Biology · Introductory biology · Concept

Neurons and the action potential

A resting neuron holds a membrane potential of about −70 mV, negative inside, kept by the sodium-potassium pump and by K⁺ leaking out through open channels. A stimulus that depolarizes the membrane to threshold, about −55 mV, opens voltage-gated Na⁺ channels: Na⁺ rushes in and the inside swings to about +30 mV. The Na⁺ channels then inactivate and voltage-gated K⁺ channels open; K⁺ flows out and repolarizes the membrane, overshooting briefly below rest. The action potential is all-or-none, and it travels along the axon to the synapse.

The parts of a neuron

Dendrites receive signals from other cells, and the cell body, or soma, combines them. Where the axon leaves the soma, at the axon hillock, the neuron decides whether to fire. The axon carries the action potential to its terminals, which pass the signal to the next cell at synapses. In many neurons a myelin sheath insulates the axon, with gaps called nodes of Ranvier.

A neuron, and the voltage of one action potential

Phase	Membrane potential	What happens
Resting	About −70 mV	K⁺ leaks out; the pump keeps Na⁺ outside and K⁺ inside
Depolarization	Rises past −55 mV to about +30 mV	Voltage-gated Na⁺ channels open and Na⁺ rushes in
Repolarization	Falls back toward −70 mV	Na⁺ channels inactivate; voltage-gated K⁺ channels open and K⁺ flows out
Hyperpolarization	Dips to about −80 mV	K⁺ channels close slowly
Return to rest	About −70 mV	Leak channels and the pump restore the resting state

The resting potential

At rest the inside of the membrane is about 70 mV more negative than the outside. The sodium-potassium pump moves 3 Na⁺ out for every 2 K⁺ in, so Na⁺ is concentrated outside and K⁺ inside. The resting membrane is far more permeable to K⁺ than to Na⁺, and K⁺ leaking out leaves the inside negative.

Firing: depolarization and repolarization

A stimulus that brings the axon hillock to threshold opens voltage-gated Na⁺ channels. Na⁺ rushes in down its gradient, and the inside becomes positive. Within about a millisecond the Na⁺ channels inactivate and voltage-gated K⁺ channels open, so K⁺ flows out and the potential falls back. The K⁺ channels close slowly, so the membrane briefly overshoots below rest.

All-or-none and the refractory period

A stimulus below threshold fades without an action potential; any stimulus that reaches threshold makes a full one. A stronger stimulus makes action potentials more often, not bigger ones. Just after an action potential the inactivated Na⁺ channels cannot reopen, the refractory period, so the signal travels only forward along the axon.

Conduction and the synapse

Each patch of membrane that fires depolarizes the next. In a myelinated axon the action potential jumps from node to node, which is called saltatory conduction and is much faster. At an axon terminal, Ca²⁺ entering the cell triggers vesicles to release neurotransmitter, which crosses the synapse and binds receptors on the next cell.

Common mistakes

Thinking a stronger stimulus makes a bigger action potential: every action potential is the same size; a stronger stimulus makes them more often.
Saying K⁺ rushes in during depolarization: Na⁺ flows in; K⁺ flows out during repolarization.
Crediting the sodium-potassium pump with repolarizing each action potential: K⁺ channels do that; the pump maintains the gradients over time.
Placing the threshold at 0 mV: it is about −55 mV, and the peak is about +30 mV.

Key terms

Neuron: A nerve cell, built to carry electrical signals: dendrites receive input, the soma adds it up, and the axon carries action potentials to the terminals.
Dendrite: A branch of a neuron that receives signals from other cells and carries them toward the soma.
Soma: The main body of a neuron, containing its nucleus and much of its biosynthetic machinery. It integrates many inputs received through the neuron’s processes.
Axon: The long extension that carries action potentials away from the soma to the axon terminals, where the neuron signals the next cell at synapses.
Myelin: A fatty insulating sheath wrapped around many axons by glial cells. The signal jumps between its gaps, the nodes of Ranvier, so myelinated axons conduct much faster.
Membrane potential: The voltage across a cell membrane, inside relative to outside. A resting neuron sits near −70 mV, kept there by the sodium–potassium pump and K⁺ leak channels.
Action potential: A brief, all-or-none reversal of a neuron’s membrane voltage. At threshold (about −55 mV) Na⁺ channels open and the inside rises to about +30 mV; then K⁺ channels bring it back. It travels along the axon.
Neural threshold: The membrane voltage, about −55 mV in a typical neuron, at which voltage-gated Na⁺ channels open and an action potential fires. Below threshold nothing fires: an action potential is all or none.
Depolarization: A rise in membrane voltage toward zero or above. In an action potential, Na⁺ rushing in depolarizes the membrane from threshold to about +30 mV.
Repolarization: The fall of membrane voltage back toward rest after depolarization, as voltage-gated K⁺ channels open and K⁺ flows out of the cell.
Hyperpolarization: A drop of membrane voltage below the resting potential, as when K⁺ channels stay open briefly after an action potential (the undershoot).
Voltage-gated ion channel: Describes an ion channel that opens or closes as the membrane voltage changes. Voltage-gated Na⁺ and K⁺ channels produce the action potential.
Refractory period: The short time after an action potential when a neuron cannot fire again (absolute) or needs a stronger stimulus (relative). It keeps the signal moving one way along the axon.
Saltatory conduction: The jumping of an action potential from one node of Ranvier to the next along a myelinated axon. Myelin insulates the stretches between, so the signal travels much faster than along an axon without myelin.
Synapse: A junction through which a neuron communicates with another cell. Chemical synapses use neurotransmitter molecules; electrical synapses connect cells through conductive junctions.
Neurotransmitter: A chemical that a neuron releases at a synapse. It crosses the gap and binds receptors on the next cell, exciting or inhibiting it; acetylcholine, glutamate and GABA are examples.
Membrane pump: A transport protein that uses energy, usually from ATP, to move ions against their gradient. The sodium–potassium pump moves 3 Na⁺ out and 2 K⁺ in for each ATP.

Work through an example

A neuron rests at −70 mV and has a threshold of −55 mV. Three separate stimuli depolarize it by 10 mV, 15 mV and 30 mV. Which of them fire an action potential, and how do the action potentials compare? At the peak the membrane reaches +30 mV: how far does the voltage change from rest to peak?

Decide which stimuli fire a neuron →

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