Nervous System and Neuron Firing

Parts of a neuron

• Dendrites receive signals from other cells.
• The cell body (soma) contains the nucleus and combines incoming signals.
• The axon carries action potentials away from the cell body, sometimes over a metre in humans.
• Axon terminals release chemical messengers (neurotransmitters) to the next cell at a synapse.

The resting potential

A resting neuron is electrically polarized: the inside is about 65–70 mV more negative than the outside. Thisresting membrane potential exists because the sodium–potassium pump keeps Na⁺ concentrated outside and K⁺ concentrated inside, and because the resting membrane lets K⁺ leak out more easily than Na⁺ can leak in.

The action potential

1. Stimulus and threshold: a stimulus makes the inside less negative. If it reaches about −55 mV (threshold), the response becomes self-sustaining.
2. Depolarization: voltage-gated Na⁺ channels open and sodium rushes in. The membrane potential shoots up to about +40 mV within a millisecond.
3. Repolarization: Na⁺ channels inactivate, and voltage-gated K⁺ channels open. Potassium flows out and the inside becomes negative again.
4. Hyperpolarization: K⁺ channels close slowly, so the potential briefly dips below resting (the undershoot) before recovering.

The whole spike lasts only about 1–2 milliseconds. Remarkably few ions actually cross the membrane, so the concentration gradients barely change during a single spike.

How the channels work

Hodgkin and Huxley described each channel with “gates”. The sodium channel has a fast activation gate (m) that opens when the membrane depolarizes and a slower inactivation gate (h) that closes shortly afterwards. The potassium channel has a slow activation gate (n). The lower graph shows these gates: m shoots up first, then h falls and n rises — the timing difference is what creates the spike’s shape.

Refractory periods

Immediately after a spike, inactivated sodium channels cannot reopen, so no second spike is possible — theabsolute refractory period. A little later, a stronger-than-usual stimulus can trigger a spike — therelative refractory period. Refractoriness limits how fast a neuron can fire and makes signals travel in one direction only.

Propagation and myelin

An action potential in one patch of membrane depolarizes the patch next to it, which then fires too, so the spike travels down the axon like a falling line of dominoes. Many axons are wrapped in myelin, a fatty insulating layer made by glial cells. The signal then jumps between small uninsulated gaps called nodes of Ranvier — saltatory conduction — which is much faster and uses less energy. Diseases that damage myelin, such as multiple sclerosis, slow or block nerve signals.

The Hodgkin–Huxley model

Working with the giant axon of the squid, Alan Hodgkin and Andrew Huxley described the action potential with a set of equations in 1952, for which they shared the 1963 Nobel Prize. The simulator solves their equations directly:

Here g are maximum conductances, E are the equilibrium (reversal) potentials for each ion (about +50 mV for Na⁺ and −77 mV for K⁺), and m, h and n are the gates, each changing at voltage-dependent rates.

Key terms

Membrane potential

The voltage difference across the cell membrane, inside relative to outside.

Depolarization / repolarization

The membrane becoming less negative / returning to negative.

Threshold

The membrane potential at which an action potential is triggered, about −55 mV.

Voltage-gated channel

A protein pore that opens or closes in response to changes in membrane potential.

Saltatory conduction

Rapid jumping of an action potential between nodes of Ranvier on a myelinated axon.

Real-world connections

• Local anaesthetics such as lidocaine work by blocking sodium channels so pain signals cannot travel (educational context only).
• Nerve toxins: tetrodotoxin from puffer fish blocks Na⁺ channels, which is why improperly prepared puffer fish can be lethal.
• Medical tests such as nerve conduction studies and EEGs measure electrical activity from neurons.
• Neuroscience research and brain–computer interfaces build on models like Hodgkin–Huxley.

Common misconceptions

• “Stronger stimuli make bigger spikes.” Spikes are all-or-none; stronger stimuli produce more frequent spikes.
• “Nerve signals are electrons flowing like in a wire.” They are waves of ions moving across the membrane.
• “The sodium–potassium pump makes each spike.” The pump maintains gradients over time; the spike itself is driven by channels.

Key takeaways

• Neurons rest at about −65 to −70 mV, inside negative.
• Reaching threshold opens Na⁺ channels (depolarization); K⁺ channels then restore the voltage (repolarization).
• Action potentials are all-or-none and followed by refractory periods.
• Myelin makes signals jump between nodes, greatly increasing speed.