) | Less negative (depolarized) | Unchanged | Early irritability, then Na⁺ channel inactivation: weakness, arrhythmia |
| [Hypokalemia](https://durgical.com/en/conditions/hypokalaemia/) | More negative (hyperpolarized) | Unchanged | Harder to fire: weakness, ileus; cardiac repolarization delayed |
| [Hypocalcemia](https://durgical.com/en/conditions/hypocalcaemia/) | 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⁺.

## Memory aid

Potassium moves the floor, calcium moves the ceiling: excitability is the gap between them.

## 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.

[Add to my cards (4)](https://durgical.app/estudiar/?add=/en/physiology/refractory-periods-and-excitability/)

## See it in 3D

The structures of Durgical's 3D anatomy where it happens.

- [Median nerve](https://durgical.com/en/anatomy/median-nerve/)

## Related topics

- [The nerve action potential](https://durgical.com/en/physiology/the-nerve-action-potential/)
- [The resting membrane potential](https://durgical.com/en/physiology/the-resting-membrane-potential/)
- [Potassium balance](https://durgical.com/en/physiology/potassium-balance/)
- [Calcium regulation: PTH, vitamin D and calcitonin](https://durgical.com/en/physiology/calcium-regulation-pth-vitamin-d-and-calcitonin/)

## Related conditions

- [Hyperkalaemia](https://durgical.com/en/conditions/hyperkalaemia/)
- [Hypokalaemia](https://durgical.com/en/conditions/hypokalaemia/)
- [Hypocalcaemia](https://durgical.com/en/conditions/hypocalcaemia/)

## In Biochemistry

- [Calcium and phosphate](https://durgical.com/en/biochemistry/topics/calcium-and-phosphate/)

[← All of physiology](https://durgical.com/en/physiology/)

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](https://openstax.org/books/anatomy-and-physiology-2e/pages/12-4-the-action-potential) (OpenStax (Rice University), 2022); [19.2 Cardiac Muscle and Electrical Activity](https://openstax.org/books/anatomy-and-physiology-2e/pages/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).

## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake

Representative model, for educational purposes only.

---
Source: https://durgical.com/en/physiology/refractory-periods-and-excitability/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/physiology/refractory-periods-and-excitability/