Why does poking your skin sometimes hurt and sometimes just ache a beat later?
After you watchWhy does poking your skin sometimes hurt and sometimes just ache a beat later?
The short answer
A poke can feel like two things at two times because your body carries the signal on two kinds of nerve wire. Fast wires wrapped in a fatty sleeve let the signal leap between gaps and reach the brain almost instantly as a sharp sting, while thin bare wires have to re-make the signal at every step, so they crawl and deliver a dull, delayed ache.
Try this next
- What if you made the bare wire longer — like a signal coming from your toe instead of your finger? In the race, send both signals from the farthest spot and predict first: does the gap between the sharp sting and the dull ache get bigger or stay the same?
- What if a wrapped wire had its fatty sleeve damaged in just one spot, not stripped all the way? Strip only part of the wrap and predict whether the signal still leaps the rest of the gaps or stalls at the bare patch.
The whole story
How it works
A nerve is a wire that carries a tiny electric signal from your skin to your brain. Some nerve wires are wrapped in a fatty sleeve called myelin, with small bare gaps between the wraps. On a wrapped wire the signal jumps from gap to gap, skipping the insulated stretches, so it races up the wire very fast. A bare, unwrapped wire can't jump; the signal has to rebuild itself at every tiny point along the way, which is much slower. So the same pinch sends a fast signal up the wrapped wire (the sharp first sting) and a slow signal up the bare wire (the dull ache that shows up a moment later).
What people get wrong
It's easy to think a nerve is just a wire, so as long as it's connected the message arrives at full speed. But connection isn't speed. Strip the fatty wrap off a nerve and the signal can no longer leap between gaps; it has to refire at every single step, so it crawls in late and weak. The wrap, not just the wire, is what makes a warning fast.
The catch
Fast wrapped wires give a lightning-quick warning, perfect for yanking your hand off something hot, but the fatty sleeve is bulky and costly to build, so there is only room for so many. Thin bare wires are cheap, so the body can pack in lots of them to carry the lasting ache that says keep protecting this spot, but they crawl, so they can never deliver a split-second warning. Each kind of wire is good at a different job.
Questions kids ask
Why does a stubbed toe hurt twice — first sharp, then dull?
The pinch fires signals up two kinds of nerve at once. The sharp jolt rides a fast wire wrapped in fatty myelin, so it leaps to the brain first. The dull throb rides a thin bare wire that crawls, so it arrives a moment later. Same toe, two wires, two arrival times.
What does the fatty wrap on a nerve actually do?
The fatty wrap, called myelin, insulates the wire and leaves small bare gaps. The signal jumps from gap to gap instead of traveling along every point, which makes it travel many times faster. Without the wrap the signal has to rebuild itself at every step and slows way down.
Does myelin make a nerve signal faster or slower?
Faster. Myelin lets the electric signal leap between the bare gaps rather than crawling along the whole wire, so a wrapped nerve can carry a signal up to around a hundred times faster than a bare one of the same size.
What happens if a nerve loses its myelin?
Without myelin the signal can no longer leap between gaps and must refire at every step, so it travels much more slowly or stops getting through. This is what happens in diseases like multiple sclerosis, where damaged myelin slows or blocks nerve signals.
Talk about it
- When you bang your shin, you feel it twice — guess why one feeling shows up before the other.
- If you had to design a body, which nerves would you make fast and which would you let be slow, and why?
For grown-ups
The fatty wrap is myelin, and the bare gaps between wraps are the nodes of Ranvier. In a myelinated fiber the impulse only regenerates at those nodes and effectively jumps between them, called saltatory conduction, reaching speeds up to roughly 120 meters per second. Sharp first pain travels on fast thin-myelinated A-delta fibers, while the dull, delayed second pain travels on thin unmyelinated C fibers at about 1 meter per second. Myelin is an insulating sleeve of glial-cell membrane; when it is stripped, as in multiple sclerosis, conduction slows or fails.
Keep going
What else makes you wonder?
- Other animals have nerves too — does a giraffe's sting take longer to reach its brain than yours, since the wire is so much longer?
- If a wrapped wire is so much faster, why didn't the body just wrap every single nerve?
- How does your body know to build the fatty wrap around a wire while you're still growing?