1. Nociceptors: The Bare Nerve Endings in Your Skin That Detect Pain
The sensors that receive pain are called nociceptors. Their true identity is free nerve endings, which are essentially the exposed tips of nerves found under the skin. They are not only in the skin but also in joints, deep inside the body, and even in the cornea of the eye [3]. Their job is to turn strong stimuli that could harm the body into electrical signals.
However, the electrical signal itself is not the pain. We will explain that distinction in the final section.
2. How Pain Signals Travel Through the Spinal Cord and Thalamus to the Brain
The signal created by the sensor in your skin travels through nerve fibers into the spinal cord inside your backbone. Inside the spinal cord, the signal is passed to the next nerve, crosses to the opposite side of the body, and moves upward to reach the thalamus, which is deep inside the brain [2].
From the thalamus, signals are sent to the somatosensory cortex of the cerebral cortex, as well as areas called the insular cortex and the anterior cingulate cortex [4]. It is explained that these areas process the location and intensity of the pain separately from the part that handles the "unpleasant feeling." Pain does not seem to be created in just one single spot in the brain.
3. How Fast and Slow Nerve Fibers Split Pain into Two Waves
There are mainly two types of fibers that carry pain. A-delta fibers have a thin myelin sheath and transmit signals at about 5 to 30 meters per second. They carry sharp pain, known as the first pain. C fibers have no myelin sheath and are much slower, at about 0.4 to 2 meters per second. They carry the dull, spreading pain, known as the second pain [3]. Because numbers vary depending on the source, these are rough estimates.
Let us calculate this by assuming the distance from your toe to your brain is about 1.5 meters. If a fiber travels at 20 meters per second, 1.5 divided by 20 is about 0.075 seconds. If a fiber travels at 1 meter per second, 1.5 divided by 1 is about 1.5 seconds. The difference is about 1.4 seconds.
This is not a measured fact from a source but a rough estimate for explanation. Since processing time in the spinal cord and brain also takes time, this does not exactly match the real interval between the sharp and dull pains. Still, it helps you imagine why pain comes in two waves.
4. How Capsaicin in Chili Peppers Triggers the Same Sensors as Pain
Capsaicin, the spicy component in chili peppers, works by activating sensory nerves that carry pain, causing a burning sensation. In 1997, David Julius and his team extracted the gene for the receptor that detects this [5].
It was found that this receptor also works when the temperature gets high enough to cause pain. This means "spicy" and "hot" are connected to the same switch. It later became known as TRPV1. The 2021 Nobel Prize in Physiology or Medicine was awarded to Julius and Ardem Patapoutian; Patapoutian and colleagues discovered Piezo1 and Piezo2 [6]. TRPV1 is activated by heat that is painful enough to hurt.
5. Gate Control Theory: One Explanation for Why Rubbing a Bumped Spot Helps
Have you ever instinctively rubbed a spot you bumped? One theory that may explain this is the "Gate Control Theory," published by Melzack and Wall in the journal *Science* in 1965 [7].
The theory suggests that at the entrance to the spinal cord, thick fibers that carry touch signals suppress the signals from the thin fibers that carry pain. Imagine there is a "gate" before the pain signals, and touch goes to close that gate.
Today, this theory is considered too simple. However, it is said to have greatly advanced pain research. It does not mean that rubbing will always make the pain go away.
6. Why Pain Is More Than a Nerve Signal: The 2020 Definition of Pain
The international definition of pain was revised in 2020. The Japanese translation by the Japanese Society for the Study of Pain defines pain as "an unpleasant sensory and emotional experience associated with, or resembling that associated with, actual or potential tissue damage" [1].
A note adds that pain and nociception are different phenomena, and pain cannot be inferred from sensory nerve activity alone. Even if sensors send signals, it is the brain’s work that turns them into the experience of "pain." Even now that we understand the path, some ideas, like the Gate Control Theory, are still debated.
7. What to Notice the Next Time You Bump Into Something
Next time you accidentally bump into something, try to notice if a dull "throb" comes after the sharp "zap." Do not purposely bump or pinch yourself to test this.
Find a book with diagrams of the spinal cord and brain in the library. Looking at the path of pain signals in pictures is recommended.
If pain continues or is strong, tell an adult and consult a doctor.