A neuron gathers whispers from thousands of neighbors and, if they add up loud enough, fires its own signal down a long wire to the next cells. It's a tiny decision-maker, and billions of them wired together make up your thinking brain.
A cell built to signal
A neuron receives signals on its dendrites, integrates them in the cell body, and — if the sum is strong enough — sends a spike down its axon to other cells. Its trick is the membrane potential: by pumping ions across its membrane, a resting neuron holds its inside about 70 mV negative relative to outside. This charged, poised state is stored energy waiting to be released.
When incoming signals push the membrane past a threshold, voltage-gated sodium channels snap open. Positive ions rush in, the voltage briefly flips positive, and this depolarization races down the axon as an action potential. Potassium channels then restore the negative resting state.
An action potential either fires fully or not at all — there is no half-spike. Neurons encode intensity not by spike size but by spike frequency: a louder sound means faster firing, not bigger spikes.
- Sketch a voltage-vs-time graph starting at the −70 mV resting level.
- Mark the threshold (around −55 mV). Draw the sharp upstroke as sodium rushes in, peaking near +40 mV.
- Draw the downstroke as potassium exits, dipping into a brief undershoot below rest before recovering.
- Label each phase with the ion movement responsible for it.
What you should see: You reconstructed the canonical action-potential waveform and tied each segment to the ion channels driving it.