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The world, a little closer

Nervous system.

Your body runs on tiny electrical pulses racing along cells a metre long. Push a neuron past its threshold and watch a signal fire.

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01 / The experiment

All or nothing.

Raise the stimulus past the threshold and watch the pulses. Take the myelin off, then look at the ions.

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The explanation below is available while the scene loads.

One motor neuron, hugely simplified. The threshold is fixed at 40 per cent; real thresholds vary by cell and by moment.Pulses are slowed by a factor of about a thousand, and the glow is only a way of showing where the voltage flips. The axon is drawn short and thick; a real one can be ten thousand times longer than it is wide.

02 / Three parts

In, along, out.

A cell shaped like a wire, with a decision in the middle.

Input01

Dendrites

Thousands of other cells can connect here. Their signals add up at the soma; enough of them together tip it over the threshold.

Job
Collect signals
Receives from
Other neurons, or a sense organ
Shape
A branching bush
Transmission02

Axon

A single fibre that can run from your spine to your toe. Myelin wraps it in insulation so the pulse travels fast.

Job
Carry the pulse
Receives from
The soma
Shape
One long fibre, often myelinated
Output03

Terminals

The pulse releases chemical messengers into the gap to the next cell, which is nudged toward its own threshold.

Job
Pass the message on
Receives from
The arriving pulse
Shape
Fine branches ending at synapses
The big idea

A neuron fires all or nothing. Strength is carried by how often it fires, and speed by whether the axon is wrapped in myelin.

≤100 m/s

Wires made of cells

Everything you sense, decide and do runs through the nervous system: the brain, the spinal cord and the nerves that reach every corner of the body. Its working parts are cells called neurons, and there are about eighty-six billion of them in a human brain alone.

A neuron is a cell stretched into a wire. A bushy cluster of dendrites collects signals from other cells; a body, the soma, adds them up; and a single long fibre, the axon, carries the result away, sometimes a metre or more, to the next cell or to a muscle.

The threshold

Signals arriving at the dendrites nudge the cell toward firing. Too few, and nothing happens: the nudges fade and the neuron stays quiet. Enough, and the cell crosses its threshold and fires a pulse down the axon. There is no half-way. The pulse is always the same size and shape, which is why the rule is called all-or-nothing.

Strength is carried a different way. A gentle touch fires a neuron a few times a second; a hard one fires it many times a second. The message is in the rate, not the size.

Try it: set the stimulus to Faint and wait. Then to Just enough, then Strong, and count the pulses.

What the pulse is

The pulse, called an action potential, is a wave of chemistry travelling along the membrane. At rest the inside of the axon is slightly negative. When the threshold is crossed, tiny gates open and sodium ions flood in, flipping that patch positive. The flip opens the gates next door, and the patch after that, so the wave runs the length of the axon. Behind it, potassium ions flow out and reset the membrane, ready for the next pulse. Reveal Ions to watch both happening.

Insulation and speed

A bare axon carries a pulse at a metre or two a second. Most of the body’s fast nerves are wrapped in myelin, a fatty insulating sheath laid down in segments with small gaps between. Under the sheath the pulse cannot leak away, so it jumps from gap to gap, and the speed rises to a hundred metres a second or more. That is why a stubbed toe is felt in a fraction of a second, and why diseases that strip myelin away leave signals slow and scrambled.

From cell to cell

At the end of the axon the pulse cannot jump straight into the next cell. It reaches a gap, the synapse, and there it releases a puff of chemical messengers that drift across and nudge the next neuron toward its own threshold. Every thought, memory and movement is a pattern of these pulses and puffs, passed from cell to cell, billions of times a second.