Your blow warns at 20 metres per second and hurts at one

Knock your shin against a table leg and wait. The sting's warning climbs a cable sheathed in fat at around 20 metres per second; the dull ache that follows travels along a different, bare cable at barely more than a metre per second. They are two separate warnings from the same accident, and the second one takes its time to show up.

Thomas Lewis and Edward Pochin, two doctors at University College Hospital in London, published it in 1937 in Clinical Science, a clinical physiology journal, and called it the double response: a single stimulus, two pains. They pricked volunteers' skin just once and the volunteers reported two separate sensations, one after the other. What turned that into physiology rather than personal impression was the stopwatch: the gap between the two pains grew with the distance from the brain. If the second pain were a trick of memory, it would measure the same in the hand as in the foot. It does not.

The difference is in the sheathing. The fast fibres, called A-delta, are wrapped in myelin, a fatty sheath that works as insulation. Microneurography — sticking an electrode finer than a hair into a living human nerve and listening to what runs through it — has measured them at between 5 and 30 metres per second, averaging around 19. The slow ones, called C fibres, are bare, up to a hundred times finer than a hair, and conduct at barely more than a metre per second: the speed of someone strolling down a corridor in no hurry.

Each one carries a different message. The fast sting is for locating: it tells you the exact spot and makes you pull your leg back. The slow pain cannot point a finger at anything, it smarts, and it is the one that changes your behaviour over the following hours: the one that makes you limp, the one that makes you check where the table leg is every time you walk past.

These two cables should not be confused with the fastest ones you have, which do not carry pain at all: the ones that carry fine touch and limb position are thicker and conduct at 120 metres per second. In order of arrival, touch gets in six times sooner than the sting, and the sting twenty times sooner than the burn.

The fact that pain has two routes explains a dissociation seen in the clinic. In some neuropathies the sense of warmth is lost while the sense of pinprick is kept, because the damage is in the C fibres and not in the A-delta ones; in others it happens the other way round. Neurological examination makes use of this: the fine needle and cold question the A-delta fibres, and heat questions the C fibres.

If you want to time it, do it on your foot and not on your hand. From foot to spinal cord the slow warning takes about a second and the fast one some five hundredths, so the gap shows up without any instrument at all. In the hand, with almost a third less nerve ahead of it, the gap drops to about seven tenths and is harder to catch.