TL;DR:
A resting neuron holds about −70 mV inside relative to outside, kept by the sodium-potassium pump and leaky potassium channels.
An action potential is a wave: sodium channels open and the inside rises, then potassium channels open and it falls back.
At a synapse the electrical signal becomes a chemical one: neurotransmitter crosses the gap and binds receptors on the next cell.
You touch a hot pan and your hand is back before you decide to move it. In between, a signal ran along a chain of neurons. It did not travel like current in a wire. It travelled as a wave of changing voltage that regenerates itself as it goes.
The idea in plain words
A neuron has a cell body, short branches called dendrites that receive signals, and a long fibre called the axon that carries them away. Every living cell has a voltage across its membrane. In neurons that voltage can change fast, and those changes are the signal.
The signal is called an action potential. It is all or nothing: a stimulus either reaches the threshold and fires a full-sized spike, or it does not fire. A stronger stimulus makes the neuron fire more often, not bigger.
Resting potential
At rest, the inside of a typical neuron is about −70 mV compared with the outside. Two things set this. The sodium-potassium pump uses energy to push 3 Na⁺ out and 2 K⁺ in per cycle, so sodium is high outside and potassium is high inside. And the membrane is leaky to K⁺, which drifts out and leaves the inside negative.
Worked example: an action potential travelling down an axon
Follow one spot on the axon as the wave passes. The voltages are typical textbook values.
Rest, about −70 mV. Voltage-gated Na⁺ and K⁺ channels are closed.
Stimulus. Incoming signals nudge the membrane up. If it reaches the threshold, about −55 mV, the neuron fires.
Depolarization. Voltage-gated Na⁺ channels open. Na⁺ rushes in, because it is more concentrated outside and the inside is negative. The inside climbs to about +30 mV.
Repolarization. Na⁺ channels inactivate and K⁺ channels open. K⁺ flows out and the inside drops back toward negative.
Undershoot. K⁺ channels close slowly, so the voltage dips below −70 mV for a moment, then the pump and leak restore rest.
Next spot. The Na⁺ that entered spreads sideways and depolarizes the next patch past its threshold, so the wave moves on.
The wave goes one way because the patch just behind is in its refractory period: its Na⁺ channels are inactivated and cannot fire again right away. Think of dominoes that cannot be stood back up as quickly as they fall.
How a neuron decides to fire
A neuron usually receives many inputs at once. Some are excitatory and push the membrane toward threshold. Others are inhibitory and push it away. The cell body adds them up. If the total at the start of the axon reaches threshold, the neuron fires. If not, nothing is sent.
This summing is why a single neuron can act like a small decision-maker. A pain signal, for example, can be dampened if inhibitory inputs outweigh it.
Myelin and speed
Many axons are wrapped in myelin, a fatty insulating sheath with small gaps called nodes. The signal jumps from node to node, which is faster than creeping along continuously. Damage to myelin slows or blocks signals, which is the basis of some nerve diseases.
The synapse: passing the message on
Neurons do not touch. Between them is a gap called a synapse. When the action potential reaches the end of the axon, it opens calcium channels. Ca²⁺ entry makes vesicles fuse with the membrane and release a neurotransmitter into the gap. The molecules diffuse across and bind receptors on the next cell, which may make it more or less likely to fire.
So the message changes form: electrical along the axon, chemical across the gap, electrical again in the next cell. The neurotransmitter is then cleared away or taken back up, ready for the next signal.
How to check your answer
Order. Na⁺ in first (depolarize), then K⁺ out (repolarize). The reverse is a classic slip.
Direction of flow. Ions move down their concentration gradient through open channels.
Threshold. A sub-threshold stimulus gives no action potential at all.
Voltage sign. The graph should go from −70 up to about +30, down below −70, then back.
Common mistakes
Saying the action potential is a flow of electrons along the axon. It is ions crossing the membrane.
Saying bigger stimuli make bigger spikes. They make more frequent ones.
Mixing up the pump and the channels. The pump uses energy; channels do not.
Forgetting that the synapse is chemical in most neurons.
Putting it together in the hot pan reflex
The sensory neuron in your fingertip fires and sends action potentials along its axon to the spinal cord. At a synapse there, it releases neurotransmitter onto a second neuron, which can pass the signal to a motor neuron. The motor neuron fires and its axon runs to a muscle in your arm. The muscle contracts and the hand pulls back. The brain gets the news a moment later, which is why you feel the pain after you move.
Where you will use it next
Reflexes, muscle contraction, pain, drugs that act on the brain and memory all build on these steps. Links across systems show up when you compare nerve signals with hormone ones in homeostasis and feedback loops.
Frequently asked questions
Why does the signal not travel backward?
The patch behind has just fired and is in its refractory period, so it cannot fire again immediately.
Does a thicker axon carry signals faster?
Yes, in general, and myelin does it even better. Both reduce how much the signal slows along the way.
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