Lesson 1: Inside a neuron: the electrical signal
How a message travels along a single cell.
Start here
Touch your fingertip to your thumb. The instruction to do that left your brain and reached your hand in a fraction of a second — carried by cells thinner than a hair.
In this lesson you will
- Describe how a signal travels within a single neuron.
- Explain what an action potential is in everyday language.
- Identify what has to happen before a neuron fires.
A neuron does not send a message the way a wire carries electricity from a battery. Instead, a neuron uses the difference in electrical charge across its outer membrane. When enough incoming signals arrive at once, that difference changes rapidly and the change travels along the axon.
A charged membrane, ready to go
At rest, the inside of a neuron is slightly more negative than the outside because charged particles called ions are unevenly distributed across the membrane. That stored difference is what the neuron spends when it fires.
Adding up the inputs
Dendrites collect signals from many other cells. Some inputs push the neuron toward firing, others push it away. Only when the combined effect crosses a threshold does the neuron produce an action potential — a rapid change in electrical potential that sweeps along the axon.
All the way, or not at all
An action potential does not get louder or softer. Once threshold is reached, the signal travels the length of the axon at full size. Neurons convey intensity in other ways, such as how often they fire, not by making a single signal bigger.
Diagram
One action potential, start to finish
Notice that nothing happens until the combined input reaches threshold — and after that, the signal is always the same size.
Threshold: Inputs from many dendrites are added together. Only if the combined effect reaches threshold does the neuron fire.
Play the signal: one action potential
Use Play, Next stage, and Restart to step through what happens along the membrane. Nothing moves on its own until you press Play.
Stage 1 of 4 · At rest
The inside of the axon is more negative than the outside. Ions are unevenly distributed across the membrane.
This model highlights the main steps. Real neural systems involve many cells and interacting processes.
Predict: will this neuron fire?
Make a prediction first, then reveal the explanation. You can predict again afterwards.
A neuron receives several small inputs that push it toward firing, and at the same moment several inputs that push it away from firing. What happens?
Vocabulary in context
Select a term to reveal its meaning and an everyday connection. Nothing here is graded.
0 of 4 terms explored
Real-world connection: measuring signals
Because neural activity is electrical, researchers can record it. Electroencephalography (EEG) places sensors on the scalp to measure patterns of activity from large groups of neurons. Clinicians and researchers use EEG to study sleep stages, attention, and seizure activity — evidence of how directly this cell-level idea scales up to whole-brain research.
Three Key Takeaways
- 1A neuron's signal is a change in electrical potential across its membrane, not a current in a wire.
- 2Inputs are added together; the neuron fires only if the combined effect crosses threshold.
- 3An action potential is all-or-nothing — intensity is coded by firing patterns, not by signal size.