Nerve signals.

Step 1 of 7

A cell at rest

The nervous system, the brain, the spinal cord and the nerves, carries messages through the body faster than any other system. Its unit is the nerve cell (neuron): branching dendrites gather signals, the cell body adds them up, and a long axon carries the result on, sometimes a metre or more.

Even when a nerve cell is quiet, it is not switched off. Pumps in its membrane push sodium ions out and bring potassium ions in, which sets up a steady voltage between the inside of the cell and the outside. Stimulus is now fixed at zero.

Step 2 of 7

The threshold

Signals arriving at the dendrites raise the voltage in the cell body, each a little. If the voltage reaches the threshold, about −55 mV, the cell fires a nerve impulse, also called an action potential, along the axon. If not, the small effects fade and nothing goes out. The threshold of the cell in the Scene is fixed.

Step 3 of 7

All or nothing

When the cell crosses the threshold, gates in the membrane open and sodium ions rush in, until the inside of the cell turns positive for a moment. The size of the impulse is set by the gates and the ions, not by the stimulus that started it. This is the all-or-nothing rule.

Step 4 of 7

Strength as rate

If every impulse is the same size, how does the brain know whether you were touched lightly or pressed hard? Count the impulses. Stimulus is now fixed a little above the threshold.

Step 5 of 7

What travels along the axon

Show the ions is On. Watch the dots crossing the membrane just ahead of the impulse’s glow and behind it. The flip of the voltage in one patch opens the gates in the next, and so the impulse moves on.

Step 6 of 7

How fast

Most fast nerves are wrapped in myelin, a fatty sheath laid down in segments, with small gaps between them. Under the sheath the impulse cannot leak, so it jumps from gap to gap. A myelinated axon conducts at about 100 m/s. In the Scene the impulses are slowed about a thousandfold, so you can see them.

Step 7 of 7

Without myelin

A bare axon conducts an impulse at about a metre a second. In the Scene the bare impulse looks only three times slower, so that both cases fit on the screen; the real difference you will work out yourself.