Physiology chapters
The nerve action potential
The phases of the action potential and the ion channels behind each, threshold and the all-or-none law, refractory periods, how fast impulses travel, and how potassium and calcium changes alter excitability.
- An action potential is a brief reversal of membrane voltage: Na+ rushes in (depolarisation), then K+ flows out (repolarisation).
- It obeys the all-or-none law: once threshold is reached it fires fully; otherwise not at all.
- During the absolute refractory period no second impulse is possible (Na+ channels inactivated); myelinated fibres conduct fast by saltatory conduction.
On this page
Overview
An action potential is a rapid, short-lived change in membrane voltage that travels along a nerve or muscle fibre. It is how nerves carry information.
It lasts about 1 millisecond in a large nerve fibre and has the same size every time it fires.
Read membrane potential and ion channels first if you need the basics.
Phases of the action potential
| Phase | What happens | Channels |
|---|---|---|
| Resting | Membrane polarised at about −70 to −90 mV | K+ leak channels |
| Depolarisation | Voltage rises rapidly and overshoots to about +35 mV | Voltage-gated Na+ channels open; Na+ rushes in |
| Repolarisation | Voltage falls back towards rest | Na+ channels inactivate; voltage-gated K+ channels open; K+ flows out |
| After-hyperpolarisation | Voltage briefly dips below the resting level | K+ channels stay open a little too long |
The membrane then returns to rest. The Na-K pump restores the small amounts of ions moved, but it is not needed for each action potential.
The upstroke is caused by Na+ entry; the downstroke by K+ exit.
Positive feedback
Depolarisation opens Na+ channels; the Na+ entry causes more depolarisation, which opens more channels. This positive feedback makes the upstroke explosive.
Threshold and the all-or-none law
A stimulus must depolarise the membrane to a critical level, the threshold, for an action potential to start. In large nerve fibres, threshold is reached when the potential rises by about 15–30 mV (to around −65 mV).
- A subthreshold stimulus produces only a small local response that fades.
- A threshold or stronger stimulus produces a full action potential.
This is the all-or-none law. A stronger stimulus does not make a bigger action potential; it makes more frequent action potentials.
Accommodation
If the membrane is depolarised slowly, Na+ channels inactivate before threshold is reached, and no action potential fires. This is accommodation.
Refractory periods
| Period | Timing | Why | Effect |
|---|---|---|---|
| Absolute | From the start of the action potential to about one third of the way through repolarisation | Na+ channels are inactivated and cannot reopen | No stimulus, however strong, can fire a second action potential |
| Relative | Follows the absolute period, during late repolarisation and after-hyperpolarisation | Some Na+ channels have recovered; K+ channels still open | A stronger than normal stimulus can fire |
The absolute refractory period is due to inactivation of voltage-gated Na+ channels. It limits how often a nerve can fire and makes the impulse travel in one direction.
Propagation and conduction velocity
An action potential at one point depolarises the membrane next to it through local currents, so it spreads along the fibre.
Factors that increase conduction velocity
- Larger fibre diameter: less internal resistance.
- Myelination.
- Higher temperature (within normal limits).
Saltatory conduction
In myelinated fibres, the myelin sheath insulates the axon. Ion channels cluster at the nodes of Ranvier, gaps in the myelin about every 1–3 mm. The action potential jumps from node to node.
- It increases speed by 5 to 50 times.
- It saves energy, because ions move only at the nodes.
| Fibre | Myelin | Approx. velocity |
|---|---|---|
| Large myelinated (Aα) | Thick | Up to about 120 m/s |
| Small unmyelinated (C) | None | About 0.5–2 m/s |
Conduction velocity depends on diameter and myelin, not on the strength of the stimulus.
Effects of electrolyte disturbances
| Change | Effect | Why |
|---|---|---|
| Hypocalcaemia | Nerves become more excitable: tingling, muscle spasm, tetany | Ca2+ normally binds near Na+ channels and stabilises them; low Ca2+ lets them open more easily (threshold falls) |
| Hypercalcaemia | Nerves and muscle less excitable: weakness, constipation | Na+ channels are stabilised further |
| Hyperkalaemia | Depolarised resting potential; at first more excitable, then Na+ channels inactivate and cells fail to fire | Smaller K+ gradient |
| Hypokalaemia | Hyperpolarised; less excitable; muscle weakness | Larger K+ gradient |
Low calcium makes nerves hyperexcitable (tetany, Trousseau and Chvostek signs), because Ca2+ normally raises the threshold of Na+ channels.
Excitability terms
- Rheobase: the smallest stimulus strength that can fire the nerve if applied long enough.
- Chronaxie: the time needed for a stimulus of twice the rheobase to fire the nerve. It is a measure of excitability.
Clinical relevance
Local anaesthetics
How it happens: drugs such as lidocaine block voltage-gated Na+ channels.
What you see: action potentials cannot start or spread, so pain sensation is blocked. Small, unmyelinated pain fibres are usually blocked first.
Demyelinating diseases
How it happens: loss of myelin in multiple sclerosis (central) or Guillain-Barré syndrome (peripheral).
What you see: slowed or blocked conduction, with weakness, numbness or visual loss depending on the nerves affected.
Tetany
How it happens: low plasma calcium (after thyroid surgery with parathyroid injury, vitamin D deficiency) or alkalosis (which lowers ionised calcium).
What you see: tingling around the mouth and fingers, carpopedal spasm, laryngeal spasm.
Questions ask which channel causes each phase, the all-or-none law, what happens during absolute vs relative refractory periods, the basis of saltatory conduction, the factors that change conduction velocity, why hypocalcaemia causes tetany, and how local anaesthetics work.
Quick revision
- Upstroke: Na+ in. Downstroke: K+ out. After-hyperpolarisation: slow K+ channel closure.
- Threshold reached → full action potential (all-or-none); stronger stimulus → higher frequency.
- Absolute refractory: Na+ channels inactivated. Relative: stronger stimulus needed.
- Speed rises with diameter and myelin; saltatory conduction at nodes of Ranvier.
- Hypocalcaemia: hyperexcitable nerves, tetany.
- Local anaesthetics block voltage-gated Na+ channels.
Written by The House Job from standard physiology references (Guyton and Hall Textbook of Medical Physiology; BRS Physiology).
FAQ
What causes the depolarisation phase of the action potential?
The opening of voltage-gated sodium channels, which lets Na+ rush into the cell.
What is the all-or-none law?
Once threshold is reached, an action potential fires at full size; below threshold, none fires.
Why does hypocalcaemia cause tetany?
Low calcium lowers the threshold of sodium channels, so nerves fire spontaneously.
What is saltatory conduction?
Conduction in myelinated fibres in which the action potential jumps from one node of Ranvier to the next.