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The action potential and conduction

How a neuron fires an all-or-none electrical pulse and sends it along its axon, why myelin makes it faster, and the Hodgkin–Huxley equations that describe it.

Updated 2026-10-023 sources

Depth:

Why it matters

Every message in the spinal cord, up or down, travels as action potentials. How fast they travel decides how long a reflex or a voluntary movement takes ^{[1, 2]}.

What it is

An action potential is a brief reversal of the voltage across a neuron's membrane. At rest the inside is about −70 mV^[1], the value most commonly used for the resting membrane potential, though it varies between cells ^[1].

How it works

A stimulus depolarises the membrane, moving its potential toward zero ^[1].
If it reaches threshold, about −55 mV^[1], voltage-gated sodium channels open; any depolarisation that does not reach it produces no action potential ^[1].
Sodium flows in until the membrane reaches about 30 mV^[1].
Repolarisation brings it back toward rest and overshoots into hyperpolarisation while potassium channels are open ^[1].

Action potentials are all or none: either the membrane reaches threshold and the whole sequence happens, or nothing does ^[1]. For a short time afterwards the membrane is refractory, in an absolute and then a relative phase ^[1].

Hodgkin and Huxley described the squid giant axon membrane with currents through sodium, potassium and leak conductances, each gated by voltage-dependent variables; the neuron lab runs their equations ^[3].

Hodgkin–Huxley sodium current^[3]

I_{Na} = \bar g_{Na}\, m^{3} h\, (V - E_{Na})

C_m \frac{dV}{dt} = -\left(I_{Na} + I_K + I_L\right) + I_{stim}

I_K = \bar g_K\, n^4 (V - E_K), \qquad I_L = g_L (V - E_L)

\phi = Q_{10}^{(T - 6.3\,^{\circ}\mathrm{C})/10}

The sodium current through a patch of membrane is its maximum conductance, scaled by three activation gates (m) and one inactivation gate (h), times the driving force: how far the membrane potential is from the sodium reversal potential. Negative current flows into the cell. The potassium and leak currents have the same form; the capacitance equation adds them up; φ scales the gate rates with temperature.

Variables of Hodgkin–Huxley sodium current
Symbol	Meaning	Unit
$\bar g_{Na}$	maximum sodium conductance	mS/cm²
$m$	activation gate (fraction open, 0 to 1)	dimensionless
$h$	inactivation gate (fraction not inactivated, 0 to 1)	dimensionless
$V$	membrane potential	mV
$E_{Na}$	sodium reversal potential	mV
$I_{Na}$	sodium current density	µA/cm²

Valid when

Squid giant axon membrane, the preparation it was fitted to; other neurons have other channels and constants
Potentials here are shifted so that rest is −65 mV, the common modern convention (Hodgkin and Huxley measured from rest, with the opposite sign)
Gate values m and h come from their own rate equations; the example fixes them to show one instant

Worked example^[3]

With the published maximum conductance and reversal potential (in the shifted convention), and gates half open and 60% available, at −40 mV the sodium current is strongly inward.

gNa = 120 mS/cm², m = 0.5 , h = 0.6 , V = −40 mV, ENa = 50 mV → INa = −810 µA/cm²

Why it is built this way

Along an unmyelinated axon the signal spreads continuously; along a myelinated axon it jumps from node to node, which is called saltatory conduction and is faster ^[1]. Depolarisation also spreads faster down a wide axon than down a narrow one ^[1].

The numbers

For myelinated fibres, Hursh measured conduction velocity rising in proportion to fibre diameter ^[2]:

Conduction velocity of a myelinated fibre (Hursh)^[2]

v \approx k\, D, \qquad k \approx 6\ \tfrac{\mathrm{m/s}}{\mu\mathrm{m}}

For myelinated nerve fibres, conduction velocity grows in proportion to the fibre’s outside diameter. Hursh measured a ratio of about 6 metres per second for every micrometre.

Variables of Conduction velocity of a myelinated fibre (Hursh)
Symbol	Meaning	Unit
$D$	fibre diameter (outside the myelin)	µm
$v$	conduction velocity	m/s

Valid when

Myelinated peripheral fibres of the cat, where it was measured
Using it for central fibres, or for human nerves, is an approximation
Not for unmyelinated fibres, where speed grows more slowly with diameter

Worked example^[2]

A 10 µm fibre conducts at about 60 m/s by this ratio.

D = 10 µm → v = 60 m/s

Common misconceptions

Misconception: A stronger stimulus makes a bigger action potential.

Action potentials are all or none: once threshold is reached the same sequence happens ^[1].

Check yourself

What happens to a depolarisation that does not reach threshold?

It does not produce an action potential ^[1].

Why is saltatory conduction faster?

The action potential effectively jumps from one node of Ranvier to the next, and fresh sodium influx at each node renews it ^[1].

References

Betts JG, Young KA, Wise JA, Johnson E, Poe B, Kruse DH, et al.. 12.4 The Action Potential. Anatomy and Physiology 2e. OpenStax. 2022. https://openstax.org/books/anatomy-and-physiology-2e/pages/12-4-the-action-potential
Hursh JB. CONDUCTION VELOCITY AND DIAMETER OF NERVE FIBERS. American Journal of Physiology-Legacy Content. 1939;127(1):131-139. doi:10.1152/ajplegacy.1939.127.1.131
Hodgkin AL, Huxley AF. A quantitative description of membrane current and its application to conduction and excitation in nerve. The Journal of Physiology. 1952;117(4):500-544. doi:10.1113/jphysiol.1952.sp004764

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