Nerves & senses

You accidentally touch something hot and pull your hand back in a flash, even before you think about it. How can that be so fast? Your body has its own messaging system made of nerve cells that pass signals along at great speed.

Nerve cells carry signals

Nerve cells are long, thin cells. A small electrical impulse runs through them, similar to current through a cable. This impulse is only sent off when the stimulus reaches a minimum strength. That minimum strength is called the threshold. At the contact point between two nerve cells, the synapse, the message then jumps onward until it reaches its target.

The brain as control centre

All important messages come together in the brain. It is the control centre that reads the signals and sends commands back. The five classic senses are the inputs: seeing, hearing, smelling, tasting and touching constantly deliver new information about the surroundings.

Stimulus and reaction

From perception to movement a fixed chain runs its course: a sense takes in a stimulus, the nerves pass it on, the brain processes it and sends a reply. For a protective reflex like pulling back your hand it is even faster: the spinal cord triggers the movement directly, before the brain even finds out. For a single nerve cell the all-or-nothing principle applies. If the stimulus reaches the threshold, the cell fires at full strength. If it stays below, nothing happens at all.