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# Ion channels and gating

- Concept
- Exam priority: medium

Channels are protein pores that open and close to let one specific ion through in microseconds.

Updated 2026-11-01

Also called: Voltage-gated channels, Ligand-gated channels

## Key points

- Voltage-gated Na⁺ channel: an activation gate (opens fast on depolarisation) and an inactivation gate (closes shortly after). It recovers only on repolarisation.
- Voltage-gated K⁺ channel: opens more slowly and repolarises the membrane.
- Local anaesthetics block the Na⁺ channel from inside, preferring open or inactivated channels (use-dependent block).
- Sustained depolarisation (hyperkalaemia) leaves Na⁺ channels inactivated: the cell becomes inexcitable.
- Nicotinic receptor: a ligand-gated channel (acetylcholine) that passes Na⁺ and K⁺.

## What it is

An ion channel is a protein with a water-filled pore and a selectivity filter that passes almost only one kind of ion. Its opening is controlled by a gate that responds to voltage, to a ligand (neurotransmitter, second messenger), to stretch, or stays partly open (leak channels). The opening and inactivation sequence of voltage-gated Na⁺ and K⁺ channels generates the action potential, and is the target of local anaesthetics, several antiarrhythmics and many toxins.

## Key facts

- Selectivity depends on pore size and on the ion's charge and hydration shell, not only its size.
- A single channel opens all-or-none; total current depends on how many channels are open at once.
- Small unmyelinated fibres (pain, temperature, autonomic) are blocked by local anaesthetics before large motor fibres.
- Channelopathies: cystic fibrosis (CFTR Cl⁻ channel), long QT syndromes, myotonias and some epilepsies.

## How it works, step by step

- At rest the Na⁺ channel is closed and ready: activation gate shut, inactivation gate open.
- Depolarisation to threshold opens the activation gate within a fraction of a millisecond; Na⁺ enters.
- A few milliseconds later the inactivation gate closes; the channel stops conducting even though it is still depolarised.
- As the membrane repolarises, the gates reset and the channel can open again (end of the absolute refractory period).

### Action potential

**Rest** −70 mV **Threshold** −55 mV **Peak** +35 mV **Duration** ≈ 1–2 ms

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

Tap the depolarisation and repolarisation phases to see which channels are open at each moment.

## Kinds of gate

| Type | Opened by | Examples |
| --- | --- | --- |
| Voltage-gated | A change in membrane potential | Na⁺, K⁺ and Ca²⁺ channels of nerve and muscle |
| Ligand-gated | Binding of a molecule | Nicotinic receptor, GABA-A, glutamate (NMDA, AMPA) |
| Mechanically gated | Stretch or pressure | Touch receptors, inner-ear hair cells, baroreceptors |
| Leak | Open nearly all the time | K⁺ channels that set the resting potential |

## Clinical notes

- Local anaesthetics in infected tissue: acidic pH increases the ionised form, which cannot cross the membrane, so the effect weakens.
- Lidocaine as an antiarrhythmic: it blocks Na⁺ channels more strongly in ischaemic, depolarised tissue that fires fast.
- Hypocalcaemia: with less extracellular Ca²⁺, Na⁺ channels open with less stimulus, causing paraesthesia and tetany.

## Memory aid

Open, plug, reset: activation opens, inactivation plugs, repolarisation resets.

## Test yourself

Think of the answer, then tap the question to see it.

## Why can a neuron not fire another action potential during the absolute refractory period?

Its Na⁺ channels are inactivated; no depolarisation opens them until the membrane repolarises and the gates reset.

## Why do local anaesthetics block pain before motor strength?

Pain fibres are thin and little or not myelinated: blocking a short stretch of membrane is enough. Motor fibres are thick and need several nodes blocked.

## What does 'use-dependent' block mean?

The drug binds open or inactivated channels better; cells that fire more often (pain fibres, tachyarrhythmia) are blocked more.

## Why does local anaesthesia work poorly in an abscess?

Acidic pH converts more drug to the ionised form, which cannot cross the membrane to reach its site inside the channel.

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## Related topics

- [The nerve action potential](https://durgical.com/en/physiology/the-nerve-action-potential/)
- [Refractory periods and excitability](https://durgical.com/en/physiology/refractory-periods-and-excitability/)
- [The resting membrane potential](https://durgical.com/en/physiology/the-resting-membrane-potential/)
- [Synaptic transmission](https://durgical.com/en/physiology/synaptic-transmission/)
- [The neuromuscular junction](https://durgical.com/en/physiology/the-neuromuscular-junction/)
- [Membrane transport: diffusion, channels and carriers](https://durgical.com/en/physiology/membrane-transport-diffusion-channels-and-carriers/)

## Related conditions

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

## In Pharmacology

- [Local anesthetics](https://durgical.com/en/pharmacology/classes/local-anesthetics/)
- [Lidocaine](https://durgical.com/en/pharmacology/lidocaine/)

[← 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); [12.5 Communication Between Neurons](https://openstax.org/books/anatomy-and-physiology-2e/pages/12-5-communication-between-neurons) (OpenStax (Rice University), 2022); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 4). 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.

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