Why does stubbing your little toe bring a second, duller pain a moment later?
― Pain travels along nerves of two kinds: a fast line and a slow line
The instant you slam your little toe into a dresser, you know "oh no, I've done it", yet the really bad pain arrives a beat later. It is not your imagination. The news of the pain reaches your brain through two kinds of nerve with very different speeds, and it arrives in two separate deliveries.
At night, you are walking through a dark room and you smash your little toe against the corner of a dresser.
The first thing you feel is a sharp "zing". Then comes a brief moment of bracing yourself, "here it comes, here it comes", and a heavy pain spreads through the toe.
When you cut a fingertip on paper, or bump your hand, you barely notice this gap. Why does it split so clearly into two waves only with the little toe?
There are just two reasons
The nerves that carry sharp pain travel more than ten metres per second. The nerves that carry dull pain travel about one metre per second, not much faster than a person walks.
From toe to brain is about 1.5 metres in an adult. On the slow line, the signal takes more than a second to arrive. The longer the distance, the wider the gap between the two arrivals.
So the "zing" and the dull ache are the news of one single blow, split between two messengers who run at different speeds. Let's look at it step by step.
What is the difference between the fast line and the slow line?
A nerve is a long, thin cell, like an electric wire. The news travels along it as a change in electricity, passed on one stretch after another. But unlike a metal wire, the electricity does not simply rush along. Tiny gates on the surface of the line open one after another, handing the electrical change on to the next stretch bit by bit.
The difference comes from the line's "wrapping". The fast line is wrapped in sections, like sausages, with insulating sheaths made of fat. Where there is a sheath, the gates do not need to open, and the news moves by leaping from one gap in the sheath to the next. Look at the top of Figure 1.
The slow line has no such sheath. It is also very thin. As in the bottom of Figure 1, it has no choice but to open every gate one by one and move forward from one end to the other. That is why it is slow.
The sharp-pain line is said to carry signals at roughly 5 to 30 metres per second. The dull-pain line is said to manage about 0.5 to 2 metres per second. That is a difference of more than ten times.
Why does the little toe split so clearly into two?
The time for news to arrive is set by "distance ÷ speed". Taking the toe-to-brain distance as 1.5 metres, let's lay the times out on a time scale, as in Figure 2.
On the fast line, the news arrives in about 0.1 seconds. That is the "zing". On the slow line, it takes about 1.5 seconds. That is the dull ache that comes after a beat. This gap of a little over a second is what "here it comes, here it comes" really is.
From the fingers to the brain is a much shorter distance than from the toes. That shrinks the gap between the two pains, so they are more likely to feel like a single pain. The "two hits" are so clear with the little toe because it is the farthest place in the body.
When you hit something, don't you pull your foot away without thinking? That was not ordered by your brain. When the news reaches the nerves inside your spine, an order to "pull back" turns around from there directly. Before your brain feels "ouch", your body has already started moving away from danger.
The "being touched" news from rubbing travels on faster lines than pain does. One idea holds that, inside the spine, this news blocks part of the pathway for the pain news. The theory was proposed in 1965, and its details are still being researched, but it fits well with the experience that rubbing eases the pain a little.
Summary
From a single blow, pain is carried as two pieces of news. A fast line wrapped in insulating sheaths carries the "zing", and a thin line without sheaths carries the dull ache. Because the toes are far from the brain, the gap between arrivals widens to more than a second, and the pain is felt in two waves.
The "zing" and the dull ache are the same pain, sent by express and by slow post.
The little toe is where the difference is easiest to see.
For the story of how cold slows the nerve signals themselves, see Why can't you do up a single button when your hands go numb with cold? For how nerve electricity was discovered, read Why did a frog's leg twitch just from touching metal?
- The next time you bump into something, count "one, two" in your head from the "zing" to the dull ache. Compare your toe with the back of your hand and you will see the difference.
- This one needs two people. One person lightly touches the other's ankle and shoulder at the same moment. The person touched says which one they felt first. Many people are said to feel them as almost simultaneous, or to feel the shoulder first.
- Cool your hand in cold water for about a minute, then pinch a fingertip lightly. If the pain feels dull and arrives late, that is a sign the cold has slowed your nerve signals.
* Please do not bump or pinch yourself hard on purpose. Keep the cooling short, too.
For those who want to know more ― terms, formulas and links to textbooksFrom middle-school science to university specialist courses, each part says which level it is
- Middle schoolCovered in middle-school science
- High schoolCovered in high-school biology
- High school+Advanced high-school material, or textbook sidebars
- UniversityUniversity specialist courses (neurophysiology), not taught in high school
- ResearchNot taught even at university as settled fact; what researchers are investigating right now
Middle schoolTerms: this phenomenon has names
- First pain and second pain: the sharp pain that comes first is called first pain, and the dull, spreading pain that comes later is called second pain.
- Myelinated and unmyelinated nerves: a nerve wrapped in an insulating sheath (myelin sheath) is a myelinated nerve, and one without is an unmyelinated nerve.
- Reflex: a sudden bodily movement that turns around in the nerves inside the spine (the spinal cord) without going through the brain.
Middle schoolHigh schoolChecking with a formula: time for pain to reach the brain
We divide the toe-to-brain distance by the speeds of the two kinds of nerve to find the difference in arrival times.
| In symbols | t ＝ L ÷ v Δt ＝ L ÷ v_slow − L ÷ v_fast |
| In words | Travel time = length of nerve ÷ speed of the signal. The time difference between the two pains is the slow line's time minus the fast line's time |
| Where it comes from | It is "speed = distance ÷ time" solved for time. We assume the signal moves along the line at a roughly constant speed |
| Symbol | Meaning and unit |
|---|---|
| L | Length of nerve from toe to brain (m). About 1.5 m in an adult |
| v_fast | Speed of the thin myelinated line that carries sharp pain (m/s) |
| v_slow | Speed of the unmyelinated line that carries dull pain (m/s) |
| t, Δt | Travel time, and the time difference between the two pains (s) |
| Length from toe to brain | About 1.5 m (a guide for adults) |
| Speed of the sharp-pain line | Said to be 5 to 30 m per second; here we use 15 m/s |
| Speed of the dull-pain line | Said to be 0.5 to 2 m per second; here we use 1 m/s |
| Length from fingers to brain | About 1 m (a guide for adults) |
| Time for sharp pain to arrive (s) | 1.5 ÷ 15 ＝ 0.1 |
| Time for dull pain to arrive (s) | 1.5 ÷ 1 ＝ 1.5 |
| Time difference at the little toe (s) | 1.5 − 0.1 ＝ 1.4 |
| Time for dull pain to arrive from a finger (s) | 1 ÷ 1 ＝ 1 |
| Ratio of the two lines' speeds | 15 ÷ 1 ＝ 15 |
At the little toe, the two pains arrive about 1.4 seconds apart. That is long enough to count "one, two". At a finger the distance is shorter, so the gap shrinks too. However, the time the brain takes to experience it as pain is added on top, so what you actually feel is a little longer than this.
High schoolHigh school+Why does a sheath make the signal faster?
High schoolThe news in a nerve is the "action potential", a brief reversal of the electric potential between the inside and outside of the membrane. Sodium channels open and the potential changes, and that change opens the channels in the neighbouring part, in a chain that carries the signal along. In a myelinated nerve, action potentials occur only at the gaps in the myelin sheath (the nodes of Ranvier), and the signal travels by leaping from node to node. This is called saltatory conduction.
High school+Saltatory conduction is known to have been demonstrated by experiment in 1938 by the Japanese physiologist Ichiji Tasaki. The first person to measure that nerve signals have a speed at all was Helmholtz, in 1850. Working with frog nerves, he is reported to have obtained a value of a few tens of metres per second. Until then, the signal was thought to travel instantly.
UniversityClassification of nerve fibres, and what sets conduction speed
In neurophysiology, nerve fibres are classified by thickness and by whether they have myelin. Sharp pain is carried by Aδ fibres, which are thin and myelinated, and dull pain by C fibres, which are unmyelinated. Conduction speed is said to be roughly proportional to the square root of the diameter in unmyelinated fibres, and roughly proportional to the diameter in myelinated fibres. This is explained by cable theory, which considers the electrical capacitance of the membrane and the electrical resistance inside the fibre.
📖 Derivations of the formulas, and further reading: Action potential (Wikipedia) ／ Saltatory conduction (Wikipedia)
ResearchWhat is still not clearly understood
- How rubbing eases pain. The 1965 gate control theory proposed that touch signals suppress the pain pathway in the spinal cord. The broad idea is supported, but the circuit in the original theory has been revised repeatedly in its details.
- Why the dull ache feels so unpleasant. Second pain is thought to be strongly linked to brain regions involved in emotion and memory. But where the "unpleasantness" of pain arises is still under study.
- How the lines' behaviour changes in chronic pain. In long-lasting pain, the sensitivity of nerves and the spinal cord is said to change. Which changes keep the pain going is a major question, and it bears on treatment.
In other words, this article too is an "explanation of what is understood so far". It is the brain that "feels" pain, and nerve speed alone does not decide everything about it.
Links to textbooks (by level)
| Level | Subject and unit | Where in this article |
|---|---|---|
| Middle school | Science Year 2 "Stimulus and Response", Science Year 1 "Speed" | Reflex, calculating distance ÷ speed |
| High school | Biology "Nervous system and conduction of excitation" | Action potential, myelinated nerves and saltatory conduction |
| High school+ | Biology (advanced, history-of-science sidebars) | Ichiji Tasaki, Helmholtz's measurement |
| University | Neurophysiology | Aδ and C fibres, cable theory |
| Research | Pain research (algology) | Gate control theory, chronic pain |
| ― | Links to daily life | Rubbing a bump, numbness from the cold |
- Melzack R., Wall P. D. (1965) Pain Mechanisms: A New Theory. Science 150, 971–979.
- 跳躍伝導 (Saltatory conduction; Japanese Wikipedia)
- 活動電位 (Action potential; Japanese Wikipedia)
- Kandel E. R. et al., Principles of Neural Science, McGraw-Hill (pain conduction and classification of nerve fibres)
- Upper-secondary school Biology textbooks (units on the nervous system and the conduction and transmission of excitation)
* This article is a general-audience science explainer. The figures given are rough guides for understanding how things work. Nerve speeds and distances vary from person to person. If strong pain or swelling continues after a knock, it may be a fracture, so please see a medical professional.