Physiology · Cell and membrane
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
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 mVThreshold −55 mVPeak +35 mVDuration ≈ 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
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.
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.
Related topics
- The nerve action potential
- Refractory periods and excitability
- The resting membrane potential
- Synaptic transmission
- The neuromuscular junction
- Membrane transport: diffusion, channels and carriers
Related conditions
In Pharmacology
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); 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.