Touch a hot pan and you yank your hand back before you've even had time to think. It feels instant. But messages in your nerves aren't truly instantaneous. They travel at measurable speeds, and those speeds vary a lot.
Here's the short answer. Nerve signals travel anywhere from about 0.5 meters per second to more than 120 meters per second. At the top end, that's more than 260 miles per hour. The fastest signals travel along nerve fibers wrapped in an insulating coating called myelin, while the slowest crawl along thin, uncoated fibers at roughly a walking pace.
The Secret Is Insulation
How fast a signal moves depends mostly on two things: how thick the nerve fiber is, and how much myelin it has. Myelin wraps around a nerve fiber in segments, a bit like insulation on a wire.
Between those segments are tiny gaps, each about one micrometer long, called nodes of Ranvier. Instead of creeping along the whole fiber, the signal recharges at each gap and leaps ahead to the next one. Scientists call this saltatory conduction, from the Latin word for leap. It's a big reason myelinated nerves are so fast.
Those gaps are the only places along the fiber where charged particles flow in and out to renew the signal. When the message reaches the end of the line, it triggers the release of chemical messengers that pass it along to the next cell, whether that's another nerve, a muscle, or a gland.
When myelin is damaged, signals slow down or fail. In multiple sclerosis, for example, the immune system attacks myelin, which impairs the nervous system's ability to send signals.
Why a Stubbed Toe Hurts Twice
Pain gives a perfect example of different nerve speeds. Pain-sensing nerves come in two main types. One kind, called A-delta fibers, has myelin and carries signals at about 5 to 30 meters per second.
The other kind, called C fibers, is thinner and has little or no myelin. Those signals move at only about 0.4 to 2 meters per second, roughly 1 to 4.5 miles per hour.
That's why pain often comes in two waves. First comes a sharp, sudden jolt carried by the faster fibers. Then a slower, longer-lasting ache arrives through the slower ones. The next time you stub your toe, you can notice both. Long or intense input to the slow fibers can even build up in the spinal cord, an effect called wind-up that can make you more sensitive to pain.
Arms Versus Legs
Doctors measure nerve speeds with tests called nerve conduction studies. In healthy people, signals in the nerves of the legs typically travel about 40 to 45 meters per second, and signals in the arms about 50 to 65 meters per second.
Everyone's numbers are a little different. Nerve speed can depend on age, sex, temperature, and even body measurements such as height and hand size. If you ever have numbness, tingling, or weakness, talk to your doctor.
The Man Who First Clocked a Nerve
For a long time, many people believed nerve signals moved so fast they couldn't be measured. Then, in 1849, the German scientist Hermann von Helmholtz proved otherwise.
He used a frog's leg nerve and the calf muscle attached to it. To time the signal, he rigged a sensitive meter with a tiny mirror that bounced a beam of light across the room onto a scale. He measured speeds of about 24.6 to 38.4 meters per second.
So the next time you react in a flash, remember that your body is running an incredible messaging system, with express lanes and slow lanes, all working together in a fraction of a second.
